Alfred S McEwen

Interests

No activities entered.

Courses

Scholarly Contributions

Chapters

• Patterson, G. W., McEwen, A. S., Turtle, E. P., Ernst, C. M., & Kirk, R. L. (2019). Data Products from the Europa Imaging System (EIS) on Europa Clipper. In Astronomical Data Analysis Software and Systems XXVII(p. 605).

Journals/Publications

• Daubar, I. J., Garcia, R. F., Stott, A. E., Fernando, B., Collins, G. S., Dundas, C. M., W{\'ojcicka}, N., Zenh{\"ausern}, G., McEwen, A. S., St{\"ahler}, S. C., Golombek, M., Charalambous, C., Giardini, D., Lognonn{\'e}, P., & Banerdt, W. B. (2024). Seismically detected cratering on Mars: Enhanced recent impact flux?. Science Advances, 10(26), eadk7615.
• Daubar, I., Hayes, A., Collins, G., Craft, K., Rathbun, J., Spencer, J., Wyrick, D., Bland, M., Davies, A., Ernst, C., Howell, S., Leonard, E., McEwen, A., Moore, J., Phillips, C., Prockter, L., Quick, L., Scully, J., Soderblom, J., , Brooks, S., et al. (2024). Planned Geological Investigations of the Europa Clipper Mission. \ssr, 220(1), 18.
• Hamilton, C. W., McEwen, A. S., Keszthelyi, L., Carter, L. M., Davies, A. G., Kleer, K., Jessup, K. L., Jia, X., Keane, J. T., Mandt, K., Nimmo, F., Paranicas, C., Park, R. S., Perry, J. E., Pommier, A., Radebaugh, J., Sutton, S. S., Vorburger, A., Wurz, P., , Borlina, C., et al. (2024). Comparing NASA Discovery and New Frontiers Class Mission Concepts for the Io Volcano Observer (IVO). arXiv e-prints, arXiv:2408.08334.
• Hansen, C., Byrne, S., Calvin, W., Diniega, S., Dundas, C., Hayne, P., McEwen, A., McKeown, L., Piqueux, S., Portyankina, G., Schwamb, M., Titus, T., & Widmer, J. (2024). A comparison of CO$_2$ seasonal activity in Mars' northern and southern hemispheres. \icarus, 419, 115801.
• Landis, M., Dundas, C., McEwen, A., Daubar, I., Hayne, P., Byrne, S., Sutton, S., Rangarajan, V., Tornabene, L., Britton, A., & Herkenhoff, K. (2024). New, dated small impacts on the South Polar Layered Deposits (SPLD), Mars, and implications for shallow subsurface properties. \icarus, 419, 115977.
• Lucas, A., Daubar, I., Le Teuff, M., Perrin, C., Kawamura, T., Posiolova, L., Lognonn{\'e}, P., Rodriguez, S., Giardini, D., Sainton, G., Mangeney, A., & McEwen, A. (2024). Possibly seismically triggered avalanches after the S1222a Marsquake and S1000a impact event. \icarus, 411, 115942.
• McEwen, A., Byrne, S., Hansen, C., Daubar, I., Sutton, S., Dundas, C., Bardabelias, N., Baugh, N., Bergstrom, J., Beyer, R., Block, K., Bray, V., Bridges, J., Chojnacki, M., Conway, S., Delamere, W., Ebben, T., Espinosa, A., Fennema, A., , Grant, J., et al. (2024). The high-resolution imaging science experiment (HiRISE) in the MRO extended science phases (2009\textendash2023). \icarus, 419, 115795.
• Mills, M., Bickel, V., McEwen, A., & Valantinas, A. (2024). A global dataset of pitted cones on Mars. \icarus, 418, 116145.
• Mills, M., McEwen, A., Hughes, A., Kim, J., & Okubo, C. (2024). Analyzing spatial distributions and alignments of pitted cone features in Utopia Planitia on Mars. \icarus, 408, 115825.
• Pappalardo, R. T., Buratti, B. J., Korth, H., Senske, D. A., Blaney, D. L., Blankenship, D. D., Burch, J. L., Christensen, P. R., Kempf, S., Kivelson, M. G., Mazarico, E., Retherford, K. D., Turtle, E. P., Westlake, J. H., Paczkowski, B. G., Ray, T. L., Kampmeier, J., Craft, K. L., Howell, S. M., , Klima, R. L., et al. (2024). Science Overview of the Europa Clipper Mission. \ssr, 220(4), 40.
• Roth, L., Bl{\"ocker}, A., Kleer, K., Goldstein, D., Lellouch, E., Saur, J., Schmidt, C., Strobel, D., Tao, C., Tsuchiya, F., Dols, V., Huybrighs, H., Mura, A., Szalay, J., Badman, S., Pater, I., Dott, A. -., Kagitani, M., Klaiber, L., , Koga, R., et al. (2024). Mass supply from Io to Jupiter's magnetosphere. arXiv e-prints, arXiv:2403.13970.
• Turtle, E., McEwen, A., Patterson, G., Ernst, C., Elder, C., Slack, K., Hawkins, S., McDermott, J., Meyer, H., DeMajistre, R., Espiritu, R., Seifert, H., Niewola, J., Bland, M., Becker, M., Centurelli, J., Collins, G., Corlies, P., Darlington, H., , Daubar, I., et al. (2024). The Europa Imaging System (EIS) Investigation. \ssr, 220(8), 91.
• Valantinas, A., Thomas, N., Pommerol, A., Karatekin, O., Ruiz Lozano, L., Senel, C., Temel, O., Hauber, E., Tirsch, D., Bickel, V., Munaretto, G., Pajola, M., Oliva, F., Schmidt, F., Thomas, I., McEwen, A., Almeida, M., Read, M., Rangarajan, V., , El-Maarry, M., et al. (2024). Evidence for transient morning water frost deposits on the Tharsis volcanoes of Mars. Nature Geoscience, 17(7), 608-616.
• Zurek, R., Tamppari, L., Johnston, M. D., Murchie, S., McEwen, A., Byrne, S., Seu, R., Putzig, N., Kass, D., Malin, M., & Cantor, B. (2024). MRO overview: Sixteen years in Mars orbit. \icarus, 419, 116102.
• Almeida, M., Read, M., Thomas, N., Cremonese, G., Becerra, P., Borrini, G., Byrne, S., Gruber, M., Heyd, R., Marriner, C., McArthur, G., McEwen, A., Pommerol, A., Perry, J., & Schaller, C. (2023). Targeting and image acquisition of Martian surface features with TGO/CaSSIS. \planss, 231, 105697.
• Dundas, C. M., Mellon, M. T., Posiolova, L. V., Miljkovi{\'c}, K., Collins, G. S., Tornabene, L. L., Rangarajan, V. G., Golombek, M. P., Warner, N. H., Daubar, I. J., Byrne, S., McEwen, A. S., Seelos, K. D., Viola, D., Bramson, A. M., & Speth, G. (2023). A Large New Crater Exposes the Limits of Water Ice on Mars. \grl, 50(2), e2022GL100747.
• Meitzler, R., Jun, I., Blase, R., Cassidy, T., Clark, R., Cochrane, C., Fix, S., Gladstone, R., Goldsten, J., Gudipati, M., Hand, K., Henderson, B., Jia, X., Kammer, J., Kollmann, P., McEwen, A., Meyer, H., Nordheim, T., Paranicas, C., , Paty, C., et al. (2023). Investigating Europa's Radiation Environment with the Europa Clipper Radiation Monitor. \ssr, 219(7), 61.
• Roberts}, J. H., McKinnon, W. B., Elder, C. M., Tobie, G., Biersteker, J. B., Young, D., Park, R. S., Steinbr{\"ugge}, G., Nimmo, F., Howell, S. M., Castillo-Rogez, J. C., Cable, M. L., Abrahams, J. N., Bland, M. T., Chivers, C., Cochrane, C. J., Dombard, A. J., Ernst, C., Genova, A., , Gerekos, C., et al. (2023). Exploring the Interior of Europa with the Europa Clipper. \ssr, 219(6), 46.
• Vance, S. D., Craft, K. L., Shock, E., Schmidt, B. E., Lunine, J., Hand, K. P., McKinnon, W. B., Spiers, E. M., Chivers, C., Lawrence, J. D., Wolfenbarger, N., Leonard, E. J., Robinson, K. J., Styczinski, M. J., Persaud, D. M., Steinbr{\"ugge}, G., Zolotov, M. Y., Quick, L. C., Scully, J. E., , Becker, T. M., et al. (2023). Investigating Europa's Habitability with the Europa Clipper. \ssr, 219(8), 81.
• Daubar, I., Dundas, C., McEwen, A., Gao, A., Wexler, D., Piqueux, S., Collins, G., Miljkovic, K., Neidhart, T., Eschenfelder, J., Bart, G., Wagstaff, K., Doran, G., Posiolova, L., Malin, M., Speth, G., Susko, D., & Werynski, A. (2022). New Craters on Mars: An Updated Catalog. Journal of Geophysical Research (Planets), 127(7), e07145.
• MacKenzie, S. M., Neveu, M., Davila, A. F., Lunine, J. I., Cable, M. L., Phillips-Lander, C. M., Eigenbrode, J. L., Waite, J. H., Craft, K. L., Hofgartner, J. D., McKay, C. P., Glein, C. R., Burton, D., Kounaves, S. P., Mathies, R. A., Vance, S. D., Malaska, M. J., Gold, R., German, C. R., , Soderlund, K. M., et al. (2022). Science Objectives for Flagship-Class Mission Concepts for the Search for Evidence of Life at Enceladus. Astrobiology, 22(6), 685-712.
• Munaretto, G., Pajola, M., Lucchetti, A., Cremonese, G., Simioni, E., Re, C., Bertoli, S., Tornabene, L., McEwen, A., Becerra, P., Rangarajan, V., Valantinas, A., Pommerol, A., Thomas, N., & Portyankina, G. (2022). Multiband photometry of Martian Recurring Slope Lineae (RSL) and dust-removed features at Horowitz crater, Mars from TGO/CaSSIS color observations. \planss, 214, 105443.
• Perry, J. E., Heyd, R., Read, M., Tornabene, L. L., Sutton, S. S., Byrne, S., Thomas, N., Fennema, A., McEwen, A., & Berry, K. (2022). Geometric processing of TGO CaSSIS observations. \planss, 223, 105581.
• Pommerol, A., Thomas, N., Almeida, M., Read, M., Becerra, P., Cesar, C., Valantinas, A., Simioni, E., McEwen, A., Perry, J., Marriner, C., Munaretto, G., Pajola, M., Tornabene, L., M{\`ege}, D., Da Deppo, V., Re, C., & Cremonese, G. (2022). In-flight radiometric calibration of the ExoMars TGO Colour and Stereo Surface Imaging System. \planss, 223, 105580.
• Sutton, S. S., Chojnacki, M., McEwen, A. S., Kirk, R. L., Dundas, C. M., Schaefer, E. I., Conway, S. J., Diniega, S., Portyankina, G., Landis, M. E., Baugh, N. F., Heyd, R., Byrne, S., Tornabene, L. L., Ojha, L., & Hamilton, C. W. (2022). Revealing Active Mars with HiRISE Digital Terrain Models. Remote Sensing, 14(10), 2403.
• Bishop, J., Ye{\c{s}ilba\c{s}}, M., Hinman, N., Burton, Z., Englert, P., Toner, J., McEwen, A., Gulick, V., Gibson, E., & Koeberl, C. (2021). Martian subsurface cryosalt expansion and collapse as trigger for landslides. Science Advances, 7(6), eabe4459.
• Cable, M. L., Porco, C., Glein, C. R., German, C. R., MacKenzie, S. M., Neveu, M., Hoehler, T. M., Hofmann, A. E., Hendrix, A. R., Eigenbrode, J., Postberg, F., Spilker, L. J., McEwen, A., Khawaja, N., Hunter Waite, J., Wurz, P., Helbert, J., Anbar, A., Vera, J., & N{\'u\~nez}, J. (2021). The Science Case for a Return to Enceladus. \psj, 2(4), 132.
• Diniega, S., Bramson, A. M., Buratti, B., Buhler, P., Burr, D. M., Chojnacki, M., Conway, S. J., Dundas, C. M., Hansen, C. J., McEwen, A. S., Lap{\^otre}, M. G., Levy, J., Mc, K. L., Piqueux, S., Portyankina, G., Swann, C., Titus, T. N., & Widmer, J. M. (2021). Modern Mars' geomorphological activity, driven by wind, frost, and gravity. Geomorphology, 380, 107627.
• Dundas, C. M., Becerra, P., Byrne, S., Chojnacki, M., Daubar, I. J., Diniega, S., Hansen, C. J., Herkenhoff, K. E., Landis, M. E., McEwen, A. S., Portyankina, G., & Valantinas, A. (2021). Active Mars: A Dynamic World. Journal of Geophysical Research (Planets), 126(8), e06876.
• Dundas, C. M., Mellon, M. T., Conway, S. J., Daubar, I. J., Williams, K. E., Ojha, L., Wray, J. J., Bramson, A. M., Byrne, S., McEwen, A. S., Posiolova, L. V., Speth, G., Viola, D., Landis, M. E., Morgan, G. A., & Pathare, A. V. (2021). Widespread Exposures of Extensive Clean Shallow Ice in the Midlatitudes of Mars. Journal of Geophysical Research (Planets), 126(3), e06617.
• Krasner, S. M., Bruvold, K., Call, J. A., Fieseler, P. D., Klesh, A. T., Michael Kobayashi, M., Lay, N. E., Lim, R. S., Morabito, D. D., Oudrhiri, K., Wallace, M., Grimes, A. H., Palagi, T., Litton, D. K., Signori, G. M., & McEwen, A. S. (2021). Reconstruction of Entry, Descent, and Landing Communications for the InSight Mars Lander. Journal of Spacecraft and Rockets, 58(6), 1569-1581.
• McEwen, A. S., Schaefer, E. I., Dundas, C. M., Sutton, S. S., Tamppari, L. K., & Chojnacki, M. (2021). Mars: Abundant Recurring Slope Lineae (RSL) Following the Planet Encircling Dust Event (PEDE) of 2018. Journal of Geophysical Research (Planets), 126(4), e06575.
• Mills, M. M., McEwen, A. S., & Okubo, C. H. (2021). A Preliminary Regional Geomorphologic Map in Utopia Planitia of the Tianwen-1 Zhurong Landing Region. \grl, 48(18), e94629.
• Valantinas, A., Becerra, P., Pommerol, A., Tornabene, L., Affolter, L., Cremonese, G., Hauber, E., McEwen, A., Munaretto, G., Pajola, M., Bowen, A. P., Patel, M., Rangarajan, V., Schorghofer, N., & Thomas, N. (2021). CaSSIS Color and multi-angular observations of martian slope streaks. \planss, 209, 105373.
• Daubar, I., Gao, A., Wexler, D., Dundas, C., McEwen, A., Neidhart, T., Miljkovic, K., Eschenfelder, J., Collins, G., Piqueux, S., Malin, M., & Posiolova, L. (2020). New Craters on Mars: An Updated Catalog. LPI Contributions, 2251, 2069.
• Daubar, I., Lognonn{\'e}, P., Teanby, N., Collins, G., Clinton, J., St{\"ahler}, S., Spiga, A., Karakostas, F., Ceylan, S., Malin, M., McEwen, A., Maguire, R., Charalambous, C., Onodera, K., Lucas, A., Rolland, L., Vaubaillon, J., Kawamura, T., B{\"ose}, M., , Horleston, A., et al. (2020). A New Crater Near InSight: Implications for Seismic Impact Detectability on Mars. Journal of Geophysical Research (Planets), 125(8), e06382.
• Golombek, M., Williams, N., Warner, N., Parker, T., Williams, M., Daubar, I., Calef, F., Grant, J., Bailey, P., Abarca, H., Deen, R., Ruoff, N., Maki, J., McEwen, A., Baugh, N., Block, K., Tamppari, L., Call, J., Ladewig, J., , Stoltz, A., et al. (2020). Location and Setting of the Mars InSight Lander, Instruments, and Landing Site. Earth and Space Science, 7(10), e01248.
• Hansen, C., Aye, K. -., Diniega, S., Hayne, P., McEwen, A., Portyankina, G., & Schwamb, M. (2020). Dynamic Seasons on Mars \textemdash Polar Images and Investigations. LPI Contributions, 2099, 6019.
• Landis, M., McEwen, A., Daubar, I., Hayne, P., Byrne, S., Dundas, C., Sutton, S., Britton, A., & Herkenhoff, K. (2020). South Polar Layered Deposits Near-Surface Properties Inferred from a Dated Impact Crater. LPI Contributions, 2099, 6025.
• McEwen, A., Sutton, S., Bramson, A., Byrne, S., Petersen, E., Levy, J., Golombek, M., Williams, N., & Putzig, N. (2020). Phlegra Montes: Candidate Landing Site with Shallow Ice for Human Exploration. LPI Contributions, 2099, 6008.
• Moore, J., Spilker, L., Cable, M., Edgington, S., Hendrix, A., Hofstadter, M., Hurford, T., Mandt, K., McEwen, A., Paty, C., Quick, L., Rymer, A., Sayanagi, K., Schmidt, B., & Spilker, T. (2020). Exploration Strategy for the Outer Planets 2023-2032: Goals and Priorities. arXiv e-prints, arXiv:2003.11182.
• Munaretto, G., Pajola, M., Cremonese, G., Re, C., Lucchetti, A., Simioni, E., McEwen, A., Pommerol, A., Becerra, P., Conway, S., Thomas, N., & Massironi, M. (2020). Implications for the origin and evolution of Martian Recurring Slope Lineae at Hale crater from CaSSIS observations. \planss, 187, 104947.
• Park, R. S., Riedel, J. E., Ermakov, A. I., Roa, J., Castillo-Rogez, J., Davies, A. G., McEwen, A. S., & Watkins, M. M. (2020). Advanced Pointing Imaging Camera (APIC) for planetary science and mission opportunities. \planss, 194, 105095.
• Perrin, C., Rodriguez, S., Jacob, A., Lucas, A., Spiga, A., Murdoch, N., Lorenz, R., Daubar, I., Pan, L., Kawamura, T., Lognonn{\'e}, P., Banfield, D., Banks, M., Garcia, R., Newman, C., Ohja, L., Widmer-Schnidrig, R. .., McEwen, A., & Banerdt, W. (2020). Monitoring of Dust Devil Tracks Around the InSight Landing Site, Mars, and Comparison With In Situ Atmospheric Data. \grl, 47(10), e87234.
• Sayanagi, K. M., Becker, T., Brooks, S., Brueshaber, S., Dahl, E., Pater, I., Ebert, R., El, M. M., Fletcher, L., Jessup, K. L., McEwen, A., Molyneux, P. M., Moore, L., Moses, J., N{\'enon}, Q., Orton, G., Paranicas, C., Showalter, M., Spilker, L., , Tiscareno, M., et al. (2020). Priority Questions for Jupiter System Science in the 2020s and Opportunities for Europa Clipper. arXiv e-prints, arXiv:2007.08609.
• Thomas, M., McEwen, A., & Dundas, C. (2020). Present-day mass wasting in sulfate-rich sediments in the equatorial regions of Mars. \icarus, 342, 113566.
• Bart, G. D., Daubar, I. J., Ivanov, B. A., Dundas, C. M., & McEwen, A. S. (2019). Dark halos produced by current impact cratering on Mars. \icarus, 328, 45-57.
• Dundas, C., McEwen, A. S., Diniega, S., Hansen, C., Byrne, S., & McElwaine, J. (2019). The formation of gullies on Mars today. Geological Society of London Special Publications, 467, 67-94.
• Hendrix, A. R., Hurford, T. A., Barge, L. M., Bland, M. T., Bowman, J. S., Brinckerhoff, W., Buratti, B. J., Cable, M. L., Castillo-Rogez, J., Collins, G. C., Diniega, S., German, C. R., Hayes, A., Hoehler, T., Hosseini, S., Howett, C. J., McEwen, A. S., Neish, C. D., Neveu, M., , Nordheim, T. A., et al. (2019). The NASA Roadmap to Ocean Worlds. Astrobiology, 19(1), 1-27.
• Landis, M., McEwen, A., Daubar, I., Hayne, P., Byrne, S., Dundas, C., Sutton, S., Britton, A., & Herkenhoff, K. (2019). Mars' Polar Layered Deposits Geology and History as Revealed by Impact Craters. LPI Contributions, 2089, 6335.
• McEwen, A., & Thomas, N. (2019). Past and Future High-Resolution Orbital Imaging of Mars. LPI Contributions, 2089, 6193.
• Pommerol, A., Read, M., Thomas, N., McEwen, A., & Cremonese, G. (2019). Photometric Calibration of CaSSIS Images. LPI Contributions, 2089, 6213.
• Schaefer, E., McEwen, A., & Sutton, S. (2019). A case study of recurring slope lineae (RSL) at Tivat crater: Implications for RSL origins. \icarus, 317, 621-648.
• Thomas, N., Cremonese, G., Perry, J., Almeida, M., Banaszkiewicz, M., Bapst, J., Becerra, P., Bridges, J., Byrne, S., Conway, S., Deppo, V., Debei, S., El-Maarry, M., Fennema, A., Gwinner, K., Hauber, E., Heyd, R., Hansen, C., Ivanov, A., , Keszthelyi, L., et al. (2019). The Effects of Past and Current Geologic Processes Observed by the CaSSIS Imager Onboard ESA's ExoMars Trace Gas Orbiter. LPI Contributions, 2089, 6156.
• Tornabene, L., Piatek, J., Osinski, G., Barlow, N., Boyce, J., Sopocco, R., Capitan, R., McEwen, A., Robbins, S., & Watters, W. (2019). Continuous Ejecta Deposits Observed Beyond Layered Ejecta Ramparts on Mars. LPI Contributions, 2089, 6354.
• Tornabene}, L., Thomas, N., Cremonese, G., Almeida, M., Dout{\'e}, S., Grindrod, P., Heyd, R., Luchetti, A., McEwen, A., Pajola, M., Perry, J., Pilles, E., Pommerol, A., Read, M., Seelos, F., Wray, J., Science, C., & Teams, {. (2019). Colour and Stereo Surface Imaging System (CaSSIS) on the ExoMars Trace Gas Orbiter: Colour Data Products and Their Use for Scientific Investigations. LPI Contributions, 2089, 6293.
• Wang, A., Ling, Z., Yan, Y., McEwen, A. S., Mellon, M. T., Smith, M. D., Jolliff, B. L., & Head, J. (2019). Subsurface Cl-bearing salts as potential contributors to recurring slope lineae (RSL) on Mars. \icarus, 333, 464-480.
• Bierhaus, E., McEwen, A., Robbins, S., Singer, K., Dones, L., Kirchoff, M., & Williams, J. (2018). Secondary craters and ejecta across the solar system: Populations and effects on impact-crater-based chronologies. Meteoritics and Planetary Science, 53, 638-671.
• Bray, V., Atwood-Stone, C. .., Neish, C., Artemieva, N., McEwen, A., & McElwaine, J. (2018). Lobate impact melt flows within the extended ejecta blanket of Pierazzo crater. \icarus, 301, 26-36.
• Dundas, C., Bramson, A., Ojha, L., Wray, J., Mellon, M., Byrne, S., McEwen, A., Putzig, N., Viola, D., Sutton, S., Clark, E., & Holt, J. (2018). Exposed subsurface ice sheets in the Martian mid-latitudes. Science, 359, 199-201.
• McEwen, A. (2018). MRO/HiRISE Observations of Amazonian Mars. LPI Contributions, 2086, 4001.
• McEwen, A., Thomas, M., & Dundas, C. (2018). Active Gullies and Mass Wasting on Equatorial Mars. European Planetary Science Congress, 12, EPSC2018-457.
• McEwen, A., Thomas, N., Pommerol, A., Leung, C., Sutton, S., Perry, J., Scheidt, S., & Chojnacki, M. (2018). Short-Term Equatorial Albedo Changes on Mars: Deliquescence or Dust?. European Planetary Science Congress, 12, EPSC2018-455.
• Tornabene, L., Seelos, F., Pommerol, A., Thomas, N., Caudill, C., Becerra, P., Bridges, J., Byrne, S., Cardinale, M., Chojnacki, M., Conway, S., Cremonese, G., Dundas, C., El-Maarry, M., Fernando, J., Hansen, C., Hansen, K., Harrison, T., Henson, R., , Marinangeli, L., et al. (2018). Image Simulation and Assessment of the Colour and Spatial Capabilities of the Colour and Stereo Surface Imaging System (CaSSIS) on the ExoMars Trace Gas Orbiter. \ssr, 214, 18.
• Tornabene, L., Watters, W., Osinski, G., Boyce, J., Harrison, T., Ling, V., & McEwen, A. (2018). A depth versus diameter scaling relationship for the best-preserved melt-bearing complex craters on Mars. \icarus, 299, 68-83.
• Turtle, E., Perry, J., Barbara, J., Del, G. A., Rodriguez, S., Le, M. S., Sotin, C., Lora, J., Faulk, S., Corlies, P., Kelland, J., MacKenzie, S., West, R., McEwen, A., Lunine, J., Pitesky, J., Ray, T., & Roy, M. (2018). Titan's Meteorology Over the Cassini Mission: Evidence for Extensive Subsurface Methane Reservoirs. \grl, 45, 5320-5328.
• Viola, D., & McEwen, A. (2018). Geomorphological Evidence for Shallow Ice in the Southern Hemisphere of Mars. Journal of Geophysical Research (Planets), 123, 262-277.
• Bogert, C., Hiesinger, H., Dundas, C., Kr{\"uger}, T., McEwen, A., Zanetti, M., & Robinson, M. (2017). Origin of discrepancies between crater size-frequency distributions of coeval lunar geologic units via target property contrasts. \icarus, 298, 49-63.
• Dundas, C., McEwen, A., Chojnacki, M., Milazzo, M., Byrne, S., McElwaine, J., & Urso, A. (2017). Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water. Nature Geoscience, 10, 903-907.
• Ojha, L., Chojnacki, M., McDonald, G., Shumway, A., Wolff, M., Smith, M., McEwen, A., Ferrier, K., Huber, C., Wray, J., & Toigo, A. (2017). Seasonal Slumps in Juventae Chasma, Mars. Journal of Geophysical Research (Planets), 122, 2193-2214.
• Stopar, J., Robinson, M., Barnouin, O., McEwen, A., Speyerer, E., Henriksen, M., & Sutton, S. (2017). Relative depths of simple craters and the nature of the lunar regolith. \icarus, 298, 34-48.
• Sutton, S., Boyd, A., Kirk, R., Cook, D., Backer, J., Fennema, A., Heyd, R., McEwen, A., & Mirchandani, S. (2017). Correcting Spacecraft Jitter in Hirise Images. ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 141-148.
• Thomas, N., Cremonese, G., Ziethe, R., Gerber, M., Br{\"andli}, M., Bruno, G., Erismann, M., Gambicorti, L., Gerber, T., Ghose, K., Gruber, M., Gubler, P., Mischler, H., Jost, J., Piazza, D., Pommerol, A., Rieder, M., Roloff, V., Servonet, A., , Trottmann, W., et al. (2017). The Colour and Stereo Surface Imaging System (CaSSIS) for the ExoMars Trace Gas Orbiter. \ssr, 212, 1897-1944.
• Turtle, E., Perry, J., Barbara, J., West, R., Del, G. A., & McEwen, A. (2017). Seasonal Evolution in the Behavior of Titan's Clouds from Cassini ISS, 2004-2017. European Planetary Science Congress, 11, EPSC2017-352.
• Viola, D., McEwen, A., Dundas, C., & Byrne, S. (2017). Subsurface volatile content of martian double-layer ejecta (DLE) craters. \icarus, 284, 325-343.
• Bogert}, C., {Hiesinger}, H., {Zanetti}, M., {Plescia}, J., {Ostrach}, L., {Mahanti}, P., {Meyer}, H., {McEwen}, A., {Pasckert}, J., {Michael}, G., {Kneissl}, T., , M. (2016). Factors Affecting Crater Size-Frequency Distribution Measurements: Insights Supported by the LRO Mission. LPI Contributions, 1911, 6015.
• Falk, D. A., Van Leeuwen, W. J., & McEwen, A. S. (2016). Low-cost high-resolution global pyrogenic thermal sensing: Critical information for biosphere-atmosphere interactions. Decadal Survey for Earth Science and Applications from Space, National Academy of Science, ESAS 2017.
• Turtle}, E., {McEwen}, A., {Osterman}, S., {Boldt}, J., {Strohbehn}, K., & Team, {. S. (2016). The Europa Imaging System (EIS), a Camera Suite to investigate Europa's Geology, Ice Shell, and Potential for Current Activity. LPI Contributions, 1980, 4091.
• {Atwood-Stone}, C., {Bray}, V., , A. (2016). A new study of crater concentric ridges on the Moon. \icarus, 273, 196-204.
• {Chojnacki}, M., {McEwen}, A., {Dundas}, C., {Ojha}, L., {Urso}, A., , S. (2016). Geologic context of recurring slope lineae in Melas and Coprates Chasmata, Mars. Journal of Geophysical Research (Planets), 121, 1204-1231.
• {Daubar}, I., {Dundas}, C., {Byrne}, S., {Geissler}, P., {Bart}, G., {McEwen}, A., {Russell}, P., {Chojnacki}, M., , M. (2016). Changes in blast zone albedo patterns around new martian impact craters. \icarus, 267, 86-105.
• {Davies}, A., {Keszthelyi}, L., , A. (2016). Determination of eruption temperature of Io's lavas using lava tube skylights. \icarus, 278, 266-278.
• {Dundas}, C., {McEwen}, A., {Byrne}, S., {Hansen}, C., , S. (2016). HiRISE Observations of Martian Mid-Latitude Geomorphology: Implications for Recent Climate. LPI Contributions, 1926, 6102.
• {Hansen}, C., {Portyankina}, G., {Diniega}, S., {Bourke}, M., {Bridges}, N., {Byrne}, S., {Dundas}, C., {Hayne}, P., {McEwen}, A., , N. (2016). A Decade of Imaging Mars' Seasonal Polar Processes with HiRISE. LPI Contributions, 1926, 6032.
• {Mass{\'e}}, M., {Conway}, S., {Gargani}, J., {Patel}, M., {Pasquon}, K., {McEwen}, A., {Carpy}, S., {Chevrier}, V., {Balme}, M., {Ojha}, L., {Vincendon}, M., {Poulet}, F., {Costard}, F., , G. (2016). Transport processes induced by metastable boiling water under Martian surface conditions. Nature Geoscience, 9, 425-428.
• {McEwen}, A. (2016). Martian Geologic Settings of Interest to the Search for Biosignatures, as Seen from Orbit. LPI Contributions, 1912, 2057.
• {Park}, R., {Bills}, B., {Jorgensen}, J., {Jun}, I., {Maki}, J., {McEwen}, A., {Riedel}, E., {Walch}, M., , M. (2016). Advanced Pointing Imaging Camera (APIC) Concept. LPI Contributions, 1980, 4018.
• {Pommerol}, A., {Thomas}, N., {Roloff}, V., {Tornabene}, L., {Caudill}, C., {Gambicorti}, L., {Da Deppo}, V., {Ziethe}, R., {Hansen}, C., {McEwen}, A., , G. (2016). The Colour and Stereo Surface Imaging System (CaSSIS) on Board ExoMars TGO: Expected Performance and New Opportunities for the Study of Seasonal Processes at High Latitude. LPI Contributions, 1926, 6057.
• {Ding}, N., {Bray}, V., {McEwen}, A., {Mattson}, S., {Okubo}, C., {Chojnacki}, M., , L. (2015). "{The central uplift of Ritchey crater, Mars}". icarus, 252, 255-270.
• {Dundas}, C., , A. (2015). "{Slope activity in Gale crater, Mars}". icarus, 254, 213-218.
• {Dundas}, C., {Byrne}, S., , A. (2015). "{Modeling the development of martian sublimation thermokarst landforms}". icarus, 262, 154-169.
• {Dundas}, C., {Diniega}, S., , A. (2015). "{Long-term monitoring of martian gully formation and evolution with MRO/HiRISE}". icarus, 251, 244-263.
• {Hansen}, C., {Diniega}, S., {Bridges}, N., {Byrne}, S., {Dundas}, C., {McEwen}, A., , G. (2015). "{Agents of change on Mars' northern dunes: CO$_{2}$ ice and wind}". icarus, 251, 264-274.
• {Keske}, A., {Hamilton}, C., {McEwen}, A., , I. (2015). "{Episodes of fluvial and volcanic activity in Mangala Valles, Mars}". icarus, 245, 333-347.
• {Mart{'{i}}n-Torres}, F., {Zorzano}, M., {Valent{'{i}}n-Serrano}, P., {Harri}, A., {Genzer}, M., {Kemppinen}, O., {Rivera-Valentin}, E., {Jun}, I., {Wray}, J., {Bo Madsen}, M., {Goetz}, W., {McEwen}, A., {Hardgrove}, C., {Renno}, N., {Chevrier}, V., {Mischna}, M., {Navarro-Gonz{'a}lez}, R., {Mart {'{i}}nez-Fr{'{i}}as}, J., {Conrad}, P., , {McConnochie}, T., et al. (2015). "{Transient liquid water and water activity at Gale crater on Mars}". Nature Geoscience, 8, 357-361.
• {Ojha}, L., {Wilhelm}, M., {Murchie}, S., {McEwen}, A., {Wray}, J., {Hanley}, J., {Mass{'e}}, M., , M. (2015). "{Corrigendum: Spectral evidence for hydrated salts in recurring slope lineae on Mars}". Nature Geoscience.
• {Ojha}, L., {Wilhelm}, M., {Murchie}, S., {McEwen}, A., {Wray}, J., {Hanley}, J., {Mass{'e}}, M., , M. (2015). "{Spectral Evidence for Hydrated Salts in Seasonal Brine Flows on Mars}". European Planetary Science Congress 2015, held 27 September - 2 October, 2015 in Nantes, France, Online at http://meetingorganizer.copernicus.org/EPSC2015, id.EPSC2015 -838, 10, EPSC2015-838.
• {Ojha}, L., {Wilhelm}, M., {Murchie}, S., {McEwen}, A., {Wray}, J., {Hanley}, J., {Mass{'e}}, M., , M. (2015). "{Spectral evidence for hydrated salts in recurring slope lineae on Mars}". Nature Geoscience, 8, 829-832.
• {Robinson}, M., {Boyd}, A., {Denevi}, B., {Lawrence}, S., {McEwen}, A., {Moser}, D., {Povilaitis}, R., {Stelling}, R., {Suggs}, R., {Thompson}, S., , R. (2015). "{New crater on the Moon and a swarm of secondaries}". icarus, 252, 229-235.
• {Viola}, D., {McEwen}, A., {Dundas}, C., , S. (2015). "{Expanded secondary craters in the Arcadia Planitia region, Mars: Evidence for tens of Myr-old shallow subsurface ice}". icarus, 248, 190-204.
• Dundas, C. M., Byrne, S., McEwen, A. S., Mellon, M. T., Kennedy, M. R., Daubar, I. J., & Saper, L. (2014). HiRISE observations of new impact craters exposing Martian ground ice. Journal of Geophysical Research E: Planets, 119(1), 109-127.
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  Abstract: Twenty small new impact craters or clusters have been observed to excavate bright material inferred to be ice at mid-latitudes and high latitudes on Mars. In the northern hemisphere, the craters are widely distributed geographically and occur at latitudes as low as 39°N. Stability modeling suggests that this ice distribution requires a long-term average atmospheric water vapor content around 25 precipitable micrometers, more than double the present value, which is consistent with the expected effect of recent orbital variations. Alternatively, near-surface humidity could be higher than expected for current column abundances if water vapor is not well mixed with atmospheric CO 2, or the vapor pressure at the ice table could be lower due to salts. Ice in and around the craters remains visibly bright for months to years, indicating that it is clean ice rather than ice-cemented regolith. Although some clean ice may be produced by the impact process, it is likely that the original ground ice was excess ice (exceeding dry soil pore space) in many cases. Observations of the craters suggest small-scale heterogeneities in this excess ice. The origin of such ice is uncertain. Ice lens formation by migration of thin films of liquid is most consistent with local heterogeneity in ice content and common surface boulders, but in some cases, nearby thermokarst landforms suggest large amounts of excess ice that may be best explained by a degraded ice sheet. Key Points New impact craters expose ground ice at high latitudes on Mars Ice is found at latitudes as low as 39°N Ice remains visible for many months, indicating low regolith content ©2013. American Geophysical Union. All Rights Reserved.
• Masse, M., Beck, P., Schmitt, B., Pommerol, A., McEwen, A., Chevrier, V., Brissaud, O., & Sejourne, A. (2014). Spectroscopy and detectability of liquid brines on mars. Planetary and Space Science, 92, 136-149.
• McEwen, A. S., Dundas, C. M., Mattson, S. S., Toigo, A. D., Ojha, L., Wray, J. J., Chojnacki, M., Byrne, S., Murchie, S. L., & Thomas, N. (2014). Recurring slope lineae in equatorial regions of Mars. Nature Geoscience, 7(1), 53-58.
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  Abstract: The presence of liquid water is a requirement of habitability on a planet. Possible indicators of liquid surface water on Mars include intermittent flow-like features observed on sloping terrains. These recurring slope lineae are narrow, dark markings on steep slopes that appear and incrementally lengthen during warm seasons on low-albedo surfaces. The lineae fade in cooler seasons and recur over multiple Mars years. Recurring slope lineae were initially reported to appear and lengthen at mid-latitudes in the late southern spring and summer and are more common on equator-facing slopes where and when the peak surface temperatures are higher. Here we report extensive activity of recurring slope lineae in equatorial regions of Mars, particularly in the deep canyons of Valles Marineris, from analysis of data acquired by the Mars Reconnaissance Orbiter. We observe the lineae to be most active in seasons when the slopes often face the sun. Expected peak temperatures suggest that activity may not depend solely on temperature. Although the origin of the recurring slope lineae remains an open question, our observations are consistent with intermittent flow of briny water. Such an origin suggests surprisingly abundant liquid water in some near-surface equatorial regions of Mars. © 2014 Macmillan Publishers Limited.
• Ojha, L., McEwen, A., Dundas, C., Byrne, S., Mattson, S., Wray, J., Masse, M., & Schaefer, E. (2014). HiRISE observations of Recurring Slope Lineae (RSL) during southern summer on Mars. Icarus, 231, 365-376.
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  Abstract: Recurring Slope Lineae (RSL) are active features on Mars that might require flowing water. Most examples observed through 2011 formed on steep, equator-facing slopes in the southern mid-latitudes. They form and grow during warm seasons and fade and often completely disappear during colder seasons, but recur over multiple Mars years. They are recognizable by their incremental growth, relatively low albedo and downhill orientation. We examined all images acquired by HiRISE during Ls 250-10° (slightly longer than southern summer, Ls 270-360°) of Mars years 30-31 (03/2011-10/2011), and supplemented our results with data from previous studies to better understand the geologic context and characteristics of RSL. We also confirmed candidate and likely sites from previous studies and discovered new RSL sites. We report 13 confirmed RSL sites, including the 7 in McEwen et al. (McEwen et al. [2011]. Science 333(6043), 740-743]. The observed seasonality, latitudinal and slope orientation preferences, and THEMIS brightness temperatures indicate that RSL require warm temperatures to form. We conclude that RSL are a unique phenomenon on Mars, clearly distinct from other slope processes that occur at high latitudes associated with seasonal CO2 frost, and episodic mass wasting on equatorial slopes. However, only 41% (82 out of 200) of the sites that present apparently suitable conditions for RSL formation (steep, equator-facing rocky slopes with bedrock exposure) in the southern mid-latitudes (28-60°S) contain any candidate RSL, with confirmed RSL present only in 7% (13 sites) of those locations. Significant variability in abundance, size and exact location of RSL is also observed at most sites, indicating additional controls such as availability of water or salts that might be playing a crucial role. © 2013 Elsevier Inc.
• {McEwen}, A., {Turtle}, E., {Hibbard}, K., {Reynolds}, E., , E. (2014). "{Io Volcano Observer (IVO): Budget travel to the outer Solar System}". Acta Astronautica, 93, 539-544.
• {Thomas}, N., {Cremonese}, G., {Banaszkiewicz}, M., {Bridges}, J., {Byrne}, S., {da Deppo}, V., {Debei}, S., {El-Maarry}, M., {Haubner}, E., {Hansen}, C., {Ivanov}, A., {Kestay}, L., {Kirk}, R., {Kuzmini}, R., {Mangold}, N., {Marinangeli}, L., {Markiewicz}, W., {Massironi}, M., {McEwen}, A., , {Okubo}, C., et al. (2014). "{The Colour and Stereo Surface Imaging System (CaSSIS) for ESA's Trace Gas Orbiter}". European Planetary Science Congress 2014, EPSC Abstracts, Vol.~9, id.~EPSC2014-100, 9, EPSC2014-100.
• Atwood-Stone, C., & McEwen, A. S. (2013). Avalanche slope angles in low-gravity environments from active Martian sand dunes. Geophysical Research Letters, 40(12), 2929-2934.
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  Abstract: The properties of granular material have an important effect on surface landforms and processes. Recently, it has been suggested that material properties called dynamic and static angle of repose vary with gravitational acceleration, which would have a significant effect on many planetary surface processes such as crater collapse and gully formation. In order to test that hypothesis, we measured lee slopes of active aeolian sand dunes on Mars using the High Resolution Imaging Experiment (HiRISE) DTMs (Digital Terrain Model). We examined dune fields in Nili Patera, Herschel Crater, and Gale Crater. Our measurements showed that the dynamic angles of repose for the sands in these areas are 33-34° in the first region and 30-31° in the other two. These results fall within the 30° to 35° window for the dynamic angles of repose for terrestrial dunes with similar flow depths and grain properties and thus show that this angle does not significantly vary with decreasing gravity. Key Points The dynamic angle of repose of granular material does not vary with gravity Angle of dune's slipface has implications for grain roughness and angularity Slip face angle gives qualitative measure of the distance from the sand's source ©2013. American Geophysical Union. All Rights Reserved.
• Bishop, J. L., Tirsch, D., Tornabene, L. L., Jaumann, R., McEwen, A. S., McGuire, P. C., Ody, A., Poulet, F., Clark, R. N., Parente, M., McKeown, N. K., Mustard, J. F., Murchie, S. L., Voigt, J., Aydin, Z., Bamberg, M., Petau, A., Michael, G., Seelos, F. P., , Hash, C. D., et al. (2013). Mineralogy and morphology of geologic units at Libya Montes, Mars: Ancient aqueously derived outcrops, mafic flows, fluvial features, and impacts. Journal of Geophysical Research E: Planets, 118(3), 487-513.
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  Abstract: There is ample evidence of both ancient and long-lasting fluvial activity and chemical alteration in the Libya Montes region south of Isidis Basin. The region hosts Noachian to Amazonian aged surface rocks with extensive outcrops of olivine- and pyroxene-bearing material. Libya Montes also features surface outcrops and/or deposits hosting Fe/Mg-smectite, Fe/Mg-smectite mixed with carbonate and/or other Fe/Mg-rich phyllosilicates, and Al-smectite. These units likely formed through chemical alteration connected with hydrothermal activity resulting from the formation of the Isidis Basin and/or the pervasive fluvial activity throughout this region. The morphology and stratigraphy of the aqueous and mafic minerals are described using High Resolution Imaging Science Experiment and High Resolution Stereo Camera derived digital terrain models. Analyses of the Compact Reconnaissance Imaging Spectrometer for Mars spectra show variations in the chemistry of the Fe/Mg-smectite from nontronite-like exposures with spectral features near 2.29 and 2.4 μm more consistent with Fe3+2OH groups in the mineral structure, and saponite-like outcrops with spectral features near 2.31 and 2.38 μm characteristic of Mg2+3OH groups. These Fe/Mg-smectite bearing materials also have bands near 1.9 μm due to H2O and near 2.5 μm that could be due to the smectite, other phyllosilicates, and carbonates. All regions exhibiting carbonate features near 3.4-3.5 μm also have features consistent with the presence of olivine and Fe/Mg-smectite, indicating that the carbonate signatures occur in rocks likely containing a mixture of these minerals. The Al-smectite-bearing rocks have bands near 1.41, 1.91, and 2.19 μm that are more consistent with beidellite than other Al-phyllosilicates, indicating a higher-temperature or diagenetically processed origin for this material. Our interpretation of the geologic history of this region is that ancient Noachian basaltic crustal materials experienced extensive aqueous alteration at the time of the Isidis impact, during which the montes were also formed, followed by emplacement of a rough olivine-rich lava or melt, and finally the smooth pyroxene-bearing caprock unit. Key Points The Libya Montes region includes Noachian to Amazonian aged surface rocks Surface rocks contain pyroxene, olivine, Fe/Mg-clays, dolomite, and beidellite Aqueous alteration and montes formation were likely induced by the Isidis impact ©2012. American Geophysical Union. All Rights Reserved.
• Daubar, I. J., McEwen, A. S., Byrne, S., Kennedy, M. R., & Ivanov, B. (2013). The current martian cratering rate. Icarus, 225(1), 506-516.
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  Abstract: The discovery of 248 dated impact sites known to have formed within the last few decades allows us to refine the current cratering rate and slope of the production function at Mars. We use a subset of 44 of these new craters that were imaged before and after impact by Mars Reconnaissance Orbiter's Context Camera - a thoroughly searched data set that minimizes biases from variable image resolutions. We find the current impact rate is 1.65×10-6 craters with an effective diameter ≥3.9m/km2/yr, with a differential slope (power-law exponent) of -2.45±0.36. This results in model ages that are factors of three to five below the Hartmann (Hartmann, W.K. [2005]. Icarus 174, 294-320) and Neukum et al. (Neukum, G., Ivanov, B.A., Hartmann, W.K. [2001]. Space Sci. Rev. 96, 55-86)/Ivanov (Ivanov, B.A. [2001]. Space Sci. Rev. 96, 87-104) model production functions where they overlap in diameter. The best-fit production function we measure has a shallower slope than model functions at these sizes, but model function slopes are within the statistical errors. More than half of the impacts in this size range form clusters, which is another reason to use caution when estimating surface ages using craters smaller than ~50m in diameter. © 2013 Elsevier Inc.
• Diniega, S., Hansen, C. J., McElwaine, J. N., Hugenholtz, C. H., Dundas, C. M., McEwen, A. S., & Bourke, M. C. (2013). A new dry hypothesis for the formation of martian linear gullies. Icarus, 225(1), 526-537.
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  Abstract: Long, narrow grooves found on the slopes of martian sand dunes have been cited as evidence of liquid water via the hypothesis that melt-water initiated debris flows eroded channels and deposited lateral levées. However, this theory has several short-comings for explaining the observed morphology and activity of these linear gullies. We present an alternative hypothesis that is consistent with the observed morphology, location, and current activity: that blocks of CO2 ice break from over-steepened cornices as sublimation processes destabilize the surface in the spring, and these blocks move downslope, carving out levéed grooves of relatively uniform width and forming terminal pits. To test this hypothesis, we describe experiments involving water and CO2 blocks on terrestrial dunes and then compare results with the martian features. Furthermore, we present a theoretical model of the initiation of block motion due to sublimation and use this to quantitatively compare the expected behavior of blocks on the Earth and Mars. The model demonstrates that CO2 blocks can be expected to move via our proposed mechanism on the Earth and Mars, and the experiments show that the motion of these blocks will naturally create the main morphological features of linear gullies seen on Mars. © 2013 Elsevier Inc.
• Hansen, C. J., Byrne, S., Portyankina, G., Bourke, M., Dundas, C., McEwen, A., Mellon, M., Pommerol, A., & Thomas, N. (2013). Observations of the northern seasonal polar cap on Mars: I. Spring sublimation activity and processes. Icarus, 225(2), 881-897.
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  Abstract: Spring sublimation of the seasonal CO2 northern polar cap is a dynamic process in the current Mars climate. Phenomena include dark fans of dune material propelled out onto the seasonal ice layer, polygonal cracks in the seasonal ice, sand flow down slipfaces, and outbreaks of gas and sand around the dune margins. These phenomena are concentrated on the north polar erg that encircles the northern residual polar cap. The Mars Reconnaissance Orbiter has been in orbit for three Mars years, allowing us to observe three northern spring seasons. Activity is consistent with and well described by the Kieffer model of basal sublimation of the seasonal layer of ice applied originally in the southern hemisphere. Three typical weak spots have been identified on the dunes for escape of gas sublimed from the bottom of the seasonal ice layer: the crest of the dune, the interface of the dune with the interdune substrate, and through polygonal cracks in the ice. Pressurized gas flows through these vents and carries out material entrained from the dune. Furrows in the dunes channel gas to outbreak points and may be the northern equivalent of southern radially-organized channels ("araneiform" terrain), albeit not permanent. Properties of the seasonal CO2 ice layer are derived from timing of seasonal events such as when final sublimation occurs. Modification of dune morphology shows that landscape evolution is occurring on Mars today, driven by seasonal activity associated with sublimation of the seasonal CO2 polar cap. © 2012 Elsevier Inc.
• Mcewen, A., & Mcewen, A. S. (2013). Mars in motion. Scientific American, 308(5).
• Ojha, L., Wray, J. J., Murchie, S. L., McEwen, A. S., Wolff, M. J., & Karunatillake, S. (2013). Spectral constraints on the formation mechanism of recurring slope lineae. Geophysical Research Letters, 40(21), 5621-5626.
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  Abstract: Recurring slope lineae (RSL) exhibit multiple lines of evidence for a wet origin. In the southern midlatitudes, they form on steep, equator-facing slopes that are warm during southern summer. The formation temperature, seasonality, and other geomorphic characteristics are suggestive of water-related formation, perhaps dense brines. We examined Compact Reconnaissance Imaging Spectrometer for Mars images of all confirmed RSL sites from the southern midlatitudes and the equatorial region to understand the composition of RSL and/or RSL-associated deposits. We did not detect any spectral signature attributable to water; however, a distinct and consistent spectral signature is observed at most sites, indicating enhanced abundances or distinct grain sizes of both ferric and ferrous minerals in RSL-related materials compared to adjacent non-RSL slopes. Like the RSL themselves, the strength of these signatures varies as a function of season. The observed spectral changes may indicate removal of a fine-grained surface component during RSL flow, precipitation of ferric oxides, and/or wetting of the substrate. Key Points CRISM analysis of RSL Consistent and diagnostic spectral characteristics associated with RSL Spectral behavior consistent with wetting of the substrate ©2013. American Geophysical Union. All Rights Reserved.
• Pommerol, A., Thomas, N., Jost, B., Beck, P., Okubo, C., & McEwen, A. S. (2013). Photometric properties of Mars soils analogs. Journal of Geophysical Research E: Planets, 118(10), 2045-2072.
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  Abstract: We have measured the bidirectional reflectance of analogs of dry, wet, and frozen Martian soils over a wide range of phase angles in the visible spectral range. All samples were produced from two geologic samples: the standard JSC Mars-1 soil simulant and Hawaiian basaltic sand. In a first step, experiments were conducted with the dry samples to investigate the effects of surface texture. Comparisons with results independently obtained by different teams with similar samples showed a satisfying reproducibility of the photometric measurements as well as a noticeable influence of surface textures resulting from different sample preparation procedures. In a second step, water was introduced to produce wet and frozen samples and their photometry investigated. Optical microscope images of the samples provided information about their microtexture. Liquid water, even in relatively low amount, resulted in the disappearance of the backscattering peak and the appearance of a forward-scattering peak whose intensity increases with the amount of water. Specular reflections only appeared when water was present in an amount large enough to allow water to form a film at the surface of the sample. Icy samples showed a wide variability of photometric properties depending on the physical properties of the water ice. We discuss the implications of these measurements in terms of the expected photometric behavior of the Martian surface, from equatorial to circum-polar regions. In particular, we propose some simple photometric criteria to improve the identification of wet and/or icy soils from multiple observations under different geometries. Key Points Effects of surface texture on the bidirectional reflectance of dry samples Photometric signatures of liquid water in soils ©2013. American Geophysical Union. All Rights Reserved.
• Tornabene, L. L., Osinski, G. R., McEwen, A. S., Wray, J. J., Craig, M. A., Sapers, H. M., & Christensen, P. R. (2013). An impact origin for hydrated silicates on Mars: A synthesis. Journal of Geophysical Research E: Planets, 118(5), 994-1012.
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  Abstract: Recent Mars-orbiting spectrometers continue to detect surface materials containing hydrated silicates, particularly clays and amorphous phases (e.g., silica glasses), concentrated within the heavily cratered Noachian highlands crust. This paper provides a review, summary, and synthesis of observations from terrestrial impact structures with current Martian data. It is suggested that numerous and frequent impacts into the volatile-rich silicate crust of Mars, through direct and indirect impact-generated mechanisms, represent a plausible hypothesis that can explain the widespread distribution of hydrated silicates in the surface and subsurface of the heavily cratered Noachian highlands crust largely independent of climate. In addition to impact-generated hydrothermal activity, devitrification, autometamorphism, and the voluminous production of impact "damaged" materials that are susceptible to alteration must be considered. When taken together, a drastically different early climate on Mars, in which water is stable at the surface for extended periods of time, cannot be ruled out; however, it is noted here that these additional impact mechanisms can operate and thereby extend the range of possible alteration settings to include climate conditions that may have been predominately colder and drier. Such a climate would not be dissimilar to the conditions of today, with the important exceptions of a higher geothermal gradient, and punctuated thermal disturbance to the cryosphere and hydrosphere from igneous activity and an exponentially higher impact flux. Key Points Frequent impacts into the volatile-rich crust can produce hydrated silicates.Does not require drastic climate change.Additional impact-induced alteration mechanisms explained. ©2013. American Geophysical Union. All Rights Reserved.
• Adams, E., Hibbard, K., Turtle, E., Reynolds, E., Anderson, B., Paranicas, C., Rogers, G., McAdams, J., Roth, D., Christensen, P., McEwen, A., Wieser, M., Thomas, N., Wurz, P., & Janesick, J. (2012). Io Volcano Observer's (IVO) integrated approach to optimizing system design for radiation challenges. IEEE Aerospace Conference Proceedings.
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  Abstract: One of the major challenges for a mission to the Jovian system is the radiation tolerance of the spacecraft (S/C) and the payload. Moreover, being able to achieve science observations with high signal to noise ratios (SNR), while passing through the high flux radiation zones, requires additional ingenuity on the part of the instrument provider. Consequently, the radiation mitigation is closely intertwined with the payload, spacecraft and trajectory design, and requires a systems-level approach. This paper presents a design for the Io Volcano Observer (IVO), a Discovery mission concept that makes multiple close encounters with Io while orbiting Jupiter. The mission aims to answer key outstanding questions about Io, especially the nature of its intense active volcanism and the internal processes that drive it. The payload includes narrow-angle and wide-angle cameras (NAC and WAC), dual fluxgate magnetometers (FGM), a thermal mapper (ThM), dual ion and neutral mass spectrometers (INMS), and dual plasma ion analyzers (PIA). The radiation mitigation is implemented by drawing upon experiences from designs and studies for missions such as the Radiation Belt Storm Probes (RBSP) and Jupiter Europa Orbiter (JEO). At the core of the radiation mitigation is IVO's inclined and highly elliptical orbit, which leads to rapid passes through the most intense radiation near Io, minimizing the total ionizing dose (177 krads behind 100 mils of Aluminum with radiation design margin (RDM) of 2 after 7 encounters). The payload and the spacecraft are designed specifically to accommodate the fast flyby velocities (e.g. the spacecraft is radioisotope powered, remaining small and agile without any flexible appendages). The science instruments, which collect the majority of the high-priority data when close to Io and thus near the peak flux, also have to mitigate transient noise in their detectors. The cameras use a combination of shielding and CMOS detectors with extremely fast readout to minimize noise. INMS microchannel plate detectors and PIA channel electron multipliers require additional shielding. The FGM is not sensitive to noise induced by energetic particles and the ThM microbolometer detector is nearly insensitive. Detailed SNR calculations are presented. To facilitate targeting agility, all of the spacecraft components are shielded separately since this approach is more mass efficient than using a radiation vault. IVO uses proven radiation-hardened parts (rated at 100 krad behind equivalent shielding of 280 mils of Aluminum with RDM of 2) and is expected to have ample mass margin to increase shielding if needed. © 2012 IEEE.
• Bray, V. J., Atwood-Stone, C., & McEwen, A. M. (2012). Investigating the transition from central peak to peak-ring basins using central feature volume measurements from the Global Lunar DTM 100 m. Geophysical Research Letters, 39(21).
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  Abstract: Several theories have been suggested to explain the transition from peak to peak-ring crater morphology. In order to explore the transition and assess the currently advocated peak-ring formation theories, we have collected measurements of central feature volumes and heights for relatively fresh lunar impact craters. We employed the Global Lunar DTM 100 m, which has the vertical precision and spatial coverage necessary to accurately measure peak and peak-ring volumes in more craters than previously possible. The similarity in both trend and magnitude of peak and peak-ring volumes suggests that peak-ring formation is closely related to the development of central peaks as crater size increases. Our data thus lends support to those peak-ring formation theories involving peak collapse. © 2012. American Geophysical Union. All Rights Reserved.
• Bridges, N. T., Bourke, M. C., Geissler, P. E., Banks, M. E., Colon, C., Diniega, S., Golombek, M. P., Hansen, C. J., Mattson, S., Mcewen, A. S., Mellon, M. T., Stantzos, N., & Thomson, B. J. (2012). Planet-wide sand motion on mars. Geology, 40(1), 31-34.
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  Abstract: Prior to Mars Reconnaissance Orbiter data, images of Mars showed no direct evidence for dune and ripple motion. This was consistent with climate models and lander measurements indicating that winds of sufficient intensity to mobilize sand were rare in the low-density atmosphere. We show that many sand ripples and dunes across Mars exhibit movement of as much as a few meters per year, demonstrating that Martian sand migrates under current conditions in diverse areas of the planet. Most motion is probably driven by wind gusts that are not resolved in global circulation models. A past climate with a thicker atmosphere is only required to move large ripples that contain coarse grains. © 2012 Geological Society of America.
• Burleigh, K. J., Melosh, H. J., Tornabene, L. L., Ivanov, B., McEwen, A. S., & Daubar, I. J. (2012). Impact airblast triggers dust avalanches on Mars. Icarus, 217(1), 194-201.
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  Abstract: Visible images from the Mars Reconnaissance Orbiter have revealed more than 200 new impact sites on Mars (almost all in dust-mantled regions) containing 1-50. m diameter craters, often in clusters. We count approximately 65,000 small-scale slope streaks within 2 to 3. km of one such cluster and categorize them into four morphologically distinct types. Here we show that these slope streaks (interpreted as dust avalanches) are triggered by the impact event but, surprisingly, are not due to seismic shaking; instead, the dust avalanches are due to airblasts created by the supersonic meteor(s) before impact. Sixteen of the new impact sites are associated with high areal densities of dust avalanches. The observed dust avalanche frequency suggests that impact-generated airblasts constitute a locally important and previously unrecognized process for inducing slope degradation on Mars. © 2011 Elsevier Inc.
• Caudill, C. M., Tornabene, L. L., McEwen, A. S., Byrne, S., Ojha, L., & Mattson, S. (2012). Layered MegaBlocks in the central uplifts of impact craters. Icarus, 221(2), 710-720.
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  Abstract: A database of bedrock exposed in crater central uplifts has been compiled from multiple orbital datasets. In this study we focus on uplifts which show decameter-scale layers within the exposed megablocks derived from the bedrock of the preexisting target. This distinctive morphology, found in 41 craters globally, occurs mostly in regions mapped as Hesperian plains material, generally interpreted as regions of extensive flood lavas. The association with volcanic units coupled with morphology and mineralogy that is consistent with flood lava emplacement suggests that the layers are stacks of dense lava interbedded with weaker materials. Estimates for the uplift of stratigraphic sections coupled with morphologic and morphometric analyses lead to estimated thicknesses of the layered unit. These estimates indicate Hesperian flood lavas are ∼23% thicker than a previous minimum estimate, with a volume of 5.5×10 7km 3. This increases the known extrusive volcanic production for the history of Mars and volatile release to past martian atmospheres. © 2012 Elsevier Inc.
• Dundas, C. M., Diniega, S., Hansen, C. J., Byrne, S., & McEwen, A. S. (2012). Seasonal activity and morphological changes in martian gullies. Icarus, 220(1), 124-143.
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  Abstract: Recent studies of martian dune and non-dune gullies have suggested a seasonal control on present-day gully activity. The timing of current gully activity, especially activity involving the formation or modification of channels (which commonly have been taken as evidence of fluvial processes), has important implications regarding likely gully formation processes and necessary environmental conditions. In this study, we describe the results of frequent meter-scale monitoring of several active gully sites by the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO). The aim is to better assess the scope and nature of current morphological changes and to provide improved constraints on timing of gully activity on both dune and non-dune slopes. Our observations indicate that (1) gully formation on Mars is ongoing today and (2) the most significant morphological changes are strongly associated with seasonal frost and defrosting activity. Observed changes include formation of all major components of typical gully landforms, although we have not observed alcove formation in coherent bedrock. These results reduce the need to invoke recent climate change or present-day groundwater seepage to explain the many martian gullies with pristine appearance. © 2012.
• Oberst, J., Christou, A., Suggs, R., Moser, D., Daubar, I. J., McEwen, A. S., Burchell, M., Kawamura, T., Hiesinger, H., Wünnemann, K., Wagner, R., & Robinson, M. S. (2012). The present-day flux of large meteoroids on the lunar surface - A synthesis of models and observational techniques. Planetary and Space Science, 74(1), 179-193.
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  Abstract: Monitoring the lunar surface for impacts is a highly rewarding approach to study small asteroids and large meteoroids encountering the Earth-Moon System. The various effects of meteoroids impacting the Moon are described and results from different detection and study techniques are compared. While the traditional statistics of impact craters allow us to determine the cumulative meteoroid flux on the lunar surface, the recent successful identification of fresh craters in orbital imagery has the potential to directly measure the cratering rate of today. Time-resolved recordings, e.g., seismic data of impacts and impact flash detections clearly demonstrate variations of the impact flux during the lunar day. From the temporal/spatial distribution of impact events, constraints can be obtained on the meteoroid approach trajectories and velocities. The current monitoring allows us to identify temporal clustering of impacts and to study the different meteoroid showers encountering the Earth-Moon system. Though observational biases and deficiencies in our knowledge of the scaling laws are severe, there appears to be an order-of-magnitude agreement in the observed flux within the error limits. Selenographic asymmetries in the impact flux (e.g., for equatorial vs. polar areas) have been predicted. An excess of impacts on the lunar leading hemisphere can be demonstrated in current data. We expect that future missions will allow simultaneous detections of seismic events and impact flashes. The known locations and times of the flashes will allow us to constrain the seismic solutions. While the numbers of flash detections are still limited, coordinated world-wide observations hold great potential for exploiting this observation technique. The potential for identification of fresh craters in high-resolution orbital image data has just barely been tapped, but should improve significantly with the LRO extended mission. © 2012 Elsevier Ltd. All rights reserved.
• Tornabene, L. L., Osinski, G. R., McEwen, A. S., Boyce, J. M., Bray, V. J., Caudill, C. M., Grant, J. A., Hamilton, C. W., Mattson, S., & Mouginis-Mark, P. J. (2012). Widespread crater-related pitted materials on Mars: Further evidence for the role of target volatiles during the impact process. Icarus, 220(2), 348-368.
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  Abstract: Recently acquired high-resolution images of martian impact craters provide further evidence for the interaction between subsurface volatiles and the impact cratering process. A densely pitted crater-related unit has been identified in images of 204 craters from the Mars Reconnaissance Orbiter. This sample of craters are nearly equally distributed between the two hemispheres, spanning from 53°S to 62°N latitude. They range in diameter from ~1 to 150. km, and are found at elevations between -5.5 to +5.2. km relative to the martian datum. The pits are polygonal to quasi-circular depressions that often occur in dense clusters and range in size from ~10. m to as large as 3. km. Pit sizes scale with both the host crater's diameter and the host deposit size. These pits have subtle raised rims, and unlike primary and secondary impact craters, they lack well-defined ejecta deposits and overlapping stratigraphic relationships. They also lack any sign of any preferential alignment expected of volcanic or tectonic collapse features. Morphologic and stratigraphic evidence in support of an impact origin includes the observation that pitted materials primarily occur as ponded and flow-like deposits on crater floors, behind terraces, and infilling the lowest local topographic depressions atop the ejecta blanket-similar to the distribution of impact melt-bearing bodies on the Moon. Based on the observations and comparisons to terrestrial and lunar analogs, we conclude that the pit-bearing materials are impactite deposits. The presence of these deposits in older craters, where preserved, suggests that they have formed on Mars throughout most of its geologic history; thus, understanding their origin may help to constrain the hydrological and climate history of Mars. © 2012 Elsevier Inc.
• Crumpler, L. S., Arvidson, R. E., Squyres, S. W., McCoy, T., Yingst, A., Ruff, S., Farrand, W., McSween, Y., Powell, M., Ming, D. W., Morris, R. V., Bell, J. F., Grant, J., Greeley, R., Desmarais, D., Schmidt, M., Cabrol, N. A., Haldemann, A., Lewis, K. W., , Wang, A. E., et al. (2011). Field reconnaissance geologic mapping of the Columbia Hills, Mars, based on Mars Exploration Rover Spirit and MRO HiRISE observations. Journal of Geophysical Research E: Planets, 116(7).
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  Abstract: Chemical, mineralogic, and lithologic ground truth was acquired for the first time on Mars in terrain units mapped using orbital Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (MRO HiRISE) image data. Examination of several dozen outcrops shows that Mars is geologically complex at meter length scales, the record of its geologic history is well exposed, stratigraphic units may be identified and correlated across significant areas on the ground, and outcrops and geologic relationships between materials may be analyzed with techniques commonly employed in terrestrial field geology. Despite their burial during the course of Martian geologic time by widespread epiclastic materials, mobile fines, and fall deposits, the selective exhumation of deep and well-preserved geologic units has exposed undisturbed outcrops, stratigraphic sections, and structural information much as they are preserved and exposed on Earth. A rich geologic record awaits skilled future field investigators on Mars. The correlation of ground observations and orbital images enables construction of a corresponding geologic reconnaissance map. Most of the outcrops visited are interpreted to be pyroclastic, impactite, and epiclastic deposits overlying an unexposed substrate, probably related to a modified Gusev crater central peak. Fluids have altered chemistry and mineralogy of these protoliths in degrees that vary substantially within the same map unit. Examination of the rocks exposed above and below the major unconformity between the plains lavas and the Columbia Hills directly confirms the general conclusion from remote sensing in previous studies over past years that the early history of Mars was a time of more intense deposition and modification of the surface. Although the availability of fluids and the chemical and mineral activity declined from this early period, significant later volcanism and fluid convection enabled additional, if localized, chemical activity. Copyright © 2011 by the American Geophysical Union.
• Davies, A. G., Keszthelyi, L., & McEwen, A. S. (2011). Estimating eruption temperature from thermal emission spectra of lava fountain activity in the Erta'Ale (Ethiopia) volcano lava lake: Implications for observing Io's volcanoes. Geophysical Research Letters, 38(21).
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  Abstract: We have analysed high-spatial-resolution and high-temporal-resolution temperature measurements of the active lava lake at Erta'Ale volcano, Ethiopia, to derive requirements for measuring eruption temperatures at Io's volcanoes. Lava lakes are particularly attractive targets because they are persistent in activity and large, often with ongoing lava fountain activity that exposes lava at near-eruption temperature. Using infrared thermography, we find that extracting useful temperature estimates from remote-sensing data requires (a) high spatial resolution to isolate lava fountains from adjacent cooler lava and (b) rapid acquisition of multi-color data. Because existing spacecraft data of Io's volcanoes do not meet these criteria, it is particularly important to design future instruments so that they will be able to collect such data. Near-simultaneous data at more than two relatively short wavelengths (shorter than 1 μm) are needed to constrain eruption temperatures. Resolving parts of the lava lake or fountains that are near the eruption temperature is also essential, and we provide a rough estimate of the required image scale. © 2011 by the American Geophysical Union.
• Hansen, C. J., Bourke, M., Bridges, N. T., Byrne, S., Colon, C., Diniega, S., Dundas, C., Herkenhoff, K., McEwen, A., Mellon, M., Portyankina, G., & Thomas, N. (2011). Seasonal erosion and restoration of Mars' northern polar dunes. Science, 331(6017), 575-578.
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  PMID: 21292976;Abstract: Despite radically different environmental conditions, terrestrial and martian dunes bear a strong resemblance, indicating that the basic processes of saltation and grainfall (sand avalanching down the dune slipface) operate on both worlds. Here, we show that martian dunes are subject to an additional modification process not found on Earth: springtime sublimation of Mars' CO 2 seasonal polar caps. Numerous dunes in Mars' north polar region have experienced morphological changes within a Mars year, detected in images acquired by the High-Resolution Imaging Science Experiment on the Mars Reconnaissance Orbiter. Dunes show new alcoves, gullies, and dune apron extension. This is followed by remobilization of the fresh deposits by the wind, forming ripples and erasing gullies. The widespread nature of these rapid changes, and the pristine appearance of most dunes in the area, implicates active sand transport in the vast polar erg in Mars' current climate.
• Martínez-Alonso, S., Mellon, M. T., Banks, M. E., Keszthelyi, L. P., & McEwen, A. S. (2011). Evidence of volcanic and glacial activity in Chryse and Acidalia Planitiae, Mars. Icarus, 212(2), 597-621.
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  Abstract: Chryse and Acidalia Planitiae show numerous examples of enigmatic landforms previously interpreted to have been influenced by a water/ice-rich geologic history. These landforms include giant polygons bounded by kilometer-scale arcuate troughs, bright pitted mounds, and mesa-like features. To investigate the significance of the last we have analyzed in detail the region between 60°N, 290°E and 10°N, 360°E utilizing HiRISE (High Resolution Imaging Science Experiment) images as well as regional-scale data for context. The mesas may be analogous to terrestrial tuyas (emergent sub-ice volcanoes), although definitive proof has not been identified. We also report on a blocky unit and associated landforms (drumlins, eskers, inverted valleys, kettle holes) consistent with ice-emplaced volcanic or volcano-sedimentary flows. The spatial association between tuya-like mesas, ice-emplaced flows, and further possible evidence of volcanism (deflated flow fronts, volcanic vents, columnar jointing, rootless cones), and an extensive fluid-rich substratum (giant polygons, bright mounds, rampart craters), allows for the possibility of glaciovolcanic activity in the region.Landforms indicative of glacial activity on Chryse/Acidalia suggest a paleoclimatic environment remarkably different from today's. Climate changes on Mars (driven by orbital/obliquity changes) or giant outflow channel activity could have resulted in ice-sheet-related landforms far from the current polar caps. © 2011 Elsevier Inc.
• McEwen, A. S., Ojha, L., Dundas, C. M., Mattson, S. S., Byrne, S., Wray, J. J., Cull, S. C., Murchie, S. L., Thomas, N., & Gulick, V. C. (2011). Seasonal flows on warm Martian slopes. Science, 333(6043), 740-743.
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  PMID: 21817049;Abstract: Water probably flowed across ancient Mars, but whether it ever exists as a liquid on the surface today remains debatable. Recurring slope lineae (RSL) are narrow (0.5 to 5 meters), relatively dark markings on steep (25° to 40°) slopes; repeat images from the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment show them to appear and incrementally grow during warm seasons and fade in cold seasons. They extend downslope from bedrock outcrops, often associated with small channels, and hundreds of them form in some rare locations. RSL appear and lengthen in the late southern spring and summer from 48°S to 32°S latitudes favoring equator-facing slopes, which are times and places with peak surface temperatures from ∼250 to 300 kelvin. Liquid brines near the surface might explain this activity, but the exact mechanism and source of water are not understood.
• Thomas, N., Stelter, R., Ivanov, A., Bridges, N. T., Herkenhoff, K. E., & McEwen, A. S. (2011). Spectral heterogeneity on Phobos and Deimos: HiRISE observations and comparisons to Mars Pathfinder results. Planetary and Space Science, 59(13), 1281-1292.
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  Abstract: The High-Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to observe Phobos and Deimos at spatial scales of around 6 and 20 m/px, respectively. HiRISE (McEwen et al.; JGR, 112, CiteID E05S02, DOI: 10.1029/2005JE002605, 2007) has provided, for the first time, high-resolution colour images of the surfaces of the Martian moons. When processed, by the production of colour ratio images for example, the data show considerable small-scale heterogeneity, which might be attributable to fresh impacts exposing different materials otherwise largely hidden by a homogenous regolith. The bluer material that is draped over the south-eastern rim of the largest crater on Phobos, Stickney, has been perforated by an impact to reveal redder material and must therefore be relatively thin. A fresh impact with dark crater rays has been identified. Previously identified mass-wasting features in Stickney and Limtoc craters stand out strongly in colour. The interior deposits in Stickney appear more inhomogeneous than previously suspected. Several other local colour variations are also evident. Deimos is more uniform in colour but does show some small-scale inhomogeneity. The bright streamers (Thomas et al.; Icarus, 123, 536556,1996) are relatively blue. One crater to the south-west of Voltaire and its surroundings appear quite strongly reddened with respect to the rest of the surface. The reddening of the surroundings may be the result of ejecta from this impact. The spectral gradients at optical wavelengths observed for both Phobos and Deimos are quantitatively in good agreement with those found by unresolved photometric observations made by the Imager for Mars Pathfinder (IMP; Thomas et al.; JGR, 104, 90559068, 1999). The spectral gradients of the blue and red units on Phobos bracket the results from IMP. © 2010 Elsevier Ltd. All rights reserved.
• Turtle, E. P., Genio, A. D., Barbara, J. M., Perry, J. E., Schaller, E. L., McEwen, A. S., West, R. A., & Ray, T. L. (2011). Seasonal changes in Titan's meteorology. Geophysical Research Letters, 38(3).
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  Abstract: The Cassini Imaging Science Subsystem has observed Titan for ∼1/4 Titan year, and we report here the first evidence of seasonal shifts in preferred locations of tropospheric methane clouds. South-polar convective cloud activity, common in late southern summer, has become rare. North-polar and northern mid-latitude clouds appeared during the approach to the northern spring equinox in August 2009. Recent observations have shown extensive cloud systems at low latitudes. In contrast, southern mid-latitude and subtropical clouds have appeared sporadically throughout the mission, exhibiting little seasonality to date. These differences in behavior suggest that Titan's clouds, and thus its general circulation, are influenced by both the rapid temperature response of a low-thermal-inertia surface and the much longer radiative timescale of Titan's cold thick troposphere. North-polar clouds are often seen near lakes and seas, suggesting that local increases in methane concentration and/or lifting generated by surface roughness gradients may promote cloud formation. Copyright 2011 by the American Geophysical Union.
• Turtle, E. P., Perry, J. E., Hayes, A. G., & McEwen, A. S. (2011). Shoreline retreat at Titan's Ontario Lacus and Arrakis Planitia from Cassini Imaging Science Subsystem observations. Icarus, 212(2), 957-959.
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  Abstract: Recent observations by Cassini's Imaging Science Subsystem reveal that part of the shoreline of Titan's Ontario Lacus has retreated by several kilometers and may indicate that the dark area that appeared at Arrakis Planitia (80°S, 120°W) in late 2004 has subsequently faded. These changes provide constraints on aspects of Titan's methane cycle, as well as on the properties of Titan's surface materials. © 2011 Elsevier Inc.
• Turtle, E. P., Perry, J. E., Hayes, A. G., Lorenz, R. D., Barnes, J. W., McEwen, A. S., West, R. A., Genio, A. D., Barbara, J. M., Lunine, J. I., Schaller, E. L., Ray, T. L., Lopes, R. M., & Stofan, E. R. (2011). Rapid and extensive surface changes near Titan's equator: Evidence of April showers. Science, 331(6023), 1414-1417.
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  PMID: 21415347;Abstract: Although there is evidence that liquids have flowed on the surface at Titan's equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate. We report the detection by Cassini's Imaging Science Subsystem of a large low-latitude cloud system early in Titan's northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan's low latitudes are carved by seasonal precipitation.
• Banks, M. E., Byrne, S., Galla, K., McEwen, A. S., Bray, V. J., Dundas, C. M., Fishbaugh, K. E., Herkenhoff, K. E., & Murray, B. C. (2010). Crater population and resurfacing of the Martian north polar layered deposits. Journal of Geophysical Research E: Planets, 115(8).
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  Abstract: Present-day accumulation in the north polar layered deposits (NPLD) is thought to occur via deposition on the north polar residual cap. Understanding current mass balance in relation to current climate would provide insight into the climatic record of the NPLD. To constrain processes and rates of NPLD resurfacing, a search for craters was conducted using images from the Mars Reconnaissance Orbiter Context Camera. One hundred thirty craters have been identified on the NPLD, 95 of which are located within a region defined to represent recent accumulation. High Resolution Imaging Science Experiment images of craters in this region reveal a morphological sequence of crater degradation that provides a qualitative understanding of processes involved in crater removal. A classification system for these craters was developed based on the amount of apparent degradation and infilling and where possible depth/diameter ratios were determined. The temporal and spatial distribution of crater degradation is interpreted to be close to uniform. Through comparison of the size-frequency distribution of these craters with the expected production function, the craters are interpreted to be an equilibrium population with a crater of diameter D meters having a lifetime of ∼30.75D1.14 years. Accumulation rates within these craters are estimated at 7.2D -0.14 mm/yr, which corresponds to values of ∼3-4 mm/yr and are much higher than rates thought to apply to the surrounding flat terrain. The current crater population is estimated to have accumulated in the last ∼20 kyr or less. Copyright 2010 by the American Geophysical Union.
• Bray, V. J., Tornabene, L. L., Keszthelyi, L. P., McEwen, A. S., Hawke, B. R., Giguere, T. A., Kattenhorn, S. A., Garry, W. B., Rizk, B., Caudill, C. M., Gaddis, L. R., & Van, C. (2010). New insight into lunar impact melt mobility from the LRO camera. Geophysical Research Letters, 37(21).
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  Abstract: The Lunar Reconnaissance Orbiter Camera (LROC) is systematically imaging impact melt deposits in and around lunar craters at meter and sub-meter scales. These images reveal that lunar impact melts, although morphologically similar to terrestrial lava flows of similar size, exhibit distinctive features (e.g., erosional channels). Although generated in a single rapid event, the post-impact mobility and morphology of lunar impact melts is surprisingly complex. We present evidence for multi-stage influx of impact melt into flow lobes and crater floor ponds. Our volume and cooling time estimates for the post-emplacement melt movements noted in LROC images suggest that new flows can emerge from melt ponds an extended time period after the impact event. © 2010 by the American Geophysical Union.
• Bridges, N. T., Banks, M. E., Beyer, R. A., Chuang, F. C., Dobrea, E. N., Herkenhoff, K. E., Keszthelyi, L. P., Fishbaugh, K. E., McEwen, A. S., Michaels, T. I., Thomson, B. J., & Wray, J. J. (2010). Aeolian bedforms, yardangs, and indurated surfaces in the Tharsis Montes as seen by the HiRISE Camera: Evidence for dust aggregates. Icarus, 205(1), 165-182.
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  Abstract: HiRISE images of Mars with ground sampling down to 25 cm/pixel show that the dust-rich mantle covering the surfaces of the Tharsis Montes is organized into ridges whose form and distribution are consistent with formation by aeolian saltation. Other dusty areas near the volcanoes and elsewhere on the planet exhibit a similar morphology. The material composing these "reticulate" bedforms is constrained by their remote sensing properties and the threshold curve combined with the saltation/suspension boundary, both of which vary as a function of elevation (atmospheric pressure), particle size, and particle composition. Considering all of these factors, dust aggregates are the most likely material composing these bedforms. We propose that airfall dust on and near the volcanoes aggregates in situ over time, maybe due to electrostatic charging followed by cementation by salts. The aggregates eventually reach a particle size at which saltation is possible. Aggregates on the flanks are transported downslope by katabatic winds and form linear and "accordion" morphologies. Materials within the calderas and other depressions remain trapped and are subjected to multidirectional winds, forming an interlinked "honeycomb" texture. In many places on and near the volcanoes, light-toned, low thermal inertia yardangs and indurated surfaces are present. These may represent "duststone" formed when aggregates reach a particle size below the threshold curve, such that they become stabilized and subsequently undergo cementation. © 2009 Elsevier Inc.
• Delamere, W. A., Tornabene, L. L., McEwen, A. S., Becker, K., Bergstrom, J. W., Bridges, N. T., Eliason, E. M., Gallagher, D., Herkenhoff, K. E., Keszthelyi, L., Mattson, S., McArthur, G. K., Mellon, M. T., Milazzo, M., Russell, P. S., & Thomas, N. (2010). Color imaging of Mars by the High Resolution Imaging Science Experiment (HiRISE). Icarus, 205(1), 38-52.
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  Abstract: HiRISE has been producing a large number of scientifically useful color products of Mars and other planetary objects. The three broad spectral bands, coupled with the highly sensitive 14 bit detectors and time delay integration, enable detection of subtle color differences. The very high spatial resolution of HiRISE can augment the mineralogic interpretations based on multispectral (THEMIS) and hyperspectral datasets (TES, OMEGA and CRISM) and thereby enable detailed geologic and stratigraphic interpretations at meter scales. In addition to providing some examples of color images and their interpretation, we describe the processing techniques used to produce them and note some of the minor artifacts in the output. We also provide an example of how HiRISE color products can be effectively used to expand mineral and lithologic mapping provided by CRISM data products that are backed by other spectral datasets. The utility of high quality color data for understanding geologic processes on Mars has been one of the major successes of HiRISE. © 2009 Elsevier Inc.
• Diniega, S., Byrne, S., Bridges, N. T., Dundas, C. M., & McEwen, A. S. (2010). Seasonality of present-day Martian dune-gully activity. Geology, 38(11), 1047-1050.
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  Abstract: Martian slope gullies are argued to be evidence for recent liquid water flow on the surface of Mars. To explain the source of water, a wide range of environmental conditions and processes has been invoked. However, a lack of information about the environmental context or timing of gully activity makes it difficult to evaluate the theories. Here, we present new observations of extensive gully modification over the past 6 Mars years within dune gullies with slope-gully morphology. Observed activity within 18 gullies in 7 dune fields constrains timing to winter, which is consistent with observed slope-gully activity. These observations show that fluvial processes are unlikely to cause present-day Martian dune-gully activity, and imply that CO2 frost accumulation may play the dominant role. © 2010 Geological Society of America.
• Dundas, C. M., & McEwen, A. S. (2010). An assessment of evidence for pingos on Mars using HiRISE. Icarus, 205(1), 244-258.
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  Abstract: Pingos are small hills with cores of ice, formed by injection and freezing of pressurized water. The possibility of pingos on Mars is of particular interest because of the associated implications for liquid water. We have systematically searched for candidate pingos using images from the High Resolution Imaging Science Experiment (HiRISE) camera. Since pingos are expected to develop surface fractures due to extension of the frozen ground over the ice core, we have searched for fractured features and identified a variety of mounds. These features are confined to the martian mid-latitudes, in the bands where gullies are also most common. The observed fractured mounds have a variety of morphologies and are likely of multiple origins. Isolated fractured mounds found on the floors of gullied craters in the southern hemisphere match the general morphologic characteristics of terrestrial pingos and are the best candidates for martian pingos, but there is currently no direct evidence for presence of ice cores and it is difficult to produce the necessary water volumes, so these features should still be interpreted with caution. Other fractured mounds appear more likely to be erosional remnants of an unusual mantling layer or possibly thermokarst structures. Flat-topped mounds in Utopia have some characteristics (fracture pattern and latitudinal distribution) consistent with pingos but differ in other aspects such as shape and setting. While we do not rule out a pingo origin, we prefer an erosional model for these enigmatic features. © 2009 Elsevier Inc. All rights reserved.
• Dundas, C. M., Keszthelyi, L. P., Bray, V. J., & McEwen, A. S. (2010). Role of material properties in the cratering record of young platy-ridged lava on Mars. Geophysical Research Letters, 37(12).
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  Abstract: Platy-ridged surfaces in the Elysium Planitia region of Mars exhibit different crater densities on rafted plates and polygonally patterned areas between them. Rather than being indicative of different ages, these differences provide insight into the variable strength of different types of lava surface. The sizes of small craters, and the resulting sizefrequency distribution (SFD), depend on the material strength of target surfaces. Brecciated lava surfaces are likely to have higher crater densities than coherent lava. Copyright 2010 by the American Geophysical Union.
• Fishbaugh, K. E., Byrne, S., Herkenhoff, K. E., Kirk, R. L., Fortezzo, C., Russell, P. S., & McEwen, A. (2010). Evaluating the meaning of "layer" in the martian north polar layered deposits and the impact on the climate connection. Icarus, 205(1), 269-282.
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  Abstract: Using data from the High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter, we reassess the methods by which layers within the north polar layered deposits (NPLD) can be delineated and their thicknesses measured. Apparent brightness and morphology alone are insufficient for this task; high resolution topographic data are necessary. From these analyses, we find that the visible appearance of layers depends to a large degree on the distribution of younger, mantling deposits (which in turn is partially influenced by inherent layer properties) and on the shape and location of the particular outcrop. This younger mantle partially obscures layer morphology and brightness and is likely a cause of the gradational contacts between individual layers at this scale. High resolution images reveal that there are several layers similar in appearance to the well-known marker bed discovered by Malin, M., Edgett, K., 2001. J. Geophys. Res. 106, 23429-23570. The morphology, thicknesses (4 - 8 ± sqrt(2) m), and separation distances (5 - 32 ± sqrt(2) m) of these marker beds, as gleaned from a high resolution stereo digital elevation model, lend insight into the connection between stratigraphy and climate.
• Golombek, M., Robinson, K., McEwen, A., Bridges, N., Ivanov, B., Tornabene, L., & Sullivan, R. (2010). Constraints on ripple migration at Meridiani Planum from opportunity and HiRISE observations of fresh craters. Journal of Geophysical Research E: Planets, 115(11).
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  Abstract: Observations of fresh impact craters by the Opportunity rover and in high-resolution orbital images constrain the latest phase of granule ripple migration at Meridiani Planum to have occurred between ∼50 ka and ∼200 ka. Opportunity explored the fresh Resolution crater cluster and Concepción crater that are superposed on and thus younger than the ripples. These fresh craters have small dark pebbles scattered across their surfaces, which are most likely fragments of the impactor, suggesting that the dark pebbles and cobbles observed by Opportunity at Meridiani Planum are a lag of impactor-derived material (either meteoritic or secondary impactors from elsewhere on Mars). Two larger, fresh-rayed craters in Meridiani Planum bracket ripple migration; secondaries from Ada crater are clearly superposed on and secondaries from an unnamed 0.84 km diameter crater have been modified and overprinted by the ripples. Three methods were used to estimate the age of these craters and thus when the latest phase of ripple migration occurred. The inactivity of the ripples over the past ∼50 ka at Meridiani is also consistent with other evidence for the stability of the ripples, the lack of observed eolian bed forms in craters that formed in the past 20 years, and little evidence for much dune motion in the past 30 yr on Mars. Observations of crater morphology and their interaction with the ripples allow the development of a general time scale for craters in Meridiani Planum over the past million years. © 2010 by the American Geophysical Union.
• Hansen, C. J., Thomas, N., Portyankina, G., McEwen, A., Becker, T., Byrne, S., Herkenhoff, K., Kieffer, H., & Mellon, M. (2010). HiRISE observations of gas sublimation-driven activity in Mars' southern polar regions: I. Erosion of the surface. Icarus, 205(1), 283-295.
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  Abstract: The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) has imaged the sublimation of Mars' seasonal CO2 polar cap with unprecedented detail for one complete martian southern spring. In some areas of the surface, beneath the conformal coating of seasonal ice, radially-organized channels are connected in spidery patterns. The process of formation of this terrain, erosion by gas from subliming seasonal ice, has no earthly analog. The new capabilities (high resolution, color, and stereo images) of HiRISE enable detailed study of this enigmatic terrain. Two sites are analyzed in detail, one within an area expected to have translucent seasonal CO2 ice, and the other site outside that region. Stereo anaglyphs show that some channels grow larger as they go uphill - implicating gas rather than liquid as the erosive agent. Dark fans of material from the substrate are observed draped over the seasonal ice, and this material collects in thin to thick layers in the channels, possibly choking off gas flow in subsequent years, resulting in inactive crisscrossing shallow channels. In some areas there are very dense networks of channels with similar width and depth, and fewer fans emerging later in the season are observed. Subtle variations in topography affect the channel morphology. A new terminology is proposed for the wide variety of erosional features observed. © 2009 Elsevier Inc.
• Herkenhoff, K. E., Keszthelyi, L. P., & McEwen, A. S. (2010). Introduction to the High Resolution Imaging Science Experiment (HiRISE) Special Issue of Icarus. Icarus, 205(1), 1-.
• Jaeger, W. L., Keszthelyi, L. P., Skinner Jr., J. A., Milazzo, M. P., McEwen, A. S., Titus, T. N., Rosiek, M. R., Galuszka, D. M., Howington-Kraus, E., & Kirk, R. L. (2010). Emplacement of the youngest flood lava on Mars: A short, turbulent story. Icarus, 205(1), 230-243.
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  Abstract: Recently acquired data from the High Resolution Imaging Science Experiment (HiRISE), Context (CTX) imager, and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) spacecraft were used to investigate the emplacement of the youngest flood-lava flow on Mars. Careful mapping finds that the Athabasca Valles flood lava is the product of a single eruption, and it covers 250,000 km2 of western Elysium Planitia with an estimated 5000-7500 km3 of mafic or ultramafic lava. Calculations utilizing topographic data enhanced with MRO observations to refine the dimensions of the channel system show that this flood lava was emplaced turbulently over a period of only a few to several weeks. This is the first well-documented example of a turbulently emplaced flood lava anywhere in the Solar System. However, MRO data suggest that this same process may have operated in a number of martian channel systems. The magnitude and dynamics of these lava floods are similar to the aqueous floods that are generally believed to have eroded the channels, raising the intriguing possibility that mechanical erosion by lava could have played a role in their incision. © 2009.
• Keszthelyi, L. P., Jaeger, W. L., Dundas, C. M., Martínez-Alonso, S., McEwen, A. S., & Milazzo, M. P. (2010). Hydrovolcanic features on Mars: Preliminary observations from the first Mars year of HiRISE imaging. Icarus, 205(1), 211-229.
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  Abstract: We provide an overview of features indicative of the interaction between water and lava and/or magma on Mars as seen by the High Resolution Imaging Science Experiment (HiRISE) camera during the Primary Science Phase of the Mars Reconnaissance Orbiter (MRO) mission. The ability to confidently resolve meter-scale features from orbit has been extremely useful in the study of the most pristine examples. In particular, HiRISE has allowed the documentation of previously undescribed features associated with phreatovolcanic cones (formed by the interaction of lava and groundwater) on rapidly emplaced flood lavas. These include "moats" and "wakes" that indicate that the lava crust was thin and mobile, respectively [Jaeger, W.L., Keszthelyi, L.P., McEwen, A.S., Dundas, C.M., Russel, P.S., 2007. Science 317, 1709-1711]. HiRISE has also discovered entablature-style jointing in lavas that is indicative of water-cooling [Milazzo, M.P., Keszthelyi, L.P., Jaeger, W.L., Rosiek, M., Mattson, S., Verba, C., Beyer, R.A., Geissler, P.E., McEwen, A.S., and the HiRISE Team, 2009. Geology 37, 171-174]. Other observations strongly support the idea of extensive volcanic mudflows (lahars). Evidence for other forms of hydrovolcanism, including glaciovolcanic interactions, is more equivocal. This is largely because most older and high-latitude terrains have been extensively modified, masking any earlier 1-10 m scale features. Much like terrestrial fieldwork, the prerequisite for making full use of HiRISE's capabilities is finding good outcrops.
• Kolb, K. J., McEwen, A. S., & Pelletier, J. D. (2010). Investigating gully flow emplacement mechanisms using apex slopes. Icarus, 208(1), 132-142.
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  Abstract: The origin of the martian gullies has been much debated since their discovery by the Mars Orbiter Camera (MOC, Malin, M.C., Edgett, K.S. [2000]. Science 288, 2330-2335). Several previous studies have looked at slope gradients in and around gullies, but none have used Digital Elevation Models (DEMs) from the High Resolution Imaging Science Experiment (HiRISE, McEwen, A.S., and 14 colleagues [2007]. J. Geophys. Res. 112 (E05), E0505S02), which has a pixel scale down to 25. cm/pixel. We use five 1. m/post HiRISE DEMs to measure gully apex slopes, the local channel gradient at the upslope extent of the gully debris apron, which marks a shift from erosion to deposition. The apex slope provides information about whether a flow was likely a typical dry granular flow (begins depositing on slopes ~21°) or fluidized by some extra mechanism (depositing on shallower slopes). We find that 72% of the 75 gully fans studied were likely emplaced by fluidized flows. Relatively old gullies appear more likely to have hosted fluidized flows than relatively fresh gullies. This suggests a time and location dependent fluidizing agent, possibly liquid water produced in a different climate as previously proposed. Our results do not provide evidence for water-rich flows in gullies today. © 2010 Elsevier Inc.
• Kolb, K. J., Pelletier, J. D., & McEwen, A. S. (2010). Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water. Icarus, 205(1), 113-137.
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  Abstract: Our study investigates possible formation mechanisms of the very recent bright gully deposits (BGDs) observed on Mars in order to assess if liquid water was required. We use two models in our assessment: a one-dimensional (1D) kinematic model to model dry granular flows and a two-dimensional (2D) fluid-dynamic model, FLO-2D (O'Brien et al., 1993, FLO Engineering), to model water-rich and wet sediment-rich flows. Our modeling utilizes a high-resolution topographic model generated from a pair of images acquired by the High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter. For the 1D kinematic modeling of dry granular flows, we examine a range of particle sizes, flow thicknesses, initial velocities, flow densities, and upslope initiation points to examine how these parameters affect the flow run-out distances of the center of mass of a flow. Our 1D modeling results show that multiple combinations of realistic parameters could produce dry granular flows that travel to within the observed deposits' boundaries. We run the 2D fluid-dynamic model, FLO-2D, to model both water-rich and wet sediment-rich flows. We vary the inflow volume, inflow location, discharge rate, water-loss rate (water-rich models only), and simulation time and examine the resulting maximum flow depths and velocities. Our 2D modeling results suggest that both wet sediment-rich and water-rich flows could produce the observed bright deposits. Our modeling shows that the BGDs are not definitive evidence of recent liquid water on the surface of Mars. © 2009 Elsevier Inc. All rights reserved.
• McEwen, A. S., Banks, M. E., Baugh, N., Becker, K., Boyd, A., Bergstrom, J. W., Beyer, R. A., Bortolini, E., Bridges, N. T., Byrne, S., Castalia, B., Chuang, F. C., Crumpler, L. S., Daubar, I., Davatzes, A. K., Deardorff, D. G., DeJong, A., Delamere, W. A., Dobrea, E. N., , Dundas, C. M., et al. (2010). The High Resolution Imaging Science Experiment (HiRISE) during MRO's Primary Science Phase (PSP). Icarus, 205(1), 2-37.
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  Abstract: The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) acquired 8 terapixels of data in 9137 images of Mars between October 2006 and December 2008, covering ∼0.55% of the surface. Images are typically 5-6 km wide with 3-color coverage over the central 20% of the swath, and their scales usually range from 25 to 60 cm/pixel. Nine hundred and sixty stereo pairs were acquired and more than 50 digital terrain models (DTMs) completed; these data have led to some of the most significant science results. New methods to measure and correct distortions due to pointing jitter facilitate topographic and change-detection studies at sub-meter scales. Recent results address Noachian bedrock stratigraphy, fluvially deposited fans in craters and in or near Valles Marineris, groundwater flow in fractures and porous media, quasi-periodic layering in polar and non-polar deposits, tectonic history of west Candor Chasma, geometry of clay-rich deposits near and within Mawrth Vallis, dynamics of flood lavas in the Cerberus Palus region, evidence for pyroclastic deposits, columnar jointing in lava flows, recent collapse pits, evidence for water in well-preserved impact craters, newly discovered large rayed craters, and glacial and periglacial processes. Of particular interest are ongoing processes such as those driven by the wind, impact cratering, avalanches of dust and/or frost, relatively bright deposits on steep gullied slopes, and the dynamic seasonal processes over polar regions. HiRISE has acquired hundreds of large images of past, present and potential future landing sites and has contributed to scientific and engineering studies of those sites. Warming the focal-plane electronics prior to imaging has mitigated an instrument anomaly that produces bad data under cold operating conditions. © 2009 Elsevier Inc.
• Robinson, M. S., Brylow, S. M., Tschimmel, M., Humm, D., Lawrence, S. J., Thomas, P. C., Denevi, B. W., Bowman-Cisneros, E., Zerr, J., Ravine, M. A., Caplinger, M. A., Ghaemi, F. T., Schaffner, J. A., Malin, M. C., Mahanti, P., Bartels, A., Anderson, J., Tran, T. N., Eliason, E. M., , McEwen, A. S., et al. (2010). Lunar reconnaissance orbiter camera (LROC) instrument overview. Space Science Reviews, 150(1-4), 81-124.
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  Abstract: The Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) and Narrow Angle Cameras (NACs) are on the NASA Lunar Reconnaissance Orbiter (LRO). The WAC is a 7-color push-frame camera (100 and 400 m/pixel visible and UV, respectively), while the two NACs are monochrome narrow-angle linescan imagers (0.5 m/pixel). The primary mission of LRO is to obtain measurements of the Moon that will enable future lunar human exploration. The overarching goals of the LROC investigation include landing site identification and certification, mapping of permanently polar shadowed and sunlit regions, meter-scale mapping of polar regions, global multispectral imaging, a global morphology base map, characterization of regolith properties, and determination of current impact hazards. © 2010 Springer Science+Business Media B.V.
• Weitz, C. M., Milliken, R. E., Grant, J. A., McEwen, A. S., Williams, R. M., Bishop, J. L., & Thomson, B. J. (2010). Mars Reconnaissance Orbiter observations of light-toned layered deposits and associated fluvial landforms on the plateaus adjacent to Valles Marineris. Icarus, 205(1), 73-102.
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  Abstract: We have used data from the Mars Reconnaissance Orbiter to study 30-80 m thick light-toned layered deposits on the plateaus adjacent to Valles Marineris at five locations: (1) south of Ius Chasma, (2) south of western Melas Chasma, (3) south of western Candor Chasma, (4) west of Juventae Chasma, and (5) west of Ganges Chasma. The beds within these deposits have unique variations in brightness, color, mineralogy, and erosional properties that are not typically observed in light-toned layered deposits within Valles Marineris or many other equatorial areas on Mars. Reflectance spectra indicate these deposits contain opaline silica and Fe-sulfates, consistent with low-temperature, acidic aqueous alteration of basaltic materials. We have found valley or channel systems associated with the layered deposits at all five locations, and the volcanic plains adjacent to Juventae, Ius, and Ganges exhibit inverted channels composed of light-toned beds. Valleys, channels, and light-toned layering along the walls of Juventae and Melas Chasmata are most likely coeval to the aqueous activity that affected the adjacent plateaus and indicate some hydrological activity occurred after formation of the chasmata. Although the source of water and sediment remains uncertain, the strong correlation between fluvial landforms and light-toned layered deposits argues for sustained precipitation, surface runoff, and fluvial deposition occurring during the Hesperian on the plateaus adjacent to Valles Marineris and along portions of chasmata walls. © 2009 Elsevier Inc.
• West, R., Knowles, B., Birath, E., Charnoz, S., Nino, D. D., Hedman, M., Helfenstein, P., McEwen, A., Perry, J., Porco, C., Salmon, J., Throop, H., & Wilson, D. (2010). In-flight calibration of the Cassini imaging science sub-system cameras. Planetary and Space Science, 58(11), 1475-1488.
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  Abstract: We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow- and wide-angle cameras using data from 2004 to 2009. We report on the photometric performance of the cameras including the use of polarization filters, point spread functions over a dynamic range greater than 10 7, gain and loss of hot pixels, changes in flat fields, and an analysis of charge transfer efficiency. Hot pixel behavior is more complicated than can be understood by a process of activation by cosmic ray damage and deactivation by annealing. Point spread function (PSF) analysis revealed a ghost feature associated with the narrow-angle camera Green filter. More generally, the observed PSFs do not fall off with distance as rapidly as expected if diffraction were the primary contributor. Stray light produces significant signal far from the center of the PSF. Our photometric analysis made use of calibrated spectra from eighteen stars and the spectral shape of the satellite Enceladus. The analysis revealed a shutter offset that differed from pre-launch calibration. It affects the shortest exposures. Star photometry results are reproducible to a few percent in most filters. No degradation in charge transfer efficiency has been detected although uncertainties are large. The results of this work have been digitally archived and incorporated into our calibration software CISSCAL available online. © 2010 Elsevier B.V.. All rights reserved.
• Byrne, S., Dundas, C. M., Kennedy, M. R., Mellon, M. T., McEwen, A. S., Cull, S. C., Daubar, I. J., Shean, D. E., Seelos, K. D., Murchie, S. L., Cantor, B. A., Arvidson, R. E., Edgett, K. S., Reufer, A., Thomas, N., Harrison, T. N., Posiolova, L. V., & Seelos, F. P. (2009). Distribution of mid-latitude ground ice on mars from new impact craters. Science, 325(5948), 1674-1676.
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  PMID: 19779195;Abstract: New impact craters at five sites in the martian mid-latitudes excavated material from depths of decimeters that has a brightness and color indicative of water ice. Near-infrared spectra of the largest example confirm this composition, and repeated imaging showed fading over several months, as expected for sublimating ice. Thermal models of one site show that millimeters of sublimation occurred during this fading period, indicating clean ice rather than ice in soil pores. Our derived ice-table depths are consistent with models using higher long-term average atmospheric water vapor content than present values. Craters at most of these sites may have excavated completely through this clean ice, probing the ice table to previously unsampled depths of meters and revealing substantial heterogeneity in the vertical distribution of the ice itself.
• Golombek, M. P., Huertas, A., Marlow, J., McGrane, B., Klein, C., Martinez, M., Arvidson, R. E., Heet, T., Barry, L., Seelos, K., Adams, D., Li, W., Matijevic, J. R., Parker, T., Sizemore, H. G., Mellon, M., McEwen, A. S., Tamppari, L. K., & Cheng, Y. (2009). Size-frequency distributions of rocks on the northern plains of Mars with special reference to Phoenix landing surfaces. Journal of Geophysical Research E: Planets, 114(3).
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  Abstract: The size-frequency distributions of rocks >1.5 m diameter fully resolvable in High Resolution Imaging Science Experiment (HiRISE) images of the northern plains follow exponential models developed from lander measurements of smaller rocks and are continuous with rock distributions measured at the landing sites. Dark pixels at the resolution limit of Mars Orbiter Camera thought to be boulders are shown to be mostly dark shadows of clustered smaller rocks in HiRISE images. An automated rock detector algorithm that fits ellipses to shadows and cylinders to the rocks, accurately measured (within 1 -2 pixels) rock diameter and height (by comparison to spacecraft of known size) of ∼10 million rocks over >1500 km2 of the northern plains. Rock distributions in these counts parallel models for cumulative fractional area covered by 30-90% rocks in dense rock fields around craters, 10-30% rock coverage in less dense rock fields, and 0-10% rock coverage in background terrain away from-craters. Above ∼ 1.5 m diameter, HiRISE resolves the same population of rocks seen in lander images, and thus size-frequency distributions can be extrapolated along model curves to estimate the number of rocks at smaller diameters. Extrapolating sparse rock distributions in the Phoenix landing ellipse indicate
• Good, P. G., Johnson, M. A., Eckart, D. F., Sidney, W., Manning, R. M., Highsmith, D. E., McEwen, A., Mattson, S., & Eliason, E. (2009). Mro imaging of Phoenix descent. Advances in the Astronautical Sciences, 133, 77-96.
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  Abstract: On May 25, 2008, the Mars Reconnaissance Orbiter (MRO) used its HiRISE camera to capture a dramatic image of the Phoenix Mars Lander descending on its parachute towards the surface of Mars. This was the first time that a spacecraft has imaged the final descent of another spacecraft onto a planetary body. Capturing the image required months of planning and testing across many disciplines of the MRO operations team. This paper presents the navigational changes to the MRO orbit to support the Phoenix entry geometry, the GN&C maneuvers necessary to track Phoenix during its entry and descent, the statistical analysis to determine the likelihood of capturing the image, and the HiRISE imaging preparation and post processing to bring out the details of the Lander, parachute and back-shell.
• Kirk, R. L., Howington-Kraus, E., Rosiek, M. R., Anderson, J. A., Archinal, B. A., Becker, K. J., Cook, D. A., Galuszka, D. M., Geissler, P. E., Hare, T. M., Holmberg, I. M., Keszthelyi, L. P., Redding, B. L., Delamere, W. A., Gallagher, D., Chapel, J. D., Eliason, E. M., King, R., & McEwen, A. S. (2009). Ultrahigh resolution topographic mapping of Mars with MRO HiRISE stereo images: Meter-scale slopes of candidate Phoenix landing sites. Journal of Geophysical Research E: Planets, 114(3).
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  Abstract: The objectives of this paper are twofold: first, to report our estimates of the meter-to-decameter-scale topography and slopes of candidate landing sites for the Phoenix mission, based on analysis of Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images with a typical pixel scale of 3 m and Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) images at 0.3 m pixel-1 and, second, to document in detail the geometric calibration, software, and procedures on which the photogrammetric analysis of HiRISE data is based. A combination of optical design modeling, laboratory observations, star images, and Mars images form the basis for software in the U.S. Geological Survey Integrated Software for Imagers and Spectrometers (ISIS) 3 system that corrects the images for a variety of distortions with single-pixel or subpixel accuracy. Corrected images are analyzed in the commercial photogrammetric software SOCET SET (®BAE Systems), yielding digital topographic models (DTMs) with a grid spacing of 1 m (3-4 pixels) that require minimal interactive editing. Photoclinometry yields DTMs with single-pixel grid spacing. Slopes from MOC and HiRISE are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully; only the Mars Exploration Rover (MER) B site in Meridiani Planum is smoother. MOC results at multiple locations have root-mean-square (RMS) bidirectional slopes of 0.8-4.5° at baselines of 3-10 m. HiRISE stereopairs (one per final candidate site and one in the former site) yield 1.8-2.8° slopes at 1-m baseline. Slopes at 1 m from photoclinometry are also in the range 2-3° after correction for image blur. Slopes exceeding the 16° Phoenix safety limit are extremely rare. Copyright 2008 by the American Geophysical Union.
• Lefort, A., Russell, P. S., Thomas, N., McEwen, A. S., Dundas, C. M., & Physikalisches, R. K. (2009). Observations of periglacial landforms in utopia planitia with the high resolution imaging science experiment (HiRISE). Journal of Geophysical Research E: Planets, 114(4).
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  Abstract: The region of western Utopia Planitia (80-105°E, 40-55°N) displays several types of landforms similar to Earth periglacial features, including scallop-shaped depressions and networks of polygonal terrains. The scalloped depressions have been proposed to originate from thermokarstic processes such as sublimation and/or melting of near-surface ground ice. Using HiRISE imagery, we characterize these depressions and several associated, distinct polygon networks in unprecedented morphologic and topographic detail and investigate support for an ice-based degradation process. The scalloped depressions and interior polygons and ridges are found to evolve together, mainly influenced by sublimation, local proximity of ground ice to the surface, and obliquity variations. Copyright 2009 by the American Geophysical Union.
• Metz, J. M., Grotzinger, J. P., Mohrig, D., Milliken, R., Prather, B., Pirmez, C., McEwen, A. S., & Weitz, C. M. (2009). Sublacustrine depositional fans in southwest Melas Chasma. Journal of Geophysical Research E: Planets, 114(10).
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  Abstract: Two depositional fan complexes have been identified on the floor of southwest Melas Chasma. The western fan complex is located near the center of an enclosed basin in southwest Melas Chasma and is composed of multiple lobes with dendritic finger-like terminations. These fans are very flat and have a morphology unlike any other fan that has been previously identified on Mars. On the basis of the morphologic similarity of the western fan complex to the Mississippi submarine fan complex, we suggest that it may be a deep subaqueous fan depositional system. There are numerous channels on the surface of the western fan complex, and measurements of channel length, width, and sinuosity are consistent with channels observed on terrestrial submarine fans. The eastern Melas depositional fans are less well preserved and may be of deltaic or sublacustrine origin. Recognition of the fans supports earlier suggestions for the presence of a former lake in Melas Chasma and indicates that a significant body of water was present and stable at the surface of Mars for at least 10 2 to 104 years. Copyright 2009 by the American Geophysical Union.
• Milazzo, M. P., Keszthelyi, L. P., Jaeger, W. L., Rosiek, M., Mattson, S., Verba, C., Beyer, R. A., Geissler, P. E., & McEwen, A. S. (2009). Discovery of columnar jointing on Mars. Geology, 37(2), 171-174.
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  Abstract: We report on the discovery of columnar jointing in Marte Valles, Mars. These columnar lavas were discovered in the wall of a pristine, 16-km-diameter impact crater and exhibit the features of terrestrial columnar basalts. There are discontinuous outcrops along the entire crater wall, suggesting that the columnar rocks covered a surface area of at least 200 km2, assuming that the rocks obliterated by the impact event were similarly jointed. We also see columns in the walls of other fresh craters in the nearby volcanic plains of Elysium Planitia-Amazonis Planitia, which include Marte Vallis, and in a well-preserved crater in northeast Hellas. © 2009 The Geological Society of America.
• Murchie, S. L., Mustard, J. F., Ehlmann, B. L., Milliken, R. E., Bishop, J. L., McKeown, N. K., Z., E., Seelos, F. P., Buczkowski, D. L., Wiseman, S. M., Arvidson, R. E., Wray, J. J., Swayze, G., Clark, R. N., J., D., McEwen, A. S., & Bibring, J. (2009). A synthesis of Martian aqueous mineralogy after 1 Mars year of observations from the Mars Reconnaissance Orbiter. Journal of Geophysical Research E: Planets, 114(9).
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  Abstract: Martian aqueous mineral deposits have been examined and characterized using data acquired during Mars Reconnaissance Orbiter's (MRO) primary science phase, including Compact Reconnaissance Imaging Spectrometer for Mars hyperspectral images covering the 0.4-3.9 μm wavelength range, coordinated with higher-spatial resolution HiRISE and Context Imager images. MRO's new high-resolution measurements, combined with earlier data from Thermal Emission Spectrometer; Thermal Emission Imaging System; and Observatoire pour la Minéralogie, L'Eau, les Glaces et l'Activitié on Mars Express, indicate that aqueous minerals are both diverse and widespread on the Martian surface. The aqueous minerals occur in 9-10 classes of deposits characterized by distinct mineral assemblages, morphologies, and geologic settings. Phyllosilicates occur in several settings: in compositionally layered blankets hundreds of meters thick, superposed on eroded Noachian terrains; in lower layers of intracrater depositional fans; in layers with potential chlorides in sediments on intercrater plains; and as thousands of deep exposures in craters and escarpments. Carbonate-bearing rocks form a thin unit surrounding the Isidis basin. Hydrated silica occurs with hydrated sulfates in thin stratified deposits surrounding Valles Marineris. Hydrated sulfates also occur together with crystalline ferric minerals in thick, layered deposits in Terra Meridiani and in Valles Marineris and together with kaolinite in deposits that partially infill some highland craters. In this paper we describe each of the classes of deposits, review hypotheses for their origins, identify new questions posed by existing measurements, and consider their implications for ancient habitable environments. On the basis of current data, two to five classes of Noachian-aged deposits containing phyllosilicates and carbonates may have formed in aqueous environments with pH and water activities suitable for life. Copyright 2009 by the American Geophysical Union.
• Turtle, E. P., Perry, J. E., McEwen, A. S., DelGenio, A. D., Barbara, J., West, R. A., Dawson, D. D., & Porco, C. C. (2009). Cassini imaging of Titan's high-latitude lakes, clouds, and south-polar surface changes. Geophysical Research Letters, 36(2).
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  Abstract: Cassini's Imaging Science Subsystem (ISS) has been observing Titan since April 2004, compiling a nearly global surface map and monitoring the surface and atmosphere for activity. Early images of the south-polar region revealed numerous dark surface features and contemporaneous convective cloud systems, suggesting the presence of hydrocarbon lakes similar to those later detected at Titan's North Pole. Intriguingly, repeated south-polar imaging by ISS revealed differences consistent with ponding of hydrocarbon liquids on the surface due to precipitation from a large storm. More recent ISS images of high northern latitudes illustrate the full extents (>500,000 km2) of hydrocarbon seas, sections of which have been observed by Cassini's RADAR. These observations demonstrate dynamic processes at work on Titan and that the poles harbor liquid-hydrocarbon reservoirs, the extents of which differ from pole to pole and which may be coupled to seasonally varying circulation. Copyright 2009 by the American Geophysical Union.
• Wray, J. J., Dobrea, E. N., Arvidson, R. E., Wiseman, S. M., Squyres, S. W., McEwen, A. S., Mustard, J. F., & Murchie, S. L. (2009). Phyllosilicates and sulfates at Endeavour Crater, Meridiani Planum, Mars. Geophysical Research Letters, 36(21).
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  Abstract: Phyllosilicates have been identified on the Martian surface from orbit, and are hypothesized to have formed under wet, non-acidic conditions early in the planet's history. Exposures of these minerals have not yet been examined by a landed mission. Using Mars Reconnaissance Orbiter data, we report the detection of phyllosilicate-bearing outcrops that may be accessible by the Mars Exploration Rover Opportunity currently exploring Meridiani Planum. The phyllosilicates are associated with layered, polygonally fractured rocks exposed in the rim of the 20 km diameter crater Endeavour. These rocks may have formed via regional or global-scale processes of aqueous alteration that predated the period of acid sulfate formation recorded in the rocks that Opportunity has studied to date. Detailed characterization by Opportunity could better constrain the conditions under which these phyllosilicates formed. Hydrated sulfates are also detected from orbit in the plains adjacent to Endeavour's rim, providing the first opportunity for ground truth of these detections. Copyright 2009 by the American Geophysical Union.
• Dundas, C. M., Mellon, M. T., McEwen, A. S., Lefort, A., Keszthelyi, L. P., & Thomas, N. (2008). HiRISE observations of fractured mounds: Possible Martian pingos. Geophysical Research Letters, 35(4).
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  Abstract: Early images from the High Resolution Imaging Science Experiment (HiRISE) camera have revealed small fractured mounds in the Martian mid-latitudes. HiRISE resolves fractures on the mound surfaces, indicating uplift, and shows that the mound surface material resembles that of the surrounding landscape. Analysis of Mars Orbiter Camera (MOC) images shows that in Utopia Planitia the mounds lie almost exclusively between 35-45°N. This range coincides with the peak-abundance latitudes of several landforms attributed to ground water or ice, including gullies, and suggests a ground ice-related origin. The best terrestrial analogues for the observed mound morphology are pingos, although some differences are noted. The presence of uncollapsed. pingos would indicate the presence of near-surface ground ice in the Martian mid-latitudes, at depths greater than the ∼1 meter sampled by orbital spectrometers. Pingo formation may require near-surface liquid water, which is consistent with a shallow groundwater model for the origin of gullies. Copyright 2008 by the American Geophysical Union.
• Giese, B., Denk, T., Neukum, G., Roatsch, T., Helfenstein, P., Thomas, P. C., Turtle, E. P., McEwen, A., & Porco, C. C. (2008). The topography of Iapetus' leading side. Icarus, 193(2), 359-371.
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  Abstract: We have used Cassini stereo images to study the topography of Iapetus' leading side. A terrain model derived at resolutions of 4-8 km reveals that Iapetus has substantial topography with heights in the range of -10 km to +13 km, much more than observed on the other middle-sized satellites of Saturn so far. Most of the topography is older than 4 Ga [Neukum, G., Wagner, R., Denk, T., Porco, C.C., 2005. Lunar Planet. Sci. XXXVI. Abstract 2034] which implies that Iapetus must have had a thick lithosphere early in its history to support this topography. Models of lithospheric deflection by topographic loads provide an estimate of the required elastic thickness in the range of 50-100 km. Iapetus' prominent equatorial ridge [Porco, C.C., and 34 colleagues, 2005. Science 307, 1237-1242] reaches widths of 70 km and heights of up to 13 km from their base within the modeled area. The morphology of the ridge suggests an endogenous origin rather than a formation by collisional accretion of a ring remnant [Ip, W.-H., 2006. Geophys. Res. Lett. 33, doi:10.1029/2005GL025386. L16203]. The transition from simple to complex central peak craters on Iapetus occurs at diameters of 11 ± 3   km. The central peaks have pronounced conical shapes with flanking slopes of typically 11° and heights that can rise above the surrounding plains. Crater depths seem to be systematically lower on Iapetus than on similarly sized Rhea, which if true, may be related to more pronounced crater-wall slumping (which widens the craters) on Iapetus than on Rhea. There are seven large impact basins with complex morphologies including central peak massifs and terraced walls, the largest one reaches 800 km in diameter and has rim topography of up to 10 km. Generally, no rings are observed with the basins consistent with a thick lithosphere but still thin enough to allow for viscous relaxation of the basin floors, which is inferred from crater depth-to-diameter measurements. In particular, a 400-km basin shows up-domed floor topography which is suggestive of viscous relaxation. A model of complex crater formation with a viscoplastic (Bingham) rheology [Melosh, H.J., 1989. Impact Cratering. Oxford Univ. Press, New York] of the impact-shocked icy material provides an estimate of the effective cohesion/viscosity at 0.04 ± 0.01   MPa/ 0.6 ± 0.2   GPa s. The local distribution of bright and dark material on the surface of Iapetus is largely controlled by topography and consistent with the dark material being a sublimation lag deposit originating from a bright icy substrate mixed with the dark components, but frost deposits are possible as well. © 2007 Elsevier Inc. All rights reserved.
• Grant, J. A., P., R., Grotzinger, J. P., Milliken, R. E., Tornabene, L. L., McEwen, A. S., Weitz, C. M., Squyres, S. W., Glotch, T. D., & Thomson, B. J. (2008). HiRISE imaging of impact megabreccia and sub-meter aqueous strata in Holden Crater, Mars. Geology, 36(3), 195-198.
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  Abstract: High Resolution Imaging Science Experiment (HiRISE) images of Holden crater, Mars, resolve impact megabreccia unconformably overlain by sediments deposited during two Noachian-age phases of aqueous activity. A lighter-toned lower unit exhibiting phyllosilicates was deposited in a long-lived, quiescent distal alluvial or lacustrine setting. An overlying darker-toned and often blocky upper unit drapes the sequence and was emplaced during later high-magnitude flooding as an impounded Uzboi Vallis lake overtopped the crater rim. The stratigraphy provides the first geologic context for phyllosilicate deposition during persistent wet and perhaps habitable conditions on early Mars. © 2008 The Geological Society of America.
• Jaeger, W. L., Keszthelyi, L. P., McEwen, A. S., Titus, T. N., Dundas, C. M., & Russell, P. S. (2008). Response to comment on "Athabasca Valles, Mars: A lava-draped channel system". Science, 320(5883), 1588c.
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  Abstract: The recent geologic history of Athabasca Valles, Mars, is controversial. Some studies report ice-rich sediment in its channels, whereas others find only lava. Data from the High-Resolution Imaging Science Experiment camera now confirm that, although certain features exhibit a superficial similarity to ice-related landforms, solidified lava coats the entire channel system.
• Keszthelyi, L., Jaeger, W., McEwen, A., Tornabene, L., Beyer, R. A., Dundas, C., & Milazzo, M. (2008). High resolution imaging science experiment (HiRISE) images of volcanic terrains from the first 6 months of the Mars reconnaissance orbiter primary science phase. Journal of Geophysical Research E: Planets, 113(4).
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  Abstract: In the first 6 months of the Mars Reconnaissance Orbiter's Primary Science Phase, the High Resolution Imaging Science Experiment (HiRISE) camera has returned images sampling the diversity of volcanic terrains on Mars. While many of these features were noted in earlier imaging, they are now seen with unprecedented clarity. We find that some volcanic vents produced predominantly effusive products while others generated mostly pyroclastics. Flood lavas were emplaced in both turbulent and gentle eruptions, producing roofed channels and inflation features. However, many areas on Mars are too heavily mantled to allow meter-scale volcanic features to be discerned. In particular, the major volcanic edifices are extensively mantled, though it is possible that some of the mantle is pyroclastic material rather than atmospheric dust. Support imaging by the Context Imager (CTX) and topographic information derived from stereo imaging are both invaluable in interpreting the HiRISE data. Copyright 2008 by the American Geophysical Union.
• Lewis, K. W., Aharonson, O., Grotzinger, J. P., Kirk, R. L., McEwen, A. S., & Suer, T. (2008). Quasi-periodic bedding in the sedimentary rock record of mars. Science, 322(5907), 1532-1535.
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  PMID: 19056983;Abstract: Widespread sedimentary rocks on Mars preserve evidence of surface conditions different from the modern cold and dry environment, although it is unknown how long conditions favorable to deposition persisted. We used 1-meter stereo topographic maps to demonstrate the presence of rhythmic bedding at several outcrops in the Arabia Terra region. Repeating beds are ∼10 meters thick, and one site contains hundreds of meters of strata bundled into larger units at a ∼10:1 thickness ratio. This repetition likely points to cyclicity in environmental conditions, possibly as a result of astronomical forcing. If deposition were forced by orbital variation, the rocks may have been deposited over tens of millions of years.
• Okubo, C. H., Lewis, K. W., McEwen, A. S., & Kirk, R. L. (2008). Relative age of interior layered deposits in southwest Candor Chasma based on high-resolution structural mapping. Journal of Geophysical Research E: Planets, 113(12).
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  Abstract: High-resolution topography generated from stereo HiRISE (High-Resolution Imaging Science Experiment) imagery reveals the meter-scale structure of interior layered deposits (ILD) in southwest Candor Chasma. This study seeks to determine the age of the local ILD relative to any normal faults that can be attributed to chasma formation. The study area is located near the contact of these ILD and the wall rock and is in an area where chasma-forming normal faults have been proposed. We find that while normal faults are found in the study area, these faults are not sufficiently large nor appropriately located or oriented to accommodate the roughly northeast-southwest extension that is required for normal faults that can be attributed to chasma formation. Additionally, bedding exposed in the local ILD generally dips toward the center of Candor Chasma, consistent with sediment deposition in a preexisting basin. Further, pit craters of Tithonia Catena, presumed to predate or be contemporaneous with the formation of west Candor Chasma, do not cut into the ILD within the study area. These independent lines of evidence support a postchasma age for the ILD exposed within the study area. Chasma-related normal faults may exist within these ILD at depth but are not exposed at the surface. Approximately 2 km of conformable stratigraphy is exposed in the study area, and therefore at least several kilometers of the local ILD were deposited subsequent to any chasma-related normal faulting that may have occurred in this part of Candor Chasma. Copyright 2008 by the American Geophysical Union.
• Pelletier, J. D., Kolb, K. J., McEwen, A. S., & Kirk, R. L. (2008). Recent bright gully deposits on Mars: Wet or dry flow?. Geology, 36(3), 211-214.
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  Abstract: Bright gully sediments attributed to liquid water flow have been deposited on Mars within the past several years. To test the liquid water flow hypothesis, we constructed a high-resolution (1 m/pixel) photogrammetric digital elevation model of a crater in the Centauri Montes region, where a bright gully deposit formed between 2001 and 2005. We conducted one-dimensional (1-D) and 2-D numerical flow modeling to test whether the deposit morphology is most consistent with liquid water or dry granular How. Liquid water flow models that incorporate freezing can match the runout distance of the flow for certain freezing rates but fail to reconstruct the distributary lobe morphology of the distal end of the deposit. Dry granular flow models can match both the observed runout distance and the distal morphology. Wet debris flows with high sediment concentrations are also consistent with the observed morphology because their rheologies are often similar to that of dry granular flows. As such, the presence of liquid water in this flow event cannot be ruled out, but the available evidence is consistent with dry landsliding. © 2008 The Geological Society of America.
• Russell, P., Thomas, N., Byrne, S., Herkenhoff, K., Fishbaugh, K., Bridges, N., Okubo, C., Milazzo, M., Daubar, I., Hansen, C., & McEwen, A. (2008). Seasonally active frost-dust avalanches on a north polar scarp of Mars captured by HiRISE. Geophysical Research Letters, 35(23).
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  Abstract: North-polar temporal monitoring by the High Resolution Imaging Science Experiment (HiRISE) orbiting Mars has discovered new, dramatic examples that Mars1 CO2-dominated seasonal volatile cycle is not limited to quiet deposition and sublimation of frost. In early northern martian spring, 2008, HiRISE captured several cases of CO2 frost and dust cascading down a steep, polar scarp in discrete clouds. Analysis of morphology and process reveals these events to be similar to terrestrial powder avalanches, sluffs, and falls of loose, dry snow. Potential material sources and initiating mechanisms are discussed in the context of the Martian polar spring environment and of additional, active, aeolian processes observed on the plateau above the scarp. The scarp events are identified as a trigger for mass wasting of bright, fractured layers within the basal unit, and may indirectly influence the retreat rate of steep polar scarps in competing ways. Copyright 2008 by the American Geophysical Union.
• Weitz, C. M., Milliken, R. E., Grant, J. A., McEwen, A. S., Williams, R. M., & Bishop, J. L. (2008). Light-toned strata and inverted channels adjacent to juventae and ganges chasmata, Mars. Geophysical Research Letters, 35(19).
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  Abstract: Light-toned layered deposits on the plains adjacent to Juventae and Ganges Chasmata have been identified and analyzed with several instruments onboard the Mars Reconnaissance Orbiter, including HiRISE, CTX, and CRISM. Beds exhibit variations in brightness, color, polygonal fracturing, and erosional properties that are not seen in light-toned layered deposits within the chasmata. At both Juventae and Ganges, the light-toned layered deposits along the plains have associated features we interpret as fluvial, including sinuous ridges (inverted channels) that exhibit light-toned layering and an extensive valley system for the Ganges deposit. The strong correlation between the inverted channels and light-toned layered deposits suggests an association with fluvial-lacustrine processes, although pyroclastic and eolian activity may also be involved in their emplacement. The identification of potential fluvial-lacustrine layered deposits and associated valley networks suggests surface water flow in the Valles Marineris region that occurred for sustained periods during the Hesperian (3.7-3.0 Gyr). Copyright 2008 by the American Geophysical Union.
• Bridges, N. T., Geissler, P. E., McEwen, A. S., Thomson, B. J., Chuang, F. C., Herkenhoff, K. E., Keszthelyi, L. P., & Martínez-Alonso, S. (2007). Windy Mars: A dynamic planet as seen by the HiRISE camera. Geophysical Research Letters, 34(23).
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  Abstract: With a dynamic atmosphere and a large supply of particulate material, the surface of Mars is heavily influenced by wind-driven, or aeolian, processes. The High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter (MRO) provides a new view of Martian geology, with the ability to see decimeter-size features. Current sand movement, and evidence for recent bedform development, is observed. Dunes and ripples generally exhibit complex surfaces down to the limits of resolution. Yardangs have diverse textures, with some being massive at HiRISE scale, others having horizontal and cross-cutting layers of variable character, and some exhibiting blocky and polygonal morphologies. "Reticulate" (fine polygonal texture) bedforms are ubiquitus in the thick mantle at the highest elevations. Copyright 2007 by the American Geophysical Union.
• Chuang, F. C., Beyer, R. A., McEwen, A. S., & Thomson, B. J. (2007). HiRISE observations of slope streaks on Mars. Geophysical Research Letters, 34(20).
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  Abstract: Images from the High Resolution Imaging Science Experiment have revealed new details on the morphologic and topographic characteristics of slope streaks on Mars. Over 1500 HiRISE images were analyzed with 78 unique image sites having slope streaks. Images with low sun illumination reveal that dark slope streaks have topographic relief where streaked surfaces are lower than their surroundings. Slope streaks often initiate below localized features such as rock outcrops, individual boulders, and impact craters. They are also abundant in great numbers within the blast zones of small young impact craters 10-50 m in diameter. These observations suggest that slope streaks can be triggered by localized disturbances such as rockfalls and impact blasts. Seismic activity from external (e.g., impacts) or internal forces could also trigger slope streaks. The topographic relief and triggering mechanisms of slope streaks seem to best fit models that involve dry dust avalanches. Martian slope streaks and meters-thick avalanche scars are part of a cotinuum of active mass-wasting features at meter to sub-meter scales. Copyright 2007 by the American Geophysical Union.
• Dundas, C. M., & McEwen, A. S. (2007). Rays and secondary craters of Tycho. Icarus, 186(1), 31-40.
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  Abstract: The large, fresh crater Tycho in the nearside lunar highlands has an extensive system of bright rays covering approximately 560,000 km2, containing dense clusters of secondary craters. Examination of crater densities in several clusters shows that Tycho produced almost 106 secondary craters larger than 63 m diameter. This is a lower limit, because small crater densities are reduced, most likely by mass wasting. We estimate a crater erasure rate of 2-6 cm/Myr, varying with crater size, and consistent with previous results. This process has removed many small craters, and it is probable that the original number of secondary craters formed by Tycho was higher. Also, we can only identify distant secondaries of Tycho where they occur in bright rays. Craters on Mars and Europa also formed large numbers of secondaries, but under possibly ideal conditions for spallation as a mechanism to produce high-velocity ejecta fragments. The results from Tycho show that large numbers of such fragments can be produced even from impact into a heavily fragmented target on which spallation is expected to be less important. © 2006 Elsevier Inc. All rights reserved.
• Herkenhoff, K. E., Byrne, S., Russell, P. S., Fishbaugh, K. E., & McEwen, A. S. (2007). Meter-scale morphology of the north polar region of mars. Science, 317(5845), 1711-1715.
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  PMID: 17885127;Abstract: Mars' north pole is covered by a dome of layered ice deposits. Detailed (∼30 centimeters per pixel) images of this region were obtained with the High-Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter (MRO). Planum Boreum basal unit scarps reveal cross-bedding and show evidence for recent mass wasting, flow, and debris accumulation. The north polar layers themselves are as thin as 10 centimeters but appear to be covered by a dusty veneer in places, which may obscure thinner layers. Repetition of particular layer types implies that quasi-periodic climate changes influenced the stratigraphic sequence in the polar layered deposits, informing models for recent climate variations on Mars.
• Jaeger, W. L., Keszthelyi, L. P., McEwen, A. S., Dundas, C. M., & Russell, P. S. (2007). Athabasca Valles, Mars: A lava-draped channel system. Science, 317(5845), 1709-1711.
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  PMID: 17885126;Abstract: Athabasca Valles is a young outflow channel system on Mars that may have been carved by catastrophic water floods. However, images acquired by the High-Resolution Imaging Science Experiment camera onboard the Mars Reconnaissance Orbiter spacecraft reveal that Athabasca Valles is now entirely draped by a thin layer of solidified lava - the remnant of a once-swollen river of molten rock. The lava erupted from a fissure, inundated the channels, and drained downstream in geologically recent times. Purported ice features in Athabasca Valles and its distal basin, Cerberus Palus, are actually composed of this lava. Similar volcanic processes may have operated in other ostensibly fluvial channels, which could explain in part why the landers sent to investigate sites of ancient flooding on Mars have predominantly found lava at the surface instead.
• McEwen, A. S., Eliason, E. M., Bergstrom, J. W., Bridges, N. T., Hansen, C. J., Delamere, W. A., Grant, J. A., Gulick, V. C., Herkenhoff, K. E., Keszthelyi, L., kirk, R. L., Mellon, M. T., Squyres, S. W., Thomas, N., & Weitz, C. M. (2007). Mars reconnaissance orbiter's high resolution imaging science experiment (HiRISE). Journal of Geophysical Research E: Planets, 112(5).
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  Abstract: The HiRISE camera features a 0.5 m diameter primary mirror, 12 m effective focal length, and a focal plane system that can acquire images containing up to 28 Gb (gigabits) of data in as little as 6 seconds. HiRISE will provide detailed images (0.25 to 1.3 m/pixel) covering ∼1% of the Martian surface during the 2-year Primary Science Phase (PSP) beginning November 2006. Most images will include color data covering 20% of the potential field of view. A top priority is to acquire ∼1000 stereo pairs and apply precision geometric corrections to enable topographic measurements to better than 25 cm vertical precision. We expect to return more than 12 Tb of HiRISE data during the 2-year PSP, and use pixel binning, conversion from 14 to 8 bit values, and a lossless compression system to increase coverage. HiRISE images are acquired via 14 CCD detectors, each with 2 output channels, and with multiple choices for pixel binning and number of Time Delay and Integration lines. HiRISE will support Mars exploration by locating and characterizing past, present, and future landing sites, unsuccessful landing sites, and past and potentially future rover traverses. We will investigate cratering, volcanism, tectonism, hydrology, sedimentary processes, stratigraphy, aeolian processes, mass wasting, landscape evolution, seasonal processes, climate change, spectrophotometry, glacial and periglacial processes, polar geology, and regolith properties. An Internet Web site (HiWeb) will enable anyone in the world to suggest HiRISE targets on Mars and to easily locate, view, and download HiRISE data products. Copyright 2007 by the American Geophysical Union.
• McEwen, A. S., Hansen, C. J., Delamere, W. A., Eliason, E. M., Herkenhoff, K. E., Keszthelyi, L., Gulick, V. C., Kirk, R. L., Mellon, M. T., Grant, J. A., Thomas, N., Weitz, C. M., Squyres, S. W., Bridges, N. T., Murchie, S. L., Seelos, F., Seelos, K., Okubo, C. H., Milazzo, M. P., , Tornabene, L. L., et al. (2007). A closer look at water-related geologic activity on Mars. Science, 317(5845), 1706-1709.
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  PMID: 17885125;Abstract: Water has supposedly marked the surface of Mars and produced characteristic landforms. To understand the history of water on Mars, we take a close look at key locations with the High-Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter, reaching fine spatial scales of 25 to 32 centimeters per pixel. Boulders ranging up to ∼2 meters in diameter are ubiquitous in the middle to high latitudes, which include deposits previously interpreted as fine-grained ocean sediments or dusty snow. Bright gully deposits identify six locations with very recent activity, but these lie on steep (20° to 35°) slopes where dry mass wasting could occur. Thus, we cannot confirm the reality of ancient oceans or water in active gullies but do see evidence of fluvial modification of geologically recent mid-latitude gullies and equatorial impact craters.
• Okubo, C. H., & McEwen, A. S. (2007). Fracture-controlled paleo-fluid flow in Candor Chasma, Mars. Science, 315(5814), 983-985.
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  PMID: 17303752;Abstract: Color observations from the High Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter reveal zones of localized fluid alteration (cementation and bleaching) along joints within layered deposits in western Candor Chasma, Mars. This fluid alteration occurred within the subsurface in the geologic past and has been exposed at the surface through subsequent erosion. These findings demonstrate that fluid flow along fractures was a mechanism by which subsurface fluids migrated through these layered deposits. Fractured layered deposits are thus promising sites for investigating the geologic history of water on Mars.
• Preblich, B. S., McEwen, A. S., & Studer, D. M. (2007). Mapping rays and secondary craters from the Martian crater Zunil. Journal of Geophysical Research E: Planets, 112(5).
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  Abstract: Zunil, a 10.1 km rayed crater in Elysium Planitia, Mars, produced more than 7 × 107 secondary craters ≥ 15 m in diameter. We mapped Zunil's rays from thermal IR THEMIS nighttime images up to 1700 km from the primary crater and mapped bright and dark ejecta craters (candidate secondaries) up to 3600 km range. Ray segments were mapped up to 450 km east of Zunil and up to 1700 km to the west. Both rays and bright ejecta craters are best detected over terrains with moderate thermal inertia, which are abundant west of but not east of Zunil. Nevertheless, our interpretation is that Zunil was created by a moderately oblique impact from the east. Zunil secondaries are abundant over all terrain types except the Medusae Fossae Formation (MFF). Given the likely age of Zunil (
• Squyres, S. W., Aharonson, O., Clark, B. C., Cohen, B. A., Crumpler, L., Souza, P. D., Farrand, W. H., Gellert, R., Grant, J., Grotzinger, J. P., Haldemann, A. F., Johnson, J. R., Klingelhöfer, G., Lewis, K. W., Li, R., McCoy, T., McEwen, A. S., McSween, H. Y., Ming, D. W., , Moore, J. M., et al. (2007). Pyroclastic activity at home plate in Gusev crater, Mars. Science, 316(5825), 738-742.
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  PMID: 17478719;Abstract: Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarse-grained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials.
• Thomas, P. C., Armstrong, J. W., Asmar, S. W., Burns, J. A., Denk, T., Giese, B., Helfenstein, P., Iess, L., Johnson, T. V., McEwen, A., Nicolaisen, L., Porco, C., Rappaport, N., Richardson, J., Somenzi, L., Tortora, P., Turtle, E. P., & Veverka, J. (2007). Hyperion's sponge-like appearance. Nature, 448(7149), 50-53.
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  PMID: 17611535;Abstract: Hyperion is Saturn's largest known irregularly shaped satellite and the only moon observed to undergo chaotic rotation. Previous work has identified Hyperion's surface as distinct from other small icy objects but left the causes unsettled. Here we report high-resolution images that reveal a unique sponge-like appearance at scales of a few kilometres. Mapping shows a high surface density of relatively well-preserved craters two to ten kilometres across. We have also determined Hyperion's size and mass, and calculated the mean density as 544 ± 50 kg m-3, which indicates a porosity of >40 per cent. The high porosity may enhance preservation of craters by minimizing the amount of ejecta produced or retained, and accordingly may be the crucial factor in crafting this unusual surface. ©2007 Nature Publishing Group.
• Thomas, P. C., Burns, J. A., Helfenstein, P., Squyres, S., Veverka, J., Porco, C., Turtle, E. P., McEwen, A., Denk, T., Giese, B., Roatsch, T., Johnson, T. V., & Jacobson, R. A. (2007). Shapes of the saturnian icy satellites and their significance. Icarus, 190(2), 573-584.
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  Abstract: The sizes and shapes of six icy saturnian satellites have been measured from Cassini Imaging Science Subsystem (ISS) data, employing limb coordinates and stereogrammetric control points. Mimas, Enceladus, Tethys, Dione and Rhea are well described by triaxial ellipsoids; Iapetus is best represented by an oblate spheroid. All satellites appear to have approached relaxed, equilibrium shapes at some point in their evolution, but all support at least 300 m of global-wavelength topography. The shape of Enceladus is most consistent with a homogeneous interior. If Enceladus is differentiated, its shape and apparent relaxation require either lateral inhomogeneities in an icy mantle and/or an irregularly shaped core. Iapetus supports a fossil bulge of over 30 km, and provides a benchmark for impact modification of shapes after global relaxation. Satellites such as Mimas that have smoother limbs than Iapetus, and are expected to have higher impact rates, must have relaxed after the shape of Iapetus was frozen. © 2007 Elsevier Inc. All rights reserved.
• McEwen, A. S., & Bierhaus, E. B. (2006). The importance of secondary cratering to age constraints on planetary surfaces. Annual Review of Earth and Planetary Sciences, 34, 535-567.
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  Abstract: Small craters (less than one kilometer diameter) can be primary craters produced by impact of interplanetary debris, or they can be secondary craters produced by fallback of high-velocity ejecta blocks from much larger but infrequent primary impacts. The prevalent assumption over recent decades has been that primaries are most abundant, so most small craters are independent random events and can be used for dating. However, recent results from Europa and Mars support the early theory that distant secondaries globally dominate the number of small lunar craters; this would invalidate part of production functions that have been widely used for age dating. Crater excavation results in higher mean ejection velocities for smaller fragments, resulting in a steeper size-frequency distribution for secondary craters than is produced by the same size-frequency distribution of interplanetary debris. This review also discusses how small craters can sometimes be used to derive meaningful age constraints. Copyright © 2006 by Annual Reviews. All rights reserved.
• Porco, C. C., Helfenstein, P., Thomas, P. C., Ingersoll, A. P., Wisdom, J., West, R., Neukum, G., Denk, T., Wagner, R., Roatsch, T., Kieffer, S., Turtle, E., McEwen, A., Johnson, T. V., Rathbun, J., Veverka, J., Wilson, D., Perry, J., Spitale, J., , Brahic, A., et al. (2006). Cassini observes the active south pole of enceladus. Science, 311(5766), 1393-1401.
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  PMID: 16527964;Abstract: Cassini has identified a geologically active province at the south pole of Saturn's moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at ∼55°S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturn's E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.
• Tornabene, L. L., Moersch, J. E., McSween Jr., H. Y., McEwen, A. S., Piatek, J. L., Milam, K. A., & Christensen, P. R. (2006). Identification of large (2-10 km) rayed craters on Mars in THEMIS thermal infrared images: Implications for possible Martian meteorite source regions. Journal of Geophysical Research E: Planets, 111(10).
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  Abstract: Four definitive and three probable rayed craters have been identified on Mars using 100-m resolution thermal infrared images obtained by the Mars Odyssey Thermal Emission Imaging System (THEMIS). These seven craters are similar to the previously discovered rayed crater Zunil and are best recognized by a distinct thermal contrast with respect to their surroundings. Martian rays, unlike their lunar counterparts, only exhibit minor contrasts in visible albedo. As a consequence, their presence on Mars most likely went unnoticed until substantial global coverage of THEMIS thermal infrared was achieved. Their presence has since been discerned in the coarser-resolution Thermal Emission Spectrometer (TES) data set, which preceded THEMIS. Observations in visible images of the primary cavities, secondaries, and rays suggest that, like lunar ray counterparts, Martian rays are invariably young geomorphic features. Martian rays are typically greater than hundreds of kilometers in length and consist of numerous densely clustered secondary craters, and thereby are a physical manifestation of high-velocity ejecta. Spallation accounts for a small fraction of the high-velocity ejecta that experiences low-shock compression due to interference from the rarefaction wave with the free surface. Spallation is currently the favored mechanism responsible for ejecting meteorites from Mars and is likely responsible for some of the ray-forming secondaries. Additional observations and inferences based on Martian rayed craters are compared with current Martian meteorite delivery models and the Martian meteorites themselves. The correlations presented here suggest that Martian rayed craters are the most plausible candidate source craters for the Martian meteorites to date. Copyright 2006 by the American Geophysical Union.
• Barnes, J. W., Brown, R. H., Turtle, E. P., McEwen, A. S., Lorenz, R. D., Janssen, M., Schaller, E. L., Brown, M. E., Buratti, B. J., Sotin, C., Griffith, C., Clark, R., Perry, J., Fussner, S., Barbara, J., West, R., Elachi, C., Bouchez, A. H., Roe, H. G., , Baines, K. H., et al. (2005). Planetary science: A 5-micron-bright spot on Titan: Evidence for surface diversity. Science, 310(5745), 92-95.
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  PMID: 16210535;Abstract: Observations from the Cassini Visual and Infrared Mapping Spectrometer show an anomalously bright spot on Titan located at 80°W and 20°S. This area is bright in reflected tight at all observed wavelengths, but is most noticeable at 5 microns. The spot is associated with a surface albedo feature identified in images taken by the Cassini Imaging Science Subsystem. We discuss various hypotheses about the source of the spot, reaching the conclusion that the spot is probably due to variation in surface composition, perhaps associated with recent geophysical phenomena.
• Beyer, R. A., & McEwen, A. S. (2005). Layering stratigraphy of eastern Coprates and northern Capri Chasmata, Mars. Icarus, 179(1), 1-23.
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  Abstract: Distinct competent layers are observed in the slopes of eastern Coprates Chasma, part of the Valles Marineris system on Mars. Our observations indicate that the stratigraphy of Coprates Chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers. The strong, competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes. The topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of Coprates Chasma less than 100 m below the southern Ophir Planum surface. The volume of the topmost emplaced layer is at least 70 km3 and may be greater than 2100 km3 if the unit underlies most of Ophir Planum. The broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within Coprates Chasma where the layer is also observed. Rim outcrop morphology and elevation differences between Ophir and Aurorae Plana may be indicative of the easternmost extent of the topmost competent layer. These observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the Valles Marineris is carved, and they present a picture of some of the last volcanic activity in this area. Furthermore, the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material. © 2005 Ross Beyer. Published by Elsevier Inc. All rights reserved.
• Christensen, P. R., Ruff, S. W., Fergason, R., Gorelick, N., Jakosky, B. M., Lane, M. D., McEwen, A. S., McSween, H. Y., Mehall, G. L., Milam, K., Moersch, J. E., Pelkey, S. M., Rogers, A. D., & Wyatt, M. B. (2005). Mars Exploration Rover candidate landing sites as viewed by THEMIS. Icarus, 176(1), 12-43.
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  Abstract: The analysis of six landing sites that were candidates for the two NASA Mars Exploration Rovers (MER) benefited from recently available image data from the Thermal Emission Imaging Spectrometer (THEMIS) onboard the 2001 Mars Odyssey spacecraft. The combination of daytime and nighttime thermal infrared images from THEMIS supplemented by additional data sets has lead to new or expanded insights into the nature of each landing site. In Meridiani Planum, a layer of lighter-toned, higher thermal inertia material is observable just below the hematite-bearing layer. Gusev Crater displays a more complex stratigraphy than previously observed, including an upper layer with lobate margins. The highest inertia unit of southern Isidis Planitia is confined to topographic lows in the rim/basin margin and does not appear to be due to highland material transported onto the basin floor. The enigmatic, ovoid, blocky terrain on the floor of Melas Chasma displays higher thermal inertia than its surroundings, an indication that it contains coarser or more indurated material than the adjacent aeolian bedforms. The myriad channels of Athabasca Valles display distinctive thermal signatures despite the presence of a bright layer of dust covering the region. The presence of alluvial fans produced from spur-and-gulley erosion of the walls of Eos Chasma demonstrates that mass movements have occurred following the canyon scouring floods. © 2004 Elsevier Inc. All rights reserved.
• McEwen, A. S., Preblich, B. S., Turtle, E. P., Artemieva, N. A., Golombek, M. P., Hurst, M., Kirk, R. L., Burr, D. M., & Christensen, P. R. (2005). The rayed crater Zunil and interpretations of small impact craters on Mars. Icarus, 176(2), 351-381.
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  Abstract: A 10-km diameter crater named Zunil in the Cerberus Plains of Mars created ∼107 secondary craters 10 to 200 m in diameter. Many of these secondary craters are concentrated in radial streaks that extend up to 1600 km from the primary crater, identical to lunar rays. Most of the larger Zunil secondaries are distinctive in both visible and thermal infrared imaging. MOC images of the secondary craters show sharp rims and bright ejecta and rays, but the craters are shallow and often noncircular, as expected for relatively low-velocity impacts. About 80% of the impact craters superimposed over the youngest surfaces in the Cerberus Plains, such as Athabasca Valles, have the distinctive characteristics of Zunil secondaries. We have not identified any other large (≥10 km diameter) impact crater on Mars with such distinctive rays of young secondary craters, so the age of the crater may be less than a few Ma. Zunil formed in the apparently youngest (least cratered) large-scale lava plains on Mars, and may be an excellent example of how spallation of a competent surface layer can produce high-velocity ejecta (Melosh, 1984, Impact ejection, spallation, and the origin of meteorites, Icarus 59, 234-260). It could be the source crater for some of the basaltic shergottites, consistent with their crystallization and ejection ages, composition, and the fact that Zunil produced abundant high-velocity ejecta fragments. A 3D hydrodynamic simulation of the impact event produced 1010 rock fragments ≥10 cm diameter, leading to up to 109 secondary craters ≥10 m diameter. Nearly all of the simulated secondary craters larger than 50 m are within 800 km of the impact site but the more abundant smaller (10-50 m) craters extend out to 3500 km. If Zunil is representative of large impact events on Mars, then secondaries should be more abundant than primaries at diameters a factor of ∼1000 smaller than that of the largest primary crater that contributed secondaries. As a result, most small craters on Mars could be secondaries. Depth/diameter ratios of 1300 small craters (10-500 m diameter) in Isidis Planitia and Gusev crater have a mean value of 0.08; the freshest of these craters give a ratio of 0.11, identical to that of fresh secondary craters on the Moon (Pike and Wilhelms, 1978, Secondary-impact craters on the Moon: topographic form and geologic process, Lunar Planet. Sci. IX, 907-909) and significantly less than the value of ∼0.2 or more expected for fresh primary craters of this size range. Several observations suggest that the production functions of Hartmann and Neukum (2001, Cratering chronology and the evolution of Mars, Space Sci. Rev. 96, 165-194) predict too many primary craters smaller than a few hundred meters in diameter. Fewer small, high-velocity impacts may explain why there appears to be little impact regolith over Amazonian terrains. Martian terrains dated by small craters could be older than reported in recent publications. © 2005 Elsevier Inc. All rights reserved.
• Milazzo, M. P., Keszthelyi, L. P., Radebaugh, J., Davies, A. G., Turtle, E. P., Geissler, P., Klaasen, K. P., Rathbun, J. A., & McEwen, A. S. (2005). Volcanic activity at Tvashtar Catena, Io. Icarus, 179(1), 235-251.
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  Abstract: Galileo's Solid State Imager (SSI) observed Tvashtar Catena four times between November 1999 and October 2001, providing a unique look at a distinctive high latitude volcanic complex on Io. The first observation (orbit I25, November 1999) resolved, for the first time, an active extraterrestrial fissure eruption; the brightness temperature was at least 1300 K. The second observation (orbit I27, February 2000) showed a large (∼ 500 km 2) region with many, small, hot, regions of active lava. The third observation was taken in conjunction with Cassini imaging in December 2000 and showed a Pele-like, annular plume deposit. The Cassini images revealed an ∼400 km high Pele-type plume above Tvashtar Catena. The final Galileo SSI observation of Tvashtar (orbit I32, October 2001), revealed that obvious (to SSI) activity had ceased, although data from Galileo's Near Infrared Mapping Spectrometer (NIMS) indicated that there was still significant thermal emission from the Tvashtar region. In this paper, we primarily analyze the style of eruption during orbit I27 (February 2000). Comparison with a lava flow cooling model indicates that the behavior of the Tvashtar eruption during I27 does not match that of simple advancing lava flows. Instead, it may be an active lava lake or a complex set of lava flows with episodic, overlapping eruptions. The highest reliable color temperature is ∼1300 K. Although higher temperatures cannot be ruled out, they do not need to be invoked to fit the observed data. The total power output from the active lavas in February 2000 was at least 1011 W. © 2005 Elsevier Inc. All rights reserved.
• Porco, C. C., Baker, E., Barbara, J., Beurle, K., Brahic, A., Burns, J. A., Charnoz, S., Cooper, N., Dawson, D. D., D., A., Denk, T., Dones, L., Dyudina, U., Evans, M. W., Fussner, S., Giese, B., Grazier, K., Helfenstein, P., Ingersoll, A. P., , Jacobson, R. A., et al. (2005). Imaging of Titan from the Cassini spacecraft. Nature, 434(7030), 159-168.
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  PMID: 15758990;Abstract: Titan, the largest moon of Saturn, is the only satellite in the Solar System with a substantial atmosphere. The atmosphere is poorly understood and obscures the surface, leading to intense speculation about Titan's nature. Here we present observations of Titan from the imaging science experiment onboard the Cassini spacecraft that address some of these issues. The images reveal intricate surface albedo features that suggest aeolian, tectonic and fluvial processes; they also show a few circular features that could be impact structures. These observations imply that substantial surface modification has occurred over Titan's history. We have not directly detected liquids on the surface to date. Convective clouds are found to be common near the south pole, and the motion of mid-latitude clouds consistently indicates eastward winds, from which we infer that the troposphere is rotating faster than the surface. A detached haze at an altitude of 500 km is 150-200 km higher than that observed by Voyager, and more tenuous haze layers are also resolved.
• Porco, C. C., Baker, E., Barbara, J., Beurle, K., Brahic, A., Burns, J. A., Charnoz, S., Cooper, N., Dawson, D. D., Genio, A. D., Denk, T., Dones, L., Dyudina, U., Evans, M. W., Giese, B., Grazier, K., Helfenstein, P., Ingersoll, A. P., Jacobson, R. A., , Johnson, T. V., et al. (2005). Cassini Imaging Science: Initial results on Phoebe and Iapetus. Science, 307(5713), 1237-1242.
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  PMID: 15731440;Abstract: The Cassini Imaging Science Subsystem acquired high-resolution imaging data on the outer Saturnian moon, Phoebe, during Cassini's close flyby on 11 June 2004 and on lapetus during a flyby on 31 December 2004. Phoebe has a heavily cratered and ancient surface, shows evidence of ice near the surface, has distinct layering of different materials, and has a mean density that is indicative of an ice-rock mixture, lapetus's dark leading side (Cassini Regio) is ancient, heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief. Local albedo variations within and bordering Cassini Regio suggest mass wasting of ballistically deposited material, the origin of which remains unknown.
• Porco, C. C., Baker, E., Barbara, J., Beurle, K., Brahic, A., Burns, J. A., Charnoz, S., Cooper, N., Dawson, D. D., Genio, A. D., Denk, T., Dones, L., Dyudina, U., Evans, M. W., Giese, B., Grazier, K., Helfenstein, P., Ingersoll, A. P., Jacobson, R. A., , Johnson, T. V., et al. (2005). Cassini Imaging Science: Initial results on Saturn's atmosphere. Science, 307(5713), 1243-1247.
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  PMID: 15731441;Abstract: The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004, From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of ∼375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning.
• Porco, C. C., Baker, E., Barbara, J., Beurle, K., Brahic, A., Burns, J. A., Charnoz, S., Cooper, N., Dawson, D. D., Genio, A. D., Denk, T., Dones, L., Dyudina, U., Evans, M. W., Giese, B., Grazier, K., Helfenstein, P., Ingersoll, A. P., Jacobson, R. A., , Johnson, T. V., et al. (2005). Cassini Imaging Science: Initial results on Saturn's rings and small satellites. Science, 307(5713), 1226-1236.
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  PMID: 15731439;Abstract: Images acquired of Saturn's rings and small moons by the Cassini Imaging Science Subsystem (ISS) during the first 9 months of Cassini operations at Saturn have produced many new findings. These include new saturnian moons; refined orbits of new and previously known moons; narrow diffuse rings in the F-ring region and embedded in gaps within the main rings; exceptionally fine-scale ring structure in moderate- to high-optical depth regions; new estimates for the masses of ring-region moons, as well as ring particle properties in the Cassini division, derived from the analysis of linear density waves; ring particle albedos in select ring regions; and never-before-seen phenomena within the rings.
• Porco, C. C., West, R. A., Squyres, S., McEwen, A., Thomas, P., Murray, C. D., Delgenio, A., Ingersoll, A. P., Johnson, T. V., Neukum, G., Veverka, J., Dones, L., Brahic, A., Burns, J. A., Haemmerle, V., Knowles, B., Dawson, D., Roatsch, T., Beurle, K., & Owen, W. (2005). Cassini imaging science: Instrument characteristics and anticipated scientific investigations at Saturn. Space Science Reviews, 115(1-4), 363-497.
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  Abstract: The Cassini Imaging Science Subsystem (ISS) is the highest-resolution two-dimensional imaging device on the Cassini Orbiter and has been designed for investigations of the bodies and phenomena found within the Saturnian planetary system. It consists of two framing cameras: a narrow angle, reflecting telescope with a 2-m focal length and a square field of view (FOV) 0.35° across, and a wide-angle refractor with a 0.2-m focal length and a FOV 3.5° across. At the heart of each camera is a charged coupled device (CCD) detector consisting of a 1024 square array of pixels, each 12 μ on a side. The data system allows many options for data collection, including choices for on-chip summing, rapid imaging and data compression. Each camera is outfitted with a large number of spectral filters which, taken together, span the electromagnetic spectrum from 200 to 1100 nm. These were chosen to address a multitude of Saturn-system scientific objectives: sounding the three-dimensional cloud structure and meteorology of the Saturn and Titan atmospheres, capturing lightning on both bodies, imaging the surfaces of Saturn's many icy satellites, determining the structure of its enormous ring system, searching for previously undiscovered Saturnian moons (within and exterior to the rings), peering through the hazy Titan atmosphere to its yet-unexplored surface, and in general searching for temporal variability throughout the system on a variety of time scales. The ISS is also the optical navigation instrument for the Cassini mission. We describe here the capabilities and characteristics of the Cassini ISS, determined from both ground calibration data and in-flight data taken during cruise, and the Saturn-system investigations that will be conducted with it. At the time of writing, Cassini is approaching Saturn and the images returned to Earth thus far are both breathtaking and promising. © 2004 Kluwer Academic Publishers.
• Abramov, O., & McEwen, A. (2004). An evaluation of interpolation methods for Mars Orbiter Laser Altimeter (MOLA) data. International Journal of Remote Sensing, 25(3), 669-676.
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  Abstract: The Mars Orbiter Laser Altimeter (MOLA) instrument on the Mars Global Surveyor (MGS) spacecraft has returned a large amount of data on the topography of Mars. It is possible to generate high resolution digital elevation models (DEMs) from these data by employing data interpolation techniques. Four interpolation algorithms were selected for testing on MOLA data: Delaunay-based linear interpolation, splining, nearest neighbour, and natural neighbour. These methods were applied to the MOLA data of Korolev crater for qualitative analysis. In addition, a DEM of a part of Iceland was used for quantitative testing by simulating MOLA data acquisition, interpolating those data, and then calculating the mean absolute error (MAE) between the interpolated and original DEM. Execution speeds were measured for the four algorithms. The natural neighbour method proved superior both quantitatively and qualitatively to other methods tested, but is relatively slow computationally.
• Bergstrom, J. W., Delamere, W. A., & McEwen, A. (2004). MRO high resolution imaging science experiment (HiRISE): Instrument test, calibration and operating constraints. International Astronautical Federation - 55th International Astronautical Congress 2004, 9, 6210-6216.
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  Abstract: EllisonHiRISE supports the Mars Reconnaissance Orbiter (MRO) Mission objectives through targeted imaging of nadir and off-nadir sites. Orbital images with high resolution and high signal to noise ratio will be obtained from Mars orbit. The images will have a scale of 25 to 32 cm per pixel from the nominal orbit of 250 × 320 km. HiRISE is a "push-broom" camera with a swath width at 300 km altitude of 6 km in a broad red spectral band and 1.2 km in bluegreen and near infrared bands. There are 14 CCD detector chips (2048 × 128 elements each) on the focal plane. The HiRISE camera has a half-meter primary mirror, yet through the use of lightweight glass optics and graphitecomposite structures, the mass of the instrument is only 65 kg. A uniform telescope temperature of20°C is maintained and it's estimated orbital average power consumption is less than 60 W. The large data volume of a single nominal 20,000 × 40,000-pixel image requires 13 minutes to download to the spacecraft and hours to transmit to Earth. Optical alignment and testing is complicated by gravity sag in the lightweight structure and an IFOV of only 1.0 μrad. Low-noise CCD performance was attained at a rate of 16 Mpix/s. 128 levels of time delay and integration (TDI) is used to achieve a signal-to-noise ratio of > 150:1, but requires precision timing in the electronics and a quiet spacecraft.
• Dorn, D., Meiers, W., Burkepile, J., Freymiller, E., Delamere, A., McEwen, A., Maggs, P., Pool, P., & Wallace, I. (2004). HiRISE focal plane for use on the Mars Reconnaissance Orbiter. Proceedings of SPIE - The International Society for Optical Engineering, 5167, 63-71.
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  Abstract: The primary mission of the upcoming HiRISE instrument on the Mars Reconnaissance Orbiter spacecraft is to better understand the geologic and climatic processes on Mars and to evaluate future landing sites. To accomplish this goal, a high resolution space-based camera is being developed that employs a 0.5m aperture Cassegrain-type telescope coupled to a large focal plane array (FPA) measuring approximately 14″ (L) × 2″ (W) × 2″ (D). The FPA is populated with 14 time delay and integrate (TDI) format custom charge-coupled device (CCD)-based detectors. The FPA includes panchromatic, near infrared, and blue-green spectral channels. The panchromatic channel has 20,000 pixels in the cross track direction. Each color channel consists of 4,000 pixels in the cross track direction. The minimum ground sampling distance of all channels is 50 cm per pixel. The instrument's instantaneous field of view is 1.43o × 0.1o. Over the 5-year mission, the FPA will map a portion of the surface of Mars with high spatial resolution and high signal-to-noise ratio (> 100:1 at all latitudes). Electronics are housed immediately behind the FPA, which yields a low noise, compact design that is both robust and fault tolerant. Test and characterization data from the FPA and custom CCD-based detectors is discussed along with the results from performance models.
• Geissler, P., McEwen, A., Phillips, C., Keszthelyi, L., & Spencer, J. (2004). Surface changes on Io during the Galileo mission. Icarus, 169(1), 29-64.
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  Abstract: A careful survey of Galileo SSI global monitoring images revealed more than 80 apparent surface changes that took place on Io during the 5 year period of observation, ranging from giant plume deposits to subtle changes in the color or albedo of Patera surfaces. Explosive volcanic activity was discovered at four previously unrecognized centers: an unnamed patera to the south of Karei that produced a Pele-sized red ring, a patera to the west of Zal that produced a small circular bright deposit, a large orange ring detected near the north pole of Io, and a small bright ring near Io's south pole. Only a handful of Io's many active volcanoes produced large scale explosive eruptions, and several of these erupted repeatedly, leaving at least 83% of Io's surface unaltered throughout the Galileo mission. Most of the hot spots detected from SSI, NIMS and ground-based thermal observations caused no noticeable surface changes greater than 10 km in extent over the five year period. Surface changes were found at every location where active plumes were identified, including Acala which was never seen in sunlight and was only detected through auroral emissions during eclipse. Two types of plumes are distinguished on the basis of the size and color of their deposits, confirming post-Voyager suggestions by McEwen and Soderblom [Icarus 55 (1983) 191]. Smaller plumes produce near-circular rings typically 150-200 km in radius that are white or yellow in color unless contaminated with silicates, and frequently coat their surroundings with frosts of fine-grained SO2. The larger plumes are much less numerous, limited to a half dozen examples, and produce oval, orange or red, sulfur-rich rings with maximum radii in the north-south direction that are typically in the range from 500 to 550 km. Both types of plumes can be either episodic or quasi-continuous over a five year period. Repeated eruptions of the smaller SO2-rich plumes likely contribute significantly to Io's resurfacing rate, whereas dust ejection is likely dominated by the tenuous giant plumes. Both types of plume deposits fade on time-scales of months to years through burial and alteration. Episodic seepages of SO2 at Haemus Montes, Zal Montes, Dorian Montes, and the plateau to the north of Pillan Patera may have been triggered by activity at nearby volcanic centers. © 2003 Elsevier Inc. All rights reserved.
• Geissler, P., McEwen, A., Porco, C., Strobel, D., Saur, J., Ajello, J., & West, R. (2004). Cassini observations of Io's visible aurorae. Icarus, 172(1 SPEC.ISS.), 127-140.
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  Abstract: More than 500 images of Io in eclipse were acquired by the Cassini spacecraft in late 2000 and early 2001 as it passed through the jovian system en route to Saturn (Porco et al., 2003, Science 299, 1541-1547). Io's bright equatorial glows were detected in Cassini's near-ultraviolet filters, supporting the interpretation that the visible emissions are predominantly due to molecular SO2. Detailed comparisons of laboratory SO2 spectra with the Cassini observations indicate that a mixture of gases contribute to the equatorial emissions. Potassium is suggested by new detections of the equatorial glows at near-infrared wavelengths from 730 to 800 nm. Neutral atomic oxygen and sodium are required to explain the brightness of the glows at visible wavelengths. The molecule S2 is postulated to emit most of the glow intensity in the wavelength interval from 390 to 500 nm. The locations of the visible emissions vary in response to the changing orientation of the external magnetic field, tracking the tangent points of the jovian magnetic field lines. Limb glows distinct from the equatorial emissions were observed at visible to near-infrared wavelengths from 500 to 850 nm, indicating that atomic O, Na, and K are distributed across Io's surface. Stratification of the atmosphere is demonstrated by differences in the altitudes of emissions at various wavelengths: SO2 emissions are confined to a region close to Io's surface, whereas neutral oxygen emissions are seen at altitudes that reach up to 900 km, or half the radius of the satellite. Pre-egress brightening demonstrates that light scattered into Jupiter's shadow by gases or aerosols in the giant planet's upper atmosphere contaminates images of Io taken within 13 minutes of entry into or emergence from Jupiter's umbra. Although partial atmospheric collapse is suggested by the longer timescale for post-ingress dimming than pre-egress brightening, Io's atmosphere must be substantially supported by volcanism to retain auroral emissions throughout the duration of eclipse. © 2004 Elsevier Inc. All rights reserved.
• Keszthelyi, L., Thordarson, T., McEwen, A., Haack, H., Guilbaud, M., Self, S., & Rossi, M. J. (2004). Icelandic analogs to Martian flood lavas. Geochemistry, Geophysics, Geosystems, 5(11).
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  Abstract: We report on new field observations from Icelandic lava flows that have the same surface morphology as many Martian flood lava flows. The Martian flood lavas are characterized by a platy-ridged surface morphology whose formation is not well understood. The examples on Mars include some of the most pristine lava on the planet and flows >1500 km long. The surfaces of the flows are characterized by (1) ridges tens of meters tall and wide and hundreds of meters long, (2) plates hundreds of meters to kilometers across that are bounded by ridges, (3) smooth surfaces broken into polygons several meters across and bowed up slightly in the center, (4) parallel grooves 1-10 km long cut into the flow surface by flow past obstacles, and (5) inflated pahoehoe margins. The Icelandic examples we examined (the 1783-1784 Laki Flow Field, the Búrfells Lava Flow Field by Lake Myvatn, and a lava flow from Krafla Volcano) have all these surface characteristics. When examined in detail, we find that the surfaces of the Icelandic examples are composed primarily of disrupted pahoehoe. In some cases the breccia consists of simple slabs of pahoehoe lava; in other cases it is a thick layer dominated by contorted fragments of pahoehoe lobes. Our field observations lead us to conclude that these breccias are formed by the disruption of an initial pahoehoe surface by a large flux of liquid lava within the flow. In the case of Laki, the lava flux was provided by surges in the erupted effusion rate. At Búrfells it appears that the rapid flow came from the sudden breaching of the margins of a large ponded lava flow. Using the observations from Iceland, we have improved our earlier thermal modeling of the Martian flood lavas. We now conclude that these platy-ridged lava flows may have been quite thermally efficient, allowing the flow to extend for >100 km under a disrupted crust that was carried on top of the flow. Copyright 2004 by the American Geophysical Union.
• Radebaugh, J., McEwen, A. S., Milazzo, M. P., Keszthelyi, L. P., Davies, A. G., Turtle, E. P., & Dawson, D. D. (2004). Observations and temperatures of Io's Pele Patera from Cassini and Galileo spacecraft images. Icarus, 169(1), 65-79.
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  Abstract: Pele has been the most intense high-temperature hotspot on Io to be continuously active during the Galileo monitoring from 1996-2001. A suite of characteristics suggests that Pele is an active lava lake inside a volcanic depression. In 2000-2001, Pele was observed by two spacecraft, Cassini and Galileo. The Cassini observations revealed that Pele is variable in activity over timescales of minutes, typical of active lava lakes in Hawaii and Ethiopia. These observations also revealed that the short-wavelength thermal emission from Pele decreases with rotation of Io by a factor significantly greater than the cosine of the emission angle, and that the color temperature becomes more variable and hotter at high emission angles. This behavior suggests that a significant portion of the visible thermal emission from Pele comes from lava fountains within a topographically confined lava body. High spatial resolution, nightside images from a Galileo flyby in October 2001 revealed a large, relatively cool (< 800 K) region, ringed by bright hotspots, and a central region of high thermal emission, which is hypothesized to be due to fountaining and convection in the lava lake. Images taken through different filters revealed color temperatures of 1500 ± 80 K from Cassini ISS data and 1605 ± 220 and 1420 ± 100 K from small portions of Galileo SSI data. Such temperatures are near the upper limit for basaltic compositions. Given the limitations of deriving lava eruption temperature in the absence of in situ measurement, it is possible that Pele has lavas with ultramafic compositions. The long-lived, vigorous activity of what is most likely an actively overturning lava lake in Pele Patera indicates that there is a strong connection to a large, stable magma source region. © 2003 Elsevier Inc. All rights reserved.
• Richardson, J., Lorenz, R. D., & McEwen, A. (2004). Titan's surface and rotation: New results from Voyager 1 images. Icarus, 170(1), 113-124.
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  Abstract: We present an analysis of images of Saturn's moon Titan, obtained by the Voyager 1 spacecraft on November 8-12, 1980. Orange filter (590-640 nm) images were photometrically corrected and a longitudinal average removed from them, leaving residual images with up to 5% contrast, and dominated by surface reflectivity. The resultant map shows the same regions observed at 673 nm by the Hubble Space Telescope (HST). Many of the same albedo features are present in both datasets, despite the short wavelength (600 nm) of the Voyager 1 images. A very small apparent longitudinal offset over the 14 year observation interval places tight constraints on Titan's rotation, which appears essentially synchronous at 15.9458±0.0016 days (orbital period =15.945421±0.000005 days). The detectability of the surface at such short wavelengths puts constraints on the optical depth, which may be overestimated by some fractal models. © 2004 Elsevier Inc. All rights reserved.
• Turtle, E. P., Keszthelyi, L. P., McEwen, A. S., Radebaugh, J., Milazzo, M., Simonelli, D. P., Geissler, P., Williams, D. A., Perry, J., Jaeger, W. L., Klaasen, K. P., Breneman, H. H., Denk, T., & Phillips, C. B. (2004). The final Galileo SSI observations of Io: Orbits G28-I33. Icarus, 169(1), 3-28.
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  Abstract: We present the observations of Io acquired by the Solid State Imaging (SSI) experiment during the Galileo Millennium Mission (GMM) and the strategy we used to plan the exploration of Io. Despite Galileo's tight restrictions on data volume and downlink capability and several spacecraft and camera anomalies due to the intense radiation close to Jupiter, there were many successful SSI observations during GMM. Four giant, high-latitude plumes, including the largest plume ever observed on Io, were documented over a period of eight months; only faint evidence of such plumes had been seen since the Voyager 2 encounter, despite monitoring by Galileo during the previous five years. Moreover, the source of one of the plumes was Tvashtar Catena, demonstrating that a single site can exhibit remarkably diverse eruption styles - from a curtain of lava fountains, to extensive surface flows, and finally a ∼ 400 km high plume - over a relatively short period of time (∼ 13 months between orbits 125 and G29). Despite this substantial activity, no evidence of any truly new volcanic center was seen during the six years of Galileo observations. The recent observations also revealed details of mass wasting processes acting on Io. Slumping and landsliding dominate and occur in close proximity to each other, demonstrating spatial variation in material properties over distances of several kilometers. However, despite the ubiquitous evidence for mass wasting, the rate of volcanic resurfacing seems to dominate; the floors of paterae in proximity to mountains are generally free of debris. Finally, the highest resolution observations obtained during Galileo's final encounters with Io provided further evidence for a wide diversity of surface processes at work on Io. © 2003 Elsevier Inc. All rights reserved.
• Beyer, R. A., McEwen, A. S., & Kirk, R. L. (2003). Meter-scale slopes of candidate MER landing sites from point photoclinometry. Journal of Geophysical Research E: Planets, 108(12), ROV 26-1 - ROV 26-31.
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  Abstract: Photoclinometry was used to analyze the small-scale roughness of areas that fall within the proposed Mars Exploration Rover (MER) 2003 landing ellipses. The landing ellipses presented in this study were those in Athabasca Valles, Elysium Planitia, Eos Chasma, Gusev Crater, Isidis Planitia, Melas Chasma, and Meridiani Planum. We were able to constrain surface slopes on length scales comparable to the image resolution (1.5 to 12 m/pixel). The MER 2003 mission has various engineering constraints that each candidate landing ellipse must satisfy. These constraints indicate that the statistical slope values at 5 m baselines are an important criterion. We used our technique to constrain maximum surface slopes across large swaths of each image, and built up slope statistics for the images in each landing ellipse. We are confident that all MER 2003 landing site ellipses in this study, with the exception of the Melas Chasma ellipse, are within the small-scale roughness constraints. Our results have provided input into the landing hazard assessment process. In addition to evaluating the safety of the landing sites, our mapping of small-scale roughnesses can also be used to better define and map morphologic units. The morphology of a surface is characterized by the slope distribution and magnitude of slopes. In looking at how slopes are distributed, we can better define landforms and determine the boundaries of morphologic units. Copyright 2003 by the American Geophysical Union.
• Christensen, P. R., Bandfield, J. L., F., J., Gorelick, N., Hamilton, V. E., Ivanov, A., Jakosky, B. M., Kieffer, H. H., Lane, M. D., Malin, M. C., McConnochie, T., McEwen, A. S., McSween Jr., H. Y., Mehall, G. L., Moersch, J. E., Nealson, K. H., Rice Jr., J. W., Richardson, M. I., Ruff, S. W., , Smith, M. D., et al. (2003). Morphology and composition of the surface of Mars: Mars Odyssey THEMIS results. Science, 300(5628), 2056-2061.
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  PMID: 12791998;Abstract: The Thermal Emission Imaging System (THEMIS) on Mars Odyssey has produced infrared to visible wavelength images of the martian surface that show lithologically distinct layers with variable thickness, implying temporal changes in the processes or environments during or after their formation. Kilometer-scale exposures of bedrock are observed; elsewhere airfall dust completely mantles the surface over thousands of square kilometers. Mars has compositional variations at 100-meter scales, for example, an exposure of olivine-rich basalt in the walls of Ganges Chasma. Thermally distinct ejecta facies occur around some craters with variations associated with crater age. Polar observations have identified temporal patches of water frost in the north polar cap. No thermal signatures associated with endogenic heat sources have been identified.
• Jaeger, W. L., Turtle, E. P., Keszthelyi, L. P., Radebaugh, J., McEwen, A. S., & Pappalardo, R. T. (2003). Orogenic tectonism on Io. Journal of Geophysical Research E: Planets, 108(8), 12-1.
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  Abstract: We catalog 143 Ionian mountains (montes) and mountain-like features (mensae, tholi, plana, and small peaks) in order to investigate orogenic tectonism on Io. From this comprehensive list, we select 96 mountains for which there are sufficient coverage and resolution to discern spatial relationships with surrounding geologic features. Three of the 96 mountains are probably volcanoes, 92 appear to be tectonic massifs, and 1 is ambiguous. Of the 92 tectonic mountains, 38 abut paterae (volcanic or volcano-tectonic craters with irregular or scalloped margins). This juxtaposition is unlikely to be a coincidence as the probability of it occurring by chance is ∼0.1%. We propose instead that orogenic faults may act as conduits for magma ascent, thus fueling patera formation near mountains. As resurfacing buries a shell of material from Io's surface to the base of the lithosphere, its effective radius is reduced and it heats up. We calculate the lithospheric volume change due to subsidence and thermal expansion as a function of lithospheric thickness. Conservation of volume dictates that this material must be uplifted at Io's surface. By estimating the total volume of the mountains, we are able to place a lower limit of 12 km on Io's lithospheric thickness. We hypothesize that, in some cases, mountain formation may be facilitated by asthenospheric diapirs impinging on the base of the lithosphere. The resulting lithospheric swell could focus the compressive stresses that drive orogenic tectonism. This model is one of several possible mechanisms for uplifting isolated mountains such as are observed on Io.
• Kargel, J., Carlson, R., Davies, A., Fegley Jr., B., Gillespie, A., Greeley, R., Howell, R., Jessup, K. L., Kamp, L., Keszthelyi, L., Lopes, R., Macintyre, T., Marchis, F., Mcewen, A., Milazzo, M., Perry, J., Radebaugh, J., Schaefer, L., Schmerr, N., , Smythe, W., et al. (2003). Extreme volcanism on io: Latest insights at the end of Galileo Era. Eos, 84(33), 313+318.
• Porco, C. C., West, R. A., McEwen, A., D., A., Ingersoll, A. P., Thomas, P., Squyres, S., Dones, L., Murray, C. D., Johnson, T. V., Burns, J. A., Brahic, A., Neukum, G., Veverka, J., Barbara, J. M., Denk, T., Evans, M., Ferrier, J. J., Geissler, P., , Heifenstein, P., et al. (2003). Cassini imaging of Jupiter's atmosphere, satellites, and rings. Science, 299(5612), 1541-1547.
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  PMID: 12624258;Abstract: The Cassini Imaging Science Subsystem acquired about 26,000 images of the Jupiter system as the spacecraft encountered the giant planet en route to Saturn. We report findings on Jupiter's zonal winds, convective storms, low-latitude upper troposphere, polar stratosphere, and northern aurora. We also describe previously unseen emissions arising from Io and Europa in eclipse, a giant volcanic plume over Io's north pole, disk-resolved images of the satellite Himalia, circumstantial evidence for a causal relation between the satellites Metis and Adrastea and the main jovian ring, and information on the nature of the ring particles.
• Burr, D. M., Grier, J. A., McEwen, A. S., & Keszthelyi, L. P. (2002). Repeated aqueous flooding from the cerberus fossae: Evidence for very recently extant, deep groundwater on Mars. Icarus, 159(1), 53-73.
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  Abstract: The geomorphology and topography of the Cerberus Plains region of Mars show three spatially and temporally distinct, young, aqueous flood channel systems. Flood geomorphology in each of these channels, as seen in Mars Orbiter Camera images, consists of streamlined forms, longitudinal lineations, and a single occurrence of transverse dunes, features similar to those in the flood-carved terrain of the Channeled Scabland in the northwestern United States. As additional geomorphic evidence of flooding, small cones (interpreted as phreatic) are found preferentially in the channels or at their distal ends. Glaciers, lava flows, and CO2-charged density flows are each inconsistent with these geomorphic features. Mars Orbiter Laser Altimeter data show two of the three channel systems (Athabasca Valles and an unnamed northern channel system) emanating from the Cerberus Fossae; we suggest that the third channel system (Marte Vallis) also originated at the fissures. The discharges for two of the three systems (Athabasca Valles and Marte Vallis) have been estimated from surface topography to have been on the order of 106 m3/s. Crater counts indicate that the channels are not only young (extreme Late Amazonian), but also were carved asynchronously. Geomorphic evidence suggests that two of the channels (Athabasca and Marte Valles) experienced more than one flood. Emanation from volcanotectonic fissures instead of chaotic terrain distinguishes these Cerberus Plains channels from the larger, older circum-Chryse channels. Groundwater must have collected in a liquid state prior to flood onset to flow at the estimated discharge rates. Lack of large-scale subsidence near the channels' origination points along the Cerberus Fossae indicates that this groundwater was at least several kilometers deep. © 2002 Elsevier Science (USA).
• Burr, D. M., McEwen, A. S., & E., S. (2002). Recent aqueous floods from the Cerberus Fossae, Mars. Geophysical Research Letters, 29(1), 13-1-13-4.
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  Abstract: Streamlined forms and longitudinal grooving seen in Mars Orbital Camera (MOC) images indicate recent aqueous flooding occurred downslope (south) of the southern-most Cerberus Fossae fissure. Topography from the Mars Orbital Laser Altimeter (MOLA), in conjunction with the absence of fluvial features in MOC images immediately to the north of the Fossa, substantiate the idea that floods emanated from this fissure. The floodwater flowed southward onto the western Cerberus Plains, where it probably percolated into existing lava flows. Thus, shallow ice may still be extant beneath young lava flows in this equatorial region.
• Burr, D., & McEwen, A. (2002). Recent extreme floods on Mars. IAHS-AISH Publication, 101-106.
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  Abstract: New high-resolution images of the large (>20 km wide) flood channels near the Cerberus Plains and in Marte Vallis, Mars, reveal streamlined islands, longitudinal grooving, and terracing, similar to features in the Channeled Scabland (USA). These are the youngest catastrophic flood channels on Mars, formed less than 200-500 × 106 years ago, and fluvial morphologies are preserved on scales of a few metres. Peak discharges are estimated as more than 107 m3 s-1. Our data indicate that the water originated in the region north of the Cerberus Plains, and was probably related to volcanic processes.
• McEwen, A. S. (2002). Planetary science: Active volcanism on Io. Science, 297(5590), 2220-2221.
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  PMID: 12351777;
• Williams, D. A., Radebaugh, J., Keszthelyi, L. P., McEwen, A. S., Lopes, R. M., Douté, S., & Greeley, R. (2002). Geologic mapping of the Chaac-Camaxtli region of Io from Galileo imaging data. Journal of Geophysical Research E: Planets, 107(9), 6-1.
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  Abstract: We produced a geologic/geomorphologic map of the Chaac-Camaxtli region of Io's leading anti-Jovian hemisphere using regional resolution (186 m/pixel) Galileo imaging data collected during orbit 127 (February 2000) integrated with lower resolution (1.4 km/ pixel) color data, along with other Galileo imaging and spectral data. This is the first regional map of Io made from Galileo data. Nine color and geomorphologic units have been mapped, and the close proximity of dark and various colored bright materials suggests that there is an intimate interaction between (presumably) silicate magmas and sulfur-bearing volatile materials that produced a variety of explosive and effusive deposits in the recent geologic past. This region of Io is dominated by 11 volcanic centers, most of which are paterae that are analogous in morphology to terrestrial calderas but larger in size. Mapping of structural features indicates that most of the active regions occur in topographic lows, and less active or inactive paterae are associated with topographic highs. This indicates that crustal thickness variations influence magma access to the surface. Surface changes in this region since the Voyager flybys (1979) are relatively minor (additional bright and dark flows, color changes), although several active vents have migrated within paterae. This observation, along with the identification of the relatively regular spacing of paterae (∼100-150 km) along a line, may indicate there are multiple interlacing fractures in the crust that serve as magma conduits from the interior. This connection between volcanism and tectonism may have implications for tidal heating mechanisms and their effect on Io's lithosphere. Some inactive patera floors may be evolving into bright plains material, which, if composed of silicates, might explain the strength of Io's crust to support steep patera walls and tall mountains.
• Davies, A. G., Keszthelyi, L. P., Williams, D. A., Phillips, C. B., McEwen, A. S., M., R., Smythe, W. D., Kamp, L. W., Soderblom, L. A., & Carlson, R. W. (2001). Thermal signature, eruption style, and eruption evolution at Pele and Pillan on Io. Journal of Geophysical Research E: Planets, 106(E12), 33079-33103.
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  Abstract: The Galileo spacecraft has been periodically monitoring volcanic activity on Io since June 1996, making it possible to chart the evolution of individual eruptions. We present results of coanalysis of Near-Infrared Mapping Spectrometer (NIMS) and solid-state imaging (SSI) data of eruptions at Pele and Pillan, especially from a particularly illuminating data set consisting of mutually constraining, near-simultaneous NIMS and SSI observations obtained during orbit C9 in June 1997. The observed thermal signature from each hot spot, and the way in which the thermal signature changes with time, tightly constrains the possible styles of eruption. Pele and Pillan have very different eruption styles. From September 1996 through May 1999, Pele demonstrates an almost constant total thermal output, with thermal emission spectra indicative of a long-lived, active lava lake. The NIMS Pillan data exhibit the thermal signature of a "Pillanian" eruption style, a large, vigorous eruption with associated open channel, or sheet flows, producing an extensive flow field by orbit C10 in September 1997. The high mass eruption rate, high liquidus temperature (at least 1870 K) eruption at Pillan is the best candidate so far for an active ultramafic (magnesium-rich, "komatiitic") flow on Io, a style of eruption never before witnessed. The thermal output per unit area from Pillan is, however, consistent with the emplacement of large, open-channel flows. Magma temperature at Pele is ≥1600 K. If the magma temperature is 1600 K, it suggests a komatiitic-basalt composition. The power output from Pele is indicative of a magma volumetric eruption rate of ∼250 to 340 m3 s-1. Although the Pele lava lake is considerably larger than its terrestrial counterparts, the power and mass fluxes per unit area are similar to active terrestrial lava lakes. Copyright 2001 by the American Geophysical Union.
• Geissler, P., McEwen, A., Phillips, C., Simonelli, D., Lopes, R. M., & Douté, S. (2001). Galileo imaging of SO₂ frosts on Io. Journal of Geophysical Research E: Planets, 106(E12), 33253-33266.
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  Abstract: Io's visible appearance changes dramatically with solar phase angle. The polar regions and some plume deposits near active volcanic centers become comparatively bright with increasing phase angle, while the equatorial band grows relatively dark. We suggest that the areas of Io that appear unusually bright at high phase are covered by thin frosts of SO2 that are transparent under normal illumination. A global disk-resolved photometric analysis indicates that the frosts exhibit more nearly isotropic or forwardscattering behavior and less opposition brightening than average Ionian materials. Comparison with Near-Infrared Mapping Spectrometer (NIMS) results suggests that these frosts have relatively strong 4.1 μm absorptions indicative of fine-grained SO2. Copyright 2001 by the American Geophysical Union.
• Grier, J. A., McEwen, A. S., Lucey, P. G., Milazzo, M., & Strom, R. G. (2001). Optical maturity of ejecta from large rayed lunar craters. Journal of Geophysical Research E: Planets, 106(E12), 32847-32862.
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  Abstract: Lucey et al. [2000] have developed a methodology for extracting an optical maturity parameter (OMAT) from multispectral Clementine images. The OMAT parameter characterizes the overall maturity of lunar soils and crater ejecta by changes in reflectance spectra. Using these OMAT images, we surveyed large craters (≥20 km diameter) on the Moon that had previously been mapped as possessing or possibly possessing rayed ejecta. We generated average radial profiles of OMAT values for rays of these large craters. From these profiles we classified the craters into three relative age groups: (1) older than Copernicus (inferred age of ∼810 Myr), (2) intermediate, and (3) as young or younger than Tycho (inferred age of ∼109 Myr). We suspect that there is a bias to our classification scheme, such that the OMAT profiles of smaller craters look like that of larger but older craters. Nevertheless, some large craters, such as Eudoxus (67 km) and Aristillus (55 km), are now known from this study to have optically mature ejecta and therefore are suspected to be older than Copernicus (this is consistent with an age of 1.3 Gyr suggested for Aristillus by Ryder et al. [1991]). Such craters were included by McEwen et al. [1997] when estimating the density of craters younger than or contemporaneous with Copernicus. Therefore the case for a modest increase in the cratering rate (in the past 800 Myr versus the previous 2.4 Gyr) indicated from that work has been weakened [Grier and McEwen, 2001]. Given current constraints on dating large and recent lunar craters, we cannot support (or disprove) the hypothesis that there has been a significant increase in the rate of large terrestrial impact events in the past 100-400 Myr. Copyright 2001 by the American Geophysical Union.
• Lanagan, P. D., McEwen, A. S., Keszthelyi, L. P., & Thordarson, T. (2001). Rootless cones on Mars indicating the presence of shallow equatorial ground ice in recent times. Geophysical Research Letters, 28(12), 2365-2367.
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  Abstract: High resolution Mars Orbiter Camera (MOC) images have revealed the existence of clusters of small cones in the Cerberus plains, Marte Valles, and Amazonis Planitia, Mars. These cones are similar in both morphology and planar dimensions to the larger of Icelandic rootless cones, which form due to explosive interactions between surficial lavas and near-surface groundwater. Impact crater size-frequency relationships indicate that surfaces upon which the cones sit are no older than 10 Ma. If martian cones form in the same manner as terrestrial rootless cones, then equatorial ground ice or ground water must have been present near the surface in geologically recent times.
• M., R., Kamp, L. W., Douté, S., Smythe, W. D., Carlson, R. W., McEwen, A. S., Geissler, P. E., Kieffer, S. W., Leader, F. E., Davies, A. G., Barbinis, E., Mehlman, R., Segura, M., Shirley, J., & Soderblom, L. A. (2001). Io in the near infrared: Near-Infrared Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999 and 2000. Journal of Geophysical Research E: Planets, 106(E12), 33053-33078.
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  Abstract: Galileo's Near-Infrared Mapping Spectrometer (NIMS) observed Io during the spacecraft's three flybys in October 1999, November 1999, and February 2000. The observations, which are summarized here, were used to map the detailed thermal structure of active volcanic regions and the surface distribution of SO2 and to investigate the origin of a yet unidentified compound showing an absorption feature at ∼1 μm. We present a summary of the observations and results, focusing on the distribution of thermal emission and of SO2 deposits. We find high eruption temperatures, consistent with ultramafic volcanism, at Pele. Such temperatures may be present at other hot spots, but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations. Loki is the site of frequent eruptions, and the low thermal emission may represent lavas cooling on the caldera's surface or the cooling crust of a lava lake. High-resolution spectral observations of Emakong caldera show thermal emission and SO2 within the same pixels, implying that patches of SO2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another. Thermal maps of Prometheus and Amirani show that these two hot spots are characterized by long lava flows. The thermal profiles of flows at both locations are consistent with insulated flows, with the Amirani flow field having more breakouts of fresh lava along its length. Prometheus and Amirani each show a white ring at visible wavelengths, while SO2 distribution maps show that the highest concentration of SO2 in both ring deposits lies outside the white portion. Visible measurements at high phase angles show that the white deposit around Prometheus extends into the SO2 ring. This suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction. SO2 mapping of the Chaac region shows that the interior of a caldera adjacent to Chaac has almost pure SO2. The deposit appears to be topographically controlled, suggesting a possible origin by liquid flow. Copyright 2001 by the American Geophysical Union.
• McEwen, A. S. (2001). Introduction to the special section: Geology and geophysics of Io. Journal of Geophysical Research E: Planets, 106(12), 32959-32961.
• Milazzo, M. P., Keszthelyi, L. P., & McEwen, A. S. (2001). Observations and initial modeling of lava-SO₂ interactions at Prometheus, Io. Journal of Geophysical Research E: Planets, 106(E12), 33121-33127.
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  Abstract: We present observations and initial modeling of the lava-SO2 interactions at the flow fronts in the Prometheus region of Io. Recent high-resolution observations of Prometheus reveal a compound flow field with many active flow lobes. Many of the flow lobes are associated with bright streaks of what is interpreted to be volatilized and recondensed SO2 radiating away from the hot lava. Lower-resolution color data show diffuse blue to violet areas, also near the active flow front, perhaps from active venting of SO2. Not clearly visible in any of the images is a single source vent for the active plume. While the size of the proposed vent is probably near the limit of the resolution, we expected to see radial or concentric albedo patterns or other evidence for gas and entrained particles above the flow field. The lack of an obvious plume vent, earlier suggestions that the Prometheus-type plumes may originate from the advancing flow lobes, and the high-resolution images showing evidence for large-scale volatilization of the SO2-rich substrate at Prometheus encouraged us to develop a model to quantify the heat transfer between a basaltic lava flow and a substrate of SO2 snow We calculate that the vaporization rate of SO2 snow is 2.5 x 10-6 m s-1 per unit area. Using an estimated 5 m2 s-1 lava coverage rate (from change detection images), we show that the gas production rate of SO2 at the flow fronts is enough to produce a resurfacing rate of ∼0.24 cm yr-1 at the annulus of Prometheus. This is much less than other estimates of resurfacing by the Prometheus plume. While not easily explaining the main Prometheus plume, our model readily accounts for the bright streaks. Copyright 2001 by the American Geophysical Union.
• Moore, J. M., Sullivan, R. J., Chuang, F. C., W., J., McEwen, A. S., Milazzo, M. P., Nixon, B. E., Pappalardo, R. T., Schenk, P. M., & Turtle, E. P. (2001). Landform degradation and slope processes on Io: The Galileo view. Journal of Geophysical Research E: Planets, 106(E12), 33223-33240.
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  Abstract: The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on Io. We recognize four major slope types observed on a number of intermediate resolution (∼250 m pixel-1) images and several additional textures on very high resolution (∼10 m pixel-1) images. Slopes and scarps on Io often show evidence of erosion, seen in the simplest form as alcove-carving slumps and slides at all scales. Many of the mass movement deposits on Io are probably mostly the consequence of block release and brittle slope failure. Sputtering plays no significant role. Sapping as envisioned by McCauley et al. [1979] remains viable. We speculate that alcove-lined canyons seen in one observation and lobed deposits seen along the bases of scarps in several locations may reflect the plastic deformation and "glacial" flow of interstitial volatiles (e.g., SO2) heated by locally high geothermal energy to mobilize the volatile. The appearance of some slopes and near-slope surface textures seen in very high resolution images is consistent with erosion from sublimation-degradation. However, a suitable volatile (e.g., H2S) that can sublimate fast enough to alter Io's youthful surface has not been identified. Disaggregation from chemical decomposition of solid S2O and other polysulfur oxides may conceivably operate on Io. This mechanism could degrade landforms in a manner that resembles degradation from sublimation, and at a rate that can compete with resurfacing. Copyright 2001 by the American Geophysical Union.
• Radebaugh, J., Keszthelyi, L. P., McEwen, A. S., Turtle, E. P., Jaeger, W., & Milazzo, M. (2001). Paterae on Io: A new type of volcanic caldera?. Journal of Geophysical Research E: Planets, 106(E12), 33005-33020.
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  Abstract: Paterae, defined by the International Astronomical Union as "irregular crater[s], or complex one[s] with scalloped edges," are some of the most prominent topographic features on Io. Paterae on Io are unique, yet in some aspects they resemble calderas known and studied on Earth, Mars, and Venus. They have steep walls, flat floors, and arcuate margins and sometimes exhibit nesting, all typical of terrestrial and Martian basalt shield calderas. However, they are much larger, many are irregular in shape, and they typically lack shields. Their great sizes (some >200 km diameter) and lack of associated volcanic edifices beg comparison with terrestrial ash flow calderas; however, there is no convincing evidence on Io for the high-silica erupted products or dome resurgence associated with this type of caldera. Ionian paterae seem to be linked with the eruption of large amounts of mafic to ultramafic lavas and colorful sulfur-rich materials that cover the floors and sometimes flow great distances away from patera margins. They are often angular in shape or are found adjacent to mountains or plateaus, indicating tectonic influences on their formation. A database of 417 paterae on Io measured from images with
• Schenk, P., Hargitai, H., Wilson, R., McEwen, A., & Thomas, P. (2001). The mountains of Io: Global and geological perspectives from Voyager and Galileo. Journal of Geophysical Research E: Planets, 106(E12), 33201-33222.
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  Abstract: To search for local and global scale geologic associations that may be related to the internal dynamics of Io, we have completed a global catalog of all mountains and volcanic centers. We have identified 115 mountain structures (covering ∼3% of the surface) and 541 volcanic centers, including paterae (calderas and dark spots) and shield volcanoes. The average length of an Ionian mountain is 157 km, with the longest being 570 km. The mean height of Ionian mountains is 6.3 km, and the highest known mountain is Boösaule Montes (17.5 ± 3 km). Five basic morphologic types of mountains have been identified; mesa, plateau, peak, ridge, and massif. Very few mountains bear any physical similarity to classic volcanic landforms, but many resemble flatiron mountains on Earth and are interpreted as tilted crustal blocks. This would be consistent with the hypothesis that most mountains are thrust blocks formed as a result of compressive stresses built up in the lower crust due to the global subsidence of volcanic layers as they are buried over time [Schenk and Bulmer, 1998]. More than one mechanism may be responsible for all Ionian mountains, however. The proximity of some mountains to paterae may indicate a direct link between some mountains and volcanism, although it is not always clear which came first. In contrast to earlier studies, a pronounced bimodal pattern is observed in the global distribution of both mountains and volcanic centers. The regions of highest areal densities of volcanic centers are near the sub- and anti-Jovian regions, but are offset roughly 90° in longitude from the two regions of greatest concentration of mountains. This anticorrelation may indicate the overprinting of a second stress field on the global compressive stresses due to subsidence, The bimodal distribution of volcanic centers and mountains is consistent with models of asthenospheric tidal heating and internal convection developed by Tackley et al. [2001]. Over regions of mantle upwelling, compressive stresses in the lower crust induced by global subsidence might be reduced, encouraging volcanism and discouraging mountain building. In regions of mantle downwelling, these compressive stresses in the lower crust might be increased, discouraging volcanism and encouraging mountain building. Alternatively, the global pattern may be related to possible (but undocumented) nonsynchronous rotation of Io, which would produce two regions each of compression and extension in the crust. Evidence of layering and of mass wasting, including landslides, block sliding, debris aprons and downslope creep, on Ionian mountains suggests that the crust of Io is essentially a layered stack of partially consolidated volcanic lavas and plume deposits, becoming more consolidated with depth. The lower crust especially may also be ductily deformed, punctuated by volcanic intrusions and faulting at paterae, and broken into blocks, some of which have been uplifted to form mountains. Copyright 2001 by the American Geophysical Union.
• James, P. B., Cantor, B. A., Malin, M. C., Edgett, K., Carr, M. H., Danielson, G. E., Ingersoll, A. P., Davies, M. E., Hartmann, W. K., McEwen, A. S., Soderblom, L. A., Thomas, P. C., & Veverka, J. (2000). The 1997 Spring Regression of the Martian South Polar Cap: Mars Orbiter Camera Observations. Icarus, 144(2), 410-418.
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  Abstract: The Mars Orbiter cameras (MOC) on Mars Global Surveyor observed the south polar cap of Mars during its spring recession in 1997. The images acquired by the wide angle cameras reveal a pattern of recession that is qualitatively similar to that observed by Viking in 1977 but that does differ in at least two respects. The 1977 recession in the 0° to 120° longitude sector was accelerated relative to the 1997 observations after LS = 240°; the Mountains of Mitchel also detached from the main cap earlier in 1997. Comparison of the MOC images with Mars Orbiter Laser Altimeter data shows that the Mountains of Mitchel feature is controlled by local topography. Relatively dark, low albedo regions well within the boundaries of the seasonal cap were observed to have red-to-violet ratios that characterize them as frost units rather than unfrosted or partially frosted ground; this suggests the possibility of regions covered by CO2 frost having different grain sizes. © 2000 Academic Press.
• Keszthelyi, L., McEwen, A. S., & Thordarson, T. (2000). Terrestrial analogs and thermal models for Martian flood lavas. Journal of Geophysical Research E: Planets, 105(E6), 15027-15049.
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  Abstract: The recent flood lavas on Mars appear to have a characteristic "platy-ridged" surface morphology different from that inferred for most terrestrial continental flood basalt flows. The closest analog we have found is a portion of the 1783-1784 Laki lava flow in Iceland that has a surface that was broken up and transported on top of moving lava during major surges in the eruption rate. We suggest that a similar process formed the Martian flood lava surfaces and attempt to place constraints on the eruption parameters using thermal modeling. Our conclusions from this modeling are (1) in order to produce flows >1000 km long with flow thicknesses of a few tens of meters, the thermophysical properties of the lava should be similar to fluid basalt, and (2) the average eruption rates were probably of the order of 104 m3/s, with the flood-like surges having flow rates of the order of 105 - 106 m3/s. We also suggest that these high eruption rates should have formed huge volumes of pyroclastic deposits which may be preserved in the Medusae Fossae Formation, the radar "stealth" region, or even the polar layered terrains. Copyright 2000 by the American Geophysical Union.
• Kieffer, S. W., Lopes-Gautier, R., McEwen, A., Smythe, W., Keszthelyi, L., & Carlson, R. (2000). Prometheus: Io's wandering plume. Science, 288(5469), 1204-1208.
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  PMID: 10817989;Abstract: Unlike any volcanic behavior ever observed on Earth, the plume from Prometheus on Io has wandered 75 to 95 kilometers west over the last 20 years since it was first discovered by Voyager and more recently observed by Galileo. Despite the source motion, the geometric and optical properties of the plume have remained constant. We propose that this can be explained by vaporization of a sulfur dioxide and/or sulfur 'snowfield' over which a lava flow is moving. Eruption of a boundary-layer slurry through a rootless conduit with sonic conditions at the intake of the melted snow can account for the constancy of plume properties.
• Lopes-Gautier, R., Douté, S., Smythe, W. D., Kamp, L. W., Carlson, R. W., Davies, A. G., Leader, F. E., McEwen, A. S., Geissler, P. E., Kieffer, S. W., Keszthelyi, L., Barbinis, E., Mehlman, R., Segura, M., Shirley, J., & Soderblom, L. A. (2000). A close-up look at Io from Galileo's near-infrared mapping spectrometer. Science, 288(5469), 1201-1204.
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  PMID: 10817988;Abstract: Infrared spectral images of Jupiter's volcanic moon Io, acquired during the October and November 1999 and February 2000 flybys of the Galileo spacecraft, were used to study the thermal structure and sulfur dioxide distribution of active volcanoes. Loki Patera, the solar system's most powerful known volcano, exhibits large expanses of dark, cooling lava on its caldera floor. Prometheus, the site of long-lived plume activity, has two major areas of thermal emission, which support ideas of plume migration. Sulfur dioxide deposits were mapped at local scales and show a more complex relationship to surface colors than previously thought, indicating the presence of other sulfur compounds.
• McEwen, A. S., Belton, M. J., Breneman, H. H., Fagents, S. A., Geissier, P., Greeley, R., Head, J. W., Hoppa, G., Jaeger, W. L., Johnson, T. V., Keszthelyi, L., Klaasen, K. P., Lopes-Gautier, R., Magee, K. P., Milazzo, M. P., Moore, J. M., Pappalardo, R. T., Phillips, C. B., Radebaugh, J., , Schubert, G., et al. (2000). Galileo at Io: Results from high-resolution imaging. Science, 288(5469), 1193-1198.
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  PMID: 10817986;Abstract: During late 1999/early 2000, the solid state imaging experiment on the Galileo spacecraft returned more than 100 high-resolution (5 to 500 meters per pixel) images of volcanically active Io. We observed an active lava lake, an active curtain of lava, active flows, calderas, mountains, plateaus, and plains. Several of the sulfur dioxide-rich plumes are erupting from distal flows, rather than from the source of silicate lava (caldera or fissure, often with red pyroclastic deposits). Most of the active flows in equatorial regions are being emplaced slowly beneath insulated crust, but rapidly emplaced channelized flows are also found at all latitudes. There is no evidence for high-viscosity lava, but some bright flows may consist of sulfur rather than mafic silicates. The mountains, plateaus, and calderas are strongly influenced by tectonics and gravitational collapse. Sapping channels and scarps suggests that many portions of the upper ~1 kilometer are in volatiles.
• Phillips, C. B., McEwen, A. S., Hoppa, G. V., Fagents, S. A., Greeley, R., Klemaszewski, J. E., Pappalardo, R. T., Klaasen, K. P., & Breneman, H. H. (2000). The search for current geologic activity on Europa. Journal of Geophysical Research E: Planets, 105(E9), 22579-22597.
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  Abstract: Observational evidence and theoretical arguments suggest that Jupiter's satellite Europa could be geologically active and possess an "ocean" of liquid water beneath its surface at the present time. We have searched for evidence of current geologic activity on Europa in the form of active plumes venting material above the surface and by comparison of Voyager and Galileo images to look for any changes on the surface. So far, we have observed no plumes and have detected no definitive changes. The lack of observed activity allows us to estimate a maximum steady state surface alteration rate of 1 km2 y-1 in the regions analyzed, assuming alterations will cover contiguous areas of at least 4 km2 over a period of 20 years. Assuming this as a constant, globally uniform resurfacing rate leads to a minimum average surface age of 30 million years. We also suggest that the lack of obvious circular albedo patterns on the surface due to plumes, coupled with the presence of bright-rayed craters such as Pwyll and the predicted sputtering erosion rate, implies that no large-scale plume activity has taken place over at least the last few thousand years. We thus conclude that if Europa's surface is currently active, any changes must be relatively small in spatial scale or episodic in nature rather than continuous. To detect potential small-scale surface changes, we need high-resolution comparisons between the Galileo data and future Europa Orbiter images. Copyright 2000 by the American Geophysical Union.
• Spencer, J. R., Rathbun, J. A., Travis, L. D., Tamppari, L. K., Barnard, L., Martin, T. Z., & McEwen, A. S. (2000). Io's thermal emission from the Galileo photopolarimeter-radiometer. Science, 288(5469), 1198-1201.
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  PMID: 10817987;Abstract: Galileo's photopolarimeter-radiometer instrument mapped Io's thermal emission during the 124, 125, and 127 flybys with a spatial resolution of 2.2 to 300 kilometers. Mapping of Loki in 124 shows uniform temperatures for most of Loki Patera and high temperatures in the southwest corner, probably resulting from an eruption that began 1 month before the observation. Most of Loki Patera was resurfaced before 127. Pele's caldera floor has a low temperature of 160 kelvin, whereas flows at Pillan and Zamama have temperatures of up to 200 kelvin. Global maps of nighttime temperatures provide a means for estimating global heat flow.
• Geissler, P. E., McEwen, A. S., Ip, W., Belton, M. J., Johnson, T. V., Smyth, W. H., & Ingersoll, A. P. (1999). Galileo imaging of atmospheric emissions from Io. Science, 285(5429), 870-874.
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  PMID: 10436151;Abstract: The Galileo spacecraft has detected diffuse optical emissions from Io in high-resolution images acquired while the sateLLite was eclipsed by Jupiter. Three distinct components make up Io's visible emissions. Bright blue glows of more than 300 kilorayleighs emanate from volcanic plumes, probably due to electron impact on molecular sulfur dioxide. Weaker red emissions, possibly due to atomic oxygen, are seen along the limbs, brighter on the pole closest to the plasma torus. A faint green glow appears concentrated on the night side of Io, possibly produced by atomic sodium. Io's disk-averaged emission diminishes with time after entering eclipse, whereas the localized blue glows brighten instead.
• Geissler, P. E., McEwen, A. S., Keszthelyi, L., Lopes-Gautier, R., Granahan, J., & Simonelli, D. P. (1999). Global Color Variations on Io. Icarus, 140(2), 265-282.
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  Abstract: Visible and near-infrared images of Io from the Galileo spacecraft reveal a surface more colorful than previously thought. Red, yellow, green, white, and black hues decorate the satellite, presumably caused by a varied composition of sulfur compounds and silicates. Almost a third of Io is covered by red and orange materials, particularly at polar latitudes above ±30°. These red regions were scarcely distinguishable in the shorter wavelength Voyager observations. Bright red pyroclastic deposits mark the locations of many hot spots, plumes, and visible surface changes, providing a prominent flag of recent volcanic activity. Io's equatorial regions are dominated by yellow materials, which occupy about 40% of the satellite's surface. White and gray materials cover about 27% of Io, primarily in equatorial areas and in localized deposits at high latitudes. These are identified with moderate-to-coarse-grained SO2 as mapped by the NIMS instrument on Galileo (R. W. Carlson et al. 1997, Geophys. Res. Lett.24, 2479-2482). Greenish-yellow materials in small isolated spots on Io's anti-Jupiter hemisphere were recently discovered in 3 km/pixel color imaging from orbit 14. Unlike other ionian terrains, these regions have a negative near-infrared spectral slope, suggesting contamination by a nonsulfur component. Only about 1.4% of Io's surface is occupied by dark materials, which display a variety of visible colors ranging from black to red and green. Most dark spots have a shallow spectral absorption feature at 0.9 μm, suggesting magnesium-rich silicates rather than black sulfur. Little large-scale alteration in the global color and albedo pattern has occurred between the Voyager and Galileo eras; 90% of the surface appears unchanged despite the vigorous volcanic activity which must have taken place in the intervening 17 years. This suggests that over a time scale of decades, the bulk of Io's resurfacing is restricted to a few small but persistently active areas. © 1999 Academic Press.
• Hartmann, W. K., Malin, M., McEwen, A., Carr, M., Soderblom, L., Thomas, P., Danielson, E., James, P., & Veverka, J. (1999). Evidence for recent volcanism on mars from crater counts. Nature, 397(6720), 586-589.
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  Abstract: Impact craters help characterize the age of a planetary surface, because they accumulate with time. They also provide useful constraints on the importance of surface erosion, as such processes will preferentially remove the smaller craters. Earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on Mars. They disagreed, however, on the age of the youngest volcanism. These earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter. Here we report an analysis, using new images obtained by the Mars Global Surveyor spacecraft, of crater populations that extend the size distribution down to about 16 m. Our results indicate a wide range of surface ages, with one region-lava flows within the Arsia Mons calderathat we estimate to be no older than 40-100 million years. We suggest that volcanism is a continuing process on Mars.
• Klaasen, K. P., Breneman, H. H., Cunningham, W. F., Kaufman, J. M., Klemaszewski, J. E., Magee, K. P., McEwen, A. S., Mortensen, H. B., Pappalardo, R. T., Senske, D. A., Sullivan, R. J., & Vasavada, A. R. (1999). Calibration and performance of the Galileo solid-state imaging system in Jupiter orbit. Optical Engineering, 38(7), 1178-1199.
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  Abstract: The solid-state imaging subsystem (SSI) on the National Aeronautics and Space Administration's (NASA's) Galileo Jupiter orbiter spacecraft has successfully completed its 2-yr primary mission exploring the Jovian system. The SSI has remained in remarkably stable calibration during the 8-yr flight, and the quality of the returned images is exceptional. Absolute spectral radiometric calibration has been determined to 4 to 6% across its eight spectral filters. Software and calibration files are available to enable radiometric, geometric, modulation transfer function (MTF), and scattered light image calibration. The charge-coupled device (CCD) detector endured the harsh radiation environment at Jupiter without significant damage and exhibited transient image noise effects at about the expected levels. A lossy integer cosine transform (ICT) data compressor proved essential to achieving the SSI science objectives given the low data transmission rate available from Jupiter due to a communication antenna failure. The ICT compressor does introduce certain artifacts in the images that must be controlled to acceptable levels by judicious choice of compression control parameter settings. The SSI team's expertise in using the compressor improved throughout the orbital operations phase and, coupled with a strategy using multiple playback passes of the spacecraft tape recorder, resulted in the successful return of 1645 unique images of Jupiter and its satellites.
• Lopes-Gautier, R., McEwen, A. S., Smythe, W. B., Geissler, P. E., Kamp, L., Davies, A. G., Spencer, J. R., Keszthelyi, L., Carlson, R., Leader, F. E., Mehlman, R., & Soderblom, L. (1999). Active Volcanism on Io: Global Distribution and Variations in Activity. Icarus, 140(2), 243-264.
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  Abstract: Io's volcanic activity has been monitored by instruments aboard the Galileo spacecraft since June 28, 1996. We present results from observations by the near-infrared mapping spectrometer (NIMS) for the first 10 orbits of Galileo, correlate them with results from the Solid State Imaging System (SSI) and from groundbased observations, and compare them to what was known about Io's volcanic activity from observations made during the two Voyager flybys in 1979. A total of 61 active volcanic centers have been identified from Voyager, groundbased, and Galileo observations. Of these, 41 are hot spots detected by NIMS and/or SSI. Another 25 locations were identified as possible active volcanic centers, mostly on the basis of observed surface changes. Hot spots are correlated with surface colors, particularly dark and red deposits, and generally anti-correlated with white, SO2-rich areas. Surface features corresponding to the hot spots, mostly calderas or flows, were identified from Galileo and Voyager images. Hot spot temperatures obtained from both NIMS and SSI are consistent with silicate volcanism, which appears to be widespread on Io. Two types of hot spot activity are present: persistent-type activity, lasting from months to years, and sporadic events, which may represent either short-lived activity or low-level activity that occasionally flares up. Sporadic events are not often detected, but may make an important contribution to Io's heat flow and resurfacing. The distribution of active volcanic centers on the surface does not show any clear correlation with latitude, longitude, Voyager-derived global topography, or heat flow patterns predicted by the asthenosphere and deep mantle tidal dissipation models. However, persistent hot spots and active plumes are concentrated toward lower latitudes, and this distribution favors the asthenosphere rather than the deep mantle tidal dissipation model. © 1999 Academic Press.
• McEwen, A. S., Malin, M. C., Carr, M. H., & Hartmann, W. K. (1999). Voluminous volcanism on early mars revealed in valles marineris. Nature, 397(6720), 584-586.
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  Abstract: The relative rates and importance of impact cratering, volcanism, erosion, and the deposition of sediments to the early geological history of Mars are poorly known. That history is recorded in the upper crust of the planet, which is best exposed along the 4,000km-long canyon system called Valles Marineris. Previous studies of the stratigraphy of this region have assumed that it consists of megabreccia and fractured bedrock resulting from impacts, overlain by or interbedded with relatively thin layers of lava, and with the layering restricted to the uppermost level of the crust. Here we report new high-resolution images that reveal ubiquitous horizontal layering to depths of at least 8 km in the canyons. Megabreccia should be only coarsely layered and fractured bedrock should be unlayered, so these observations indicate that volcanic or sedimentary processes were much more important in early martian history than previously believed. Morphological and compositional data suggest that the layers were formed mainly by volcanic flood lavas. Mars was therefore probably very volcanically active during at least the first billion years and after the period when the heaviest impact bombardment had ended.
• Thomas, P. C., Malin, M. C., Carr, M. H., Danielson, G. E., Davies, M. E., Hartmann, W. K., Ingersoll, A. P., James, P. B., McEwen, A. S., Soderblom, L. A., & Veverka, J. (1999). Bright dunes on mars. Nature, 397(6720), 592-594.
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  Abstract: Seasonal changes observed on the surface of Mars can in part be attributed to the transport of geological materials by wind. Images obtained by orbiting spacecraft in the 1970s showed large wind-formed features such as dunes, and revealed regional time-varying albedos that could be attributed to the effects of dust erosion and deposition. But the resolution of these images was insufficient to identify different types and sources of aeolian materials, nor could they reveal aeolian deposits other than large dunes or extensive surface coverings that were redistributed by dust storms. Here we present images of Mars with up to 50 times better resolution. These images show that martian dunes include at least two distinct components, the brighter of which we interpret to be composed of relatively soft minerals, possibly sulphates. We also find large areas of the martian surface that have several metres or more of aeolian mantle lacking obvious bedforms.
• Carr, M. H., McEwen, A. S., Howard, K. A., Chuang, F. C., Thomas, P., Schuster, P., Oberst, J., Neukum, G., & Schubert, G. (1998). Mountains and Calderas on Io: Possible Implications for Lithosphere Structure and Magma Generation. Icarus, 135(1), 146-165.
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  Abstract: The combination of Voyager images and newly acquired Galileo images with low illumination and resolutions ranging from 2 to 6 km/pixel now allows determination of the global distribution of mountains and volcanic centers on Io. The mountains generally do not have characteristics typical of terrestrial volcanic landforms, they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes. Relative elevations, determined by shadow measurements and stereoscopy, indicate that mountains in the newly imaged area range in elevation up to at least 7.6 km. The origin of the mountains remains uncertain. Some appear to be multitiered volcanic constructs; others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes; yet others show extensive tectonic disruption. Volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes. The low latitudes have one volcanic center per 7 × 104km2, and, on average, the centers are spaced roughly 250 km apart. The global distribution of high mountains suggests that the lithosphere over most of Io is thick. Although the thickness cannot be calculated, the previously suggested 30 km appears reasonable as a lower limit. The high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere. The slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on Io, coupled with the high rate of magma production, will likely result in larger melt fractions than is typical for source regions on Earth. The variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface. This mixture will be carried downward through the lithosphere as a consequence of the 0.5-1.5 cm/yr resurfacing rates. During descent, the more volatile components will tend to be driven off early, but complete or near-complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains. © 1998 Academic Press.
• Clark, B. E., Helfenstein, P., Veverka, J., Ockert-Bell, M., Sullivan, R. J., Geissler, P. E., Phillips, C. B., McEwen, A. S., Greeley, R., Neukum, G., Denk, T., & Klaasen, K. (1998). Multispectral Terrain Analysis of Europa from Galileo Images. Icarus, 135(1), 95-106.
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  Abstract: Galileo's Solid State Imaging camera recorded six images at wavelengths from 0.41 to 0.99 μm of Europa's trailing hemisphere (~1.6 km/pixel resolution) during the G1 orbit (1st orbit - target Ganymede) of the nominal mission. We have photometrically corrected these data and extracted spectra representing Europa's diverse geologic terrains. The goals of the analysis of these spectra are (1) to determine whether Europa's geologic units differ spectrally from one another, (2) to determine the number of color components necessary to explain Europa's dark material spectral behavior, and (3) to examine how europan dark materials may change in color with time. Our data indicate that europan dark spots, lineaments and triple band side materials represent a single "dark" and reddish endmember component. We see no evidence for more than one dark endmember; however the dark endmember seen in these G1 data is probably not a "pure" exposure of europan dark material. Mottled terrains, brighter lineaments, and aging triple bands can be modeled by a mixture of bright plains materials and the dark component. Exposures of europan dark materials may thus be brightening with time, eventually blending into the surrounding bright plains. © 1998 Academic Press.
• Davies, M. E., Colvin, T. R., Oberst, J., Zeitler, W., Schuster, P., Neukum, G., McEwen, A. S., Phillips, C. B., Thomas, P. C., Veverka, J., Belton, M. J., & Schubert, G. (1998). The Control Networks of the Galilean Satellites and Implications for Global Shape. Icarus, 135(1), 372-376.
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  Abstract: Images from the Galileo mission to Jupiter have been combined with earlier Voyager images to produce improved geodetic control networks for the Galilean satellites. The improved geometric stability and lack of scene-dependent distortions of the Galileo images compensate for their relatively small number. Triaxial solutions are derived for the principal axes of the satellites by assuming an equilibrium constrained by the gravity potential coefficients. The control network is used in the preparation of planimetric maps by relating the images to the latitude/longitude grid. © 1998 Academic Press.
• Malin, M. C., Carr, M. H., Danielson, G. E., Davies, M. E., Hartmann, W. K., Ingersoll, A. P., James, P. B., Masursky, H., McEwen, A. S., Soderblom, L. A., Thomas, P., Veverka, J., Caplinger, M. A., Ravine, M. A., Soulanille, T. A., & Warren, J. L. (1998). Early views of the Martian surface from the Mars Orbiter Camera of Mars Global Surveyor. Science, 279(5357), 1681-1685.
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  PMID: 9497280;Abstract: High-resolution images of the martian surface at scales of a few meters show ubiquitous erosional and depositional eolian landforms. Dunes, sandsheets, and drifts are prevalent and exhibit a range of morphology, composition (inferred from albedo), and age (as seen in occurrences of different dune orientations at the same location). Steep walls of topographic depressions such as canyons, valleys, and impact craters show the martian crust to be stratified at scales of a few tens of meters. The south polar layered terrain and superposed permanent ice cap display diverse surface textures that may reflect the complex interplay of volatile and non-volatile components. Low resolution regional views of the planet provide synoptic observations of polar cap retreat, condensate clouds, and the lifecycle of local and regional dust storms.
• McEwen, A. S., Keszthelyi, L., Geissler, P., Simonelli, D. P., Carr, M. H., Johnson, T. V., Klaasen, K. P., Breneman, H. H., Jones, T. J., Kaufman, J. M., Magee, K. P., Senske, D. A., J., M., & Schubert, G. (1998). Active Volcanism on Io as Seen by Galileo SSI. Icarus, 135(1), 181-219.
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  Abstract: Active volcanism on Io has been monitored during the nominal Galileo satellite tour from mid 1996 through late 1997. The Solid State Imaging (SSI) experiment was able to observe many manifestations of this active volcanism, including (1) changes in the color and albedo of the surface, (2) active airborne plumes, and (3) glowing vents seen in eclipse. About 30 large-scale (tens of kilometers) surface changes are obvious from comparison of the SSI images to those acquired by Voyager in 1979. These include new pyroclastic deposits of several colors, bright and dark flows, and caldera-floor materials. There have also been significant surface changes on Io during the Galileo mission itself, such as a new 400-km-diameter dark pyroclastic deposit around Pillan Patera. While these surface changes are impressive, the number of large-scale changes observed in the four months between the Voyager 1 and Voyager 2 flybys in 1979 suggested that over 17 years the cumulative changes would have been much more impressive. There are two reasons why this was not actually the case. First, it appears that the most widespread plume deposits are ephemeral and seem to disappear within a few years. Second, it appears that a large fraction of the volcanic activity is confined to repeated resurfacing of dark calderas and flow fields that cover only a few percent of Io's surface. The plume monitoring has revealed 10 active plumes, comparable to the 9 plumes observed by Voyager. One of these plumes was visible only in the first orbit and three became active in the later orbits. Only the Prometheus plume has been consistently active and easy to detect. Observations of the Pele plume have been particularly intriguing since it was detected only once by SSI, despite repeated attempts, but has been detected several times by the Hubble Space Telescope at 255 nm. Pele's plume is much taller (460 km) than during Voyager 1 (300 km) and much fainter at visible wavelengths. Prometheus-type plumes (50-150 km high, long-lived, associated with high-temperature hot spots) may result from silicate lava flows or shallow intrusions interacting with near-surface SO2. A major and surprising result is that ~30 of Io's volcanic vents glow in the dark at the short wavelengths of SSI. These are probably due to thermal emission from surfaces hotter than 700 K (with most hotter than 1000 K), well above the temperature of pure sulfur volcanism. Active silicate volcanism appears ubiquitous. There are also widespread diffuse glows seen in eclipse, related to the interaction of energetic particles with the atmosphere. These diffuse glows are closely associated with the most active volcanic vents, supporting suggestions that Io's atmopshere is dominated by volcanic outgassing. Globally, volcanic centers are rather evenly distributed. However, 14 of the 15 active plumes seen by Voyager and/or Galileo are within 30° of the equator, and there are concentrations of glows seen in eclipse at both the sub- and antijovian points. These patterns might be related to asthenospheric tidal heating or tidal stresses. Io will continue to be observed during the Galileo Europa Mission, which will climax with two close flybys of Io in late 1999. © 1998 Academic Press.
• McEwen, A. S., Keszthelyi, L., Spencer, J. R., Schubert, G., Matson, D. L., Lopes-Gautier, R., Klaasen, K. P., Johnson, T. V., Head, J. W., Geissler, P., Fagents, S., Davies, A. G., Carr, M. H., Breneman, H. H., & Belton, M. J. (1998). High-temperature silicate volcanism on Jupiter's moon Io. Science, 281(5373), 87-90.
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  PMID: 9651251;Abstract: Infrared wavelength observations of Io by the Galileo spacecraft show that at last 12 different vents are erupting lavas that are probably hotter than the highest temperature basaltic eruptions on Earth today. In at least one case, the eruption near Pillan Patea, two independent instruments on Galileo show that the lava temperature must have exceeded 1700 kelvin and may have reached 2000 kelvin. The most likely explanation is that these lavas are ultramafic (magnesium-rich) silicates, and this idea is supported by the tentative identification of magnesium-rich orthopyroxene in lava flows associated with thse high-temperature hot spots.
• Moore, J. M., Asphaug, E., Sullivan, R. J., Klemaszewski, J. E., Bender, K. C., Greeley, R., Geissler, P. E., McEwen, A. S., Turtle, E. P., Phillips, C. B., Tufts, B. R., W., J., Pappalardo, R. T., Jones, K. B., Chapman, C. R., J., M., Kirk, R. L., & Morrison, D. (1998). Large Impact Features on Europa: Results of the Galileo Nominal Mission. Icarus, 135(1), 127-145.
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  Abstract: The Galileo Orbiter examined several impact features on Europa at considerably better resolution than was possible from Voyager. The new data allow us to describe the morphology and infer the geology of the largest impact features on Europa, which are probes into the crust. We observe two basic types of large impact features: (1) "classic" impact craters that grossly resemble well-preserved lunar craters of similar size but are more topographically subdued (e.g., Pwyll) and (2) very flat circular features that lack the basic topographic structures of impact craters such as raised rims, a central depression, or central peaks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Callanish). Our interpretation is that the classic craters (all
• Pappalardo, R. T., Head, J. W., Collins, G. C., Kirk, R. L., Neukum, G., Oberst, J., Giese, B., Greeley, R., Chapman, C. R., Helfenstein, P., Moore, J. M., McEwen, A., Tufts, B. R., Senske, D. A., Breneman, H. H., & Klaasen, K. (1998). Grooved Terrain on Ganymede: First Results from Galileo High-Resolution Imaging. Icarus, 135(1), 276-302.
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  Abstract: High-resolution Galileo imaging has provided important insight into the origin and evolution of grooved terrain on Ganymede. The Uruk Sulcus target site was the first imaged at high resolution, and considerations of resolution, viewing geometry, low image compression, and complementary stereo imaging make this region extremely informative. Contrast variations in these low-incidence angle images are extreme and give the visual impression of topographic shading. However, photometric analysis shows that the scene must owe its character to albedo variations. A close correlation of albedo variations to topography is demonstrated by limited stereo coverage, allowing extrapolation of the observed brightness and topographic relationships to the rest of the imaged area. Distinct geological units are apparent across the region, and ridges and grooves are ubiquitous within these units. The stratigraphically lowest and most heavily cratered units ("lineated grooved terrain") generally show morphologies indicative of horst-and-graben-style normal faulting. The stratigraphically highest groove lanes ("parallel ridged terrain") exhibit ridges of roughly triangular cross section, suggesting that tilt-block-style normal faulting has shaped them. These extensional-tectonic models are supported by crosscutting relationships at the margins of groove lanes. Thus, a change in tectonic style with time is suggested in the Uruk Sulcus region, varying from horst and graben faulting for the oldest grooved terrain units to tilt block normal faulting for the latest units. The morphologies and geometries of some stratigraphically high units indicate that a strike-slip component of deformation has played an important role in shaping this region of grooved terrain. The most recent tectonic episode is interpreted as right-lateral transtension, with its tectonic pattern of two contemporaneous structural orientations superimposed on older units of grooved terrain. There is little direct evidence for cryovolcanic resurfacing in the Uruk Sulcus region; instead tectonism appears to be the dominant geological process that has shaped the terrain. A broad wavelength of deformation is indicated, corresponding to the Voyager-observed topography, and may be the result of ductile necking of the lithosphere, while a finer scale of deformation probably reflects faulting of the brittle near surface. The results here form a basis against which other Galileo grooved terrain observations can be compared. © 1998 Academic Press.
• Thomas, P. C., Davies, M. E., Colvin, T. R., Oberst, J., Schuster, P., Neukum, G., Carr, M. H., McEwen, A., Schubert, G., & Belton, M. J. (1998). The Shape of Io from Galileo Limb Measurements. Icarus, 135(1), 175-180.
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  Abstract: Galileo CCD images of the limb of Io provide improved data for determining the shape of this synchronously rotating satellite. The best ellipsoidal fit is within 0.3 km of the best equilibrium fit of 1829.7, 1819.2, 1815.8 km. The shape is consistent with substantial mass concentration in a core and with gravity measurements from tracking of the Galileo spacecraft. The surface of Io is largely plains and isolated peaks, with little long-wavelength topography over 1 km in amplitude. © 1998 Academic Press.
• Davies, A. G., McEwen, A. S., M., R., Keszthelyi, L., Carlson, R. W., & Smythe, W. D. (1997). Temperature and area constraints of the South Volund Volcano on Io from the NIMS and SSI instruments during the Galileo G1 orbit. Geophysical Research Letters, 24(20), 2447-2450.
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  Abstract: Analysis of data from darkside and eclipse observations of Io by the NIMS and SSI instruments show that the South Volund hot spot is a manifestation of high temperature active silicate volcanism. The NIMS data are fitted with a two temperature model (developed from modelling terrestrial lavas) which yields a better fit to the data than a single temperature fit. The multispectral color temperatures obtained from NIMS are compared with the brightness temperatures obtained from the SSI instrument, and show excellent agreement for the hotter of the two components fitted to the NIMS data. The two components might correspond to a cooled crust which has formed on the surface of an active flow or lava lake, at a temperature of approximately 450 K, and covering an area of about 50 km2, and a hotter and much smaller component, at a temperature of approximately 1100 K and an area of less than 0.1 km2. The hot component implies the existence of cracks in the surface crust of a flow or lake through which the hot interior radiates, a hot vent area, or breakouts of lava forming new flow lobes. The ratio of these areas is consistent with the crack-to-crust ratio of some lava flows and lava lakes on Earth. Copyright 1997 by the American Geophysical Union.
• Grier, J. A., & McEwen, A. S. (1997). The small-comet hypothesis: An upper limit to the current impact rate on the Moon. Geophysical Research Letters, 24(24), 3105-3108.
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  Abstract: Frank et al. [1986b] and Frank and Sigwarth [1993] hypothesized the intense bombardment of the terrestrial atmosphere by small comets. Their model requires that the Moon is impacted by small comets (107-108 g) at a rate of almost one per minute. We calculate that ah object of this mass, even with an exceedingly low density and relatively low velocity, will nevertheless produce a crater at least 50 in in diameter. These craters will excavate immature lunar soil and produce a very bright spot with a diameter of at least 150 m. If low-density comets exist that might not create deep craters [O'Keefe and Ahrens, 1982], they will nevertheless disturb the regolith sufficiently to create detectable bright spots. If the small-comet, hypothesis is correct then the near-global lunar imaging returned by Clementine in 1994 should reveal ∼107 bright spots in locations where craters are not present in images acquired in the 1960's and early 1970's. We find no new bright spots in a carefully-studied area of 5.2×104 km2, so an upper limit to the current cratering rate by small comets is 33/yr, ∼104 below that expected if the small-comet hypothesis were valid: Copyright 1997 by the American Geophysical Union.
• Keszthelyi, L., & McEwen, A. (1997). Magmatic differentiation of Io. Icarus, 130(2), 437-448.
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  Abstract: If Io has been volcanically active through much of its history, it must be highly differentiated. We present an initial attempt to quantify the differentiation of the silicate portion of Io. We suggest that, on average, each part of Io has undergone about 400 episodes of partial melting. We employ a widely used thermodynamic model of silicate melts to examine the effect of such repeated differentiation. Despite many caveats, including a grossly oversimplistic model of the differentiation process, uncertainties in the initial composition of the mantle, and the failure to model more than four episodes of partial melting, we are able to make some robust conclusions. Io should have a roughly 50 km thick, low density (2600-2900 kg m-3), alkali-rich, siliceous crust composed primarily of feldspars and nepheline. The crustal magmas should have relatively low melting temperatures (
• Keszthelyil, L., & McEwen, A. (1997). Thermal models for basaltic volcanism on Io. Geophysical Research Letters, 24(20), 2463-2466.
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  Abstract: We present a new model for the thermal emissions from active basaltic eruptions on Io. While our methodology shares many similarities with previous work, it is significantly different in that (1) it uses a field tested cooling model and (2) the model is more applicable to pahoehoe flows and lava lakes than fountain-fed, channelized, 'a'a flows. This model demonstrates the large effect lava porosity has on the surface cooling rate (with denser flows cooling more slowly) and provides a preliminary tool for examining some of the hot spots on Io. The model infrared signature of a basaltic eruption is largely controlled by a single parameter, τ, the average survival time for a lava surface. During an active eruption surfaces are quickly covered or otherwise destroyed and typical values of τ for a basaltic eruption are expected to be on the order of 10 seconds to 10 minutes. Our model suggests that the Galileo SSI eclipse data are consistent with moderately active to quiescent basaltic lava lakes but are not diagnostic of such activity. Copyright 1997 by the American Geophysical Union.
• Klaasen, K. P., J., M., Breneman, H. H., McEwen, A. S., Davies, M. E., Sullivan, R. J., Chapman, C. R., Neukum, G., & Heffernan, C. M. (1997). Inflight performance characteristics, calibration, and utilization of the Galileo solid-state imaging camera. Optical Engineering, 36(11), 3001-3027.
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  Abstract: The solid-state imaging subsystem (SSI) on NASA's Galileo Jupiter orbiter spacecraft has already demonstrated its superior performance as a scientific imager by returning stunning pictures of several planetary bodies as well as detailed inflight calibration data during its cruise to Jupiter. The SSI inflight performance remains excellent; the instrument calibration is stable and accurate. Improved determinations of the SSI's absolute spectral radiometric response and scattered-light properties have been made. Evaluation of the camera's point spread function suggests that the focus setting may be slightly nonoptimum, but the spatial resolution in returned images is still very good. The shielding of the SSI's CCD detector against energetic particle radiation appears to be adequate for operation in Jupiter's intense radiation field. New camera modes, onboard editing and data compression capabilities, and an adaptive mission operations plan have been implemented for the Jupiter orbital mission phase in order to mitigate the effects of a spacecraft anomaly that limits the allowable data return rate from Jupiter. These new capabilities are expected to allow the accomplishment of a historic scientific investigation of the Jupiter system using the SSI. © 1997 Society of Photo-Optical Instrumentation Engineers.
• McEwen, A. S., & Robinson, M. S. (1997). Mapping of the Moon by Clementine. Advances in Space Research, 19(10), 1523-1533.
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  Abstract: The "faster, cheaper, better" Clementine spacecraft mission mapped the Moon from February 19 to May 3, 1994. Global coverage was acquired in 11 spectral bandpasses from 415 to 2792 nm and at resolutions of 80-330 m/pixel; a thermal-infrared camera sampled ∼20% of the surface; a high-resolution camera sampled selected areas (especially the polar regions); and a lidar altimeter mapped the large-scale topography up to latitudes of ±75°. The spacecraft was in a polar, elliptical orbit, 400-450 km periselene altitude. Periselene latitude was -28.5° for the first month of mapping, then moved to +28.5°. NASA is supporting the archiving, systematic processing, and analysis of the ∼1.8 million lunar images and other datasets. A new global positional network has been constructed from 43,000 images and ∼0.5 million match points; new digital maps will facilitate future lunar exploration. In-flight calibrations now enable photometry to a high level of precision for the uv-visible CCD camera. Early science results include: (1) global models of topography, gravity, and crustal thicknesses; (2) new information on the topography and structure of multiring impact basins; (3) evidence suggestive of water ice in large permanent shadows near the south pole; (4) global mapping of iron abundances; and (5) new constraints on the Phanerozoic cratering rate of the Earth. Many additional results are expected following completion of calibration and systematic processing efforts. © 1997 COSPAR. Published by Elsevier Science Ltd.
• McEwen, A. S., Moore, J. M., & Shoemaker, E. M. (1997). The phanerozoic impact cratering rate: Evidence from the farside of the Moon. Journal of Geophysical Research E: Planets, 102(E4), 9231-9242.
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  Abstract: The relatively recent (< 1 b.y.) flux of asteroids and comets forming large craters on the Earth and Moon may be accurately recorded by craters with bright rays on the Moon's farside. Many previously unknown farside rayed craters are clearly distinguished in the low-phase-angle images returned by the Clementine spacecraft. Some large rayed craters on the lunar nearside are probably significantly older than 1 Ga; rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity. Most of the farside crust has a more homogeneous composition and only immature rays are visible. The size-frequency distribution of farside rayed craters is similar to that measured for Eratosthenian craters (up to 3.2 b.y.) at diameters larger than 15 km. The areal density of farside rayed craters matches that of a corrected tabulation of nearside Copernican craters. Hence the presence of bright rays due to immature soils around large craters provides a consistent time-stratigraphic basis for defining the base of the Copernican System. The density of large craters less than ∼3.2 b.y. old is ∼3.2 times higher than that of large farside rayed craters alone. This observation can be interpreted in two ways: (1) the average cratering rate has been constant over the past 3.2 b.y. and the base of the Copernican is ∼1 Ga, or (2) the cratering rate has increased in recent geologic time and the base of the Copernican is less than 1 Ga. We favor the latter interpretation because the rays of Copernicus (800-850 m.y. old) appear to be very close to optical maturity, suggesting that the average Copernican cratering rate was ∼35% higher than the average Eratosthenian rate. Other lines of evidence for an increase in the Phanerozoic (545 Ga) cratering rate are (1) the densities of small craters superimposed on Copernicus and Apollo landing sites, (2) the rates estimated from well-dated terrestrial craters (≤ 120 m.y.) and from present-day astronomical observations, and (3) the Proterozoic rate suggested by the crater record of Australia. The hypothesis most consistent with several key observations is that the cratering rate has increased by ∼2x during the past ∼300 m.y. Copyright 1997 by the American Geophysical Union.
• McEwen, A. S., Simonelli, D. P., Senske, D. R., Klaasen, K. P., Keszthelyi, L., Johnson, T. V., Geissler, P. E., Carr, M. H., & J., M. (1997). High-temperature hot spots on Io as seen by the Galileo solid state imaging (SSI) experiment. Geophysical Research Letters, 24(20), 2443-2446.
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  Abstract: High-temperature hot spots on Io have been imaged at ∼50 km spatial resolution by Galileo's CCD imaging system (SSI). Images were acquired during eclipses (Io in Jupiter's shadow) via the SSI clear filter (∼0.4-1.0 μm), detecting emissions from both small intense hot spots and diffuse extended glows associated with Io's atmosphere and plumes. A total of 13 hot spots have been detected over ∼70% of Io's surface. Each hot spot falls precisely on a low-albedo feature corresponding to a caldera floor and/or lava flow. The hot-spot temperatures must exceed ∼700 K for detection by SSI. Observations at wavelengths longer than those available to SSI require that most of these hot spots actually have significantly higher temperatures (∼1000 K or higher) and cover small areas. The high-temperature hot spots probably mark the locations of active silicate volcanism, supporting suggestions that the eruption and near-surface movement of silicate magma drives the heat flow and volcanic activity of Io. Copyright 1997 by the American Geophysical Union.
• Schenk, P. M., McEwen, A., Davies, A. G., Davenport, T., Jones, K., & Fessler, B. (1997). Geology and topography of Ra Patera, Io, in the Voyager era: Prelude to eruption. Geophysical Research Letters, 24(20), 2467-2470.
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  Abstract: Voyager era stereo images are used to map the geology and topography of Ra Patera (a major active volcanic center and possible site of sulfur eruptions on Io). The summit of Ra Patera reaches only ∼1 km above the surrounding plains. Pre-Voyager-era lava flows occur on slopes of 0.1-0.3°, comparable to the lunar mare. These flows were emplaced at either low viscosities, high eruption rates, or both. A 600-km-long ridged mountain unit (rising to ∼8 km near Carancho Patera) forms a 60 by 90 km wide plateau ∼0.5 km high 50 km east of Ra Patera. The new lava flows observed by Galileo flowed around the southern edge of this plateau. Copyright 1997 by the American Geophysical Union.
• Simonelli, D. P., Veverka, J., & McEwen, A. S. (1997). Io: Galileo evidence for major variations in regolith properties. Geophysical Research Letters, 24(20), 2475-2478.
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  Abstract: Galileo SSI images taken at different times early in the Jupiter orbital tour show striking, widespread changes in Io's color and albedo patterns that result from observing the satellite's surface at different lighting and viewing geometries. Such changes indicate significant variations in the photometric or light-scattering behavior of the Ionian surface from region to region, and thereby provide strong evidence for major lateral variations in regolith properties. In particular, the regolith particles in Io's bright equatorial band appear to be larger than, or have a higher density of internal scatterers than, those in the satellite's higher-latitude regions. Such heterogeneity is not surprising on Io, given the rapid rate of volcanic resurfacing. Copyright 1997 by the American Geophysical Union.
• Spencer, J. R., McEwen, A. S., McGrath, M. A., Sartoretti, P., Nash, D. B., Noll, K. S., & Gilmore, D. (1997). Volcanic resurfacing of Io: Post-repair HST imaging. Icarus, 127(1), 221-237.
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  Abstract: In March 1994, we used the newly refurbished Hubble Space Telescope (HST) to obtain global imaging of Io at five wavelengths between 0.34 and 1.02 μm, with a spatial resolution of 160 km. The images provided the clearest view of Io since Voyager and the first systematic observations in the wavelength range 0.7-1.0 μm. We have produced absolutely calibrated global mosaics of Io's reflectance in all our five wavelengths. The near-infrared images reveal that the 0.55-to 0.7-μm absorption edge seen in Io's disk-integrated spectrum has a very different spatial distribution from the better-known 0.40-to 0.50-μm absorption edge studied by Voyager, and must be generated by a different chemical species. The 0.55-to 0.7-μm absorption edge is strongly concentrated in the pyroclastic ejecta blanket of the volcano Pele, at a few much smaller discrete spots, and probably also in the polar regions. The Pele ejecta spectrum is consistent with the idea that S2O, partially decomposed to S4 (and probably S3), may be the species responsible for the 0.55-to 0.7-μm absorption edge at Pele and elsewhere on Io, though S4 generated by other processes may also be a possibility. S2O can be produced by high-temperature decomposition of SO2 gas, and the high temperature of the Pele volcano may account for its concentration there. Spectral anomalies of comparable size and prominence are not seen around the other "Pele-type" volcanos Surt and Aten (A. S. McEwen and L. A. Soderblom, 1983, Icarus 55, 191-217), suggesting that these volcanos, if chemically similar to Pele, are much less active. The spectrum of high-latitude regions is similar to that of quenched red sulfur glass, and if this similarity is not coincidental, the glass may be preserved here by the low polar surface temperatures. Alternatively, the low polar temperatures may preserve sulfur that has been reddened by radiation. There are many changes in albedo patterns in the 15 years between Voyager and these HST observations, but these are generally subtle at HST resolution and are not strongly concentrated in longitude; however there was a major brightening of a 400-km-diameter region centered on Ra Patera between March 1994 and repeat HST observations in July 1995, which was a larger albedo change than any seen in the previous 15 years. This was presumably due to a large eruption at Ra Patera, as confirmed by Galileo images. Long-exposure eclipse images of Io at 1.02 μm on March 6, 1994, place strong limits on the area of exposed silicate magma on Io at the time of the observations. © 1997 Academic Press.
• Spencer, J. R., Sartoretti, P., Ballester, G. E., McEwen, A. S., Clarke, J. T., & McGrath, M. A. (1997). The Pele plume (Io): Observations with the Hubble Space Telescope. Geophysical Research Letters, 24(20), 2471-2474.
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  Abstract: In July 1996, with the Hubble Space Telescope (HST), we observed the Pele plume silhouetted against Jupiter at a wavelength of 0.27 μm, the first definitive observation of an Io plume from Earth. The height, 420 ± 40 km, was greater than any plume observed by Voyager. The plume had significantly smaller optical depth at 0.34 and 0.41 μm, where it was not detected. The wavelength dependence of the optical depth can be matched by a plume either of fine dust, with minimum mass of 1.2 × 10 9 g and maximum particle size of 0.08 μm, or of SO 2 gas with a column density of 3.7 × 10 17 cm -2 and total mass of 1.1 × 10 11 g. Our models suggest that early Voyager imaging estimates of the minimum mass of the Loki plume [Collins, 1981] may have been too large by a factor of ∼ 100. We may have detected the Pele plume in reflected sunlight, at 0.27 μm, in July 1995, but did not see it 21 hours earlier, so the plume may be capable of rapid changes. Copyright 1997 by the American Geophysical Union.
• Helfenstein, P., Veverka, J., Thomas, P. C., Simonelli, D. P., Klaasen, K., Johnson, T. V., Fanale, F., Granahan, J., McEwen, A. S., Belton, M., & Chapman, C. (1996). Galileo photometry of asteroid 243 Ida. Icarus, 120(1), 48-65.
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  Abstract: Galileo imaging observations over phase angles 19.5° to 109.8° are combined with near-opposition Earth-based data to derive the photometric properties of Ida. To first order these properties are uniform over the surface and well modeled at λ = 0.55 μm by Hapke parameters ω̃0 = 0.22, h = 0.020, B0 = 1.5, g = -0.33, and θ = 18° with corresponding geometric albedo p = 0.21±0.030.01 and Bond albedo AB = 0.081±0.0170.008. Ida's photometric properties are more similar to those of "average S-asteroids" (P. Helfenstein and J. Veverka 1989, Asteroids II, Univ. of Arizona Press, Tucson) than are those of 951 Gaspra. Two primary color units are identified on Ida: Terrain A exhibits a spectrum with relatively shallower 1-μm absorption and a relatively steeper red spectral slope than average Ida, while Terrain B has a deeper 1-μm absorption and a less steep red slope. The average photometric properties of Ida and Terrain A are similar while those of Terrain B differ mostly in having a slightly higher value of ω̃0 (0.22 versus 0.21), suggesting that Terrain B consists of slightly brighter, more transparent regolith particles. Galileo observations of Ida's satellite Dactyl over phase angles 19.5° to 47.6° suggest photometric characteristics similar to those of Ida, the major difference being Dactyl's slightly lower albedo (0.20 compared to 0.21). © 1990 Academic Press, Inc.
• J., M., Chapman, C. R., Klaasen, K. P., Harch, A. P., Thomas, P. C., Veverka, J., McEwen, A. S., & Pappalardo, R. T. (1996). Galileo's encounter with 243 Ida: An overview of the imaging experiment. Icarus, 120(1), 1-19.
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  Abstract: We provide an overview of the execution, data, and results of the solid state imaging (SSI) experiment at the encounter of the Galileo spacecraft with the asteroid 243 Ida. Ninety-six images of the asteroid, representing 18 time samples during a rotation period (4.633 h), were transmitted to Earth as a result of the UT 1993 August 28.70284 encounter. This provided coverage of ∼95% of the surface and achieved ground resolutions as high as 25 m/pixel. Coverage of most of Ida's surface is available in four colors, with limited regions in five colors, at resolutions up to 105 m/pixel. A natural satellite of Ida, called Dactyl, was discovered in a prograde (with respect to Ida's spin), near-equatorial, orbit moving slowly (∼ 6 m/sec) with a separation of 85 km from Ida. Ida's shape is highly irregular; by comparison, Dactyl's global topography is quite smooth. The best fit ellipsoid to Ida's shape has principal dimensions 59.8 × 25.4 × 18.6 km, mean radius 15.7 km, and volume 16,100 ± 1900 km3. Dactyl's mean radius is only 0.7 km. Ida's spin axis (right ascension: 348.76° ± 7.5°; declination: 87.10° ± 0.4°; J2000) was found to align with the principal axis of inertia to within the error of measurement. This is consistent with a homogeneous density distribution. Dactyl's rotation rate is unknown, but its long axis was pointed in the direction of Ida at the time of observation, suggesting synchronism of its orbital motion and spin. Constraints on Dactyl's orbit yield 4.2 ± 0.6 × 1019 g for Ida's mass and 2.6 ± 0.5 g/cm3 for its bulk density. Unless Ida's bulk porosity is exceptionally high, Ida has moderate to low NiFe content. Subtle color variations across the surface of Ida are associated with fresh craters, but, unlike the case for Asteroid 951 Gaspra, are not correlated with topographic features such as ridges. This difference may be a reflection of a deeper and/or more mobile regolith on Ida. Dactyl's spectral reflectance is similar to, but quantitatively distinct from the surface of Ida itself. This difference may reflect compositional differences between Dactyl and Ida, which in turn may have originated in an only partially differentiated Koronis parent body. Results on the origin, collisional history, and geology of Ida and Dactyl are the subject of many of the papers in this special issue. There is general agreement that these asteroids originated in the catastrophic breakup of the Koronis parent body and that the formation of asteroid-satellite systems may be relatively common in such events. The age and collisional history of the pair present a dilemma: using standard interpretations of the cratering record on Ida's surface, an age > 1 byr. is indicated. However, the lifetime of Dactyl against collisional disruption is many times less than this. Novel ideas are presented concerning the collisional history of these two small objects that may resolve this dilemma. These ideas result from analysis of the geological record on the surface of Ida, Dactyl, and, by comparison, Gaspra - all of which are examined in this special issue. The execution of the Galileo flybys of Gaspra, Ida, and Dactyl provide important lessons for future flybys of small bodies. We present our views on the limitations faced by the Galileo imaging experimenters and indicate how future missions can be made more quantitative and productive through the application of innovative electronic control systems and detector technology. © 1996 Academic Press, Inc.
• Veverka, J., Helfenstein, P., Lee, P., Thomas, P., McEwen, A., Belton, M., Klaasen, K., Johnson, T. V., Granahan, J., Fanale, F., Geissler, P., & III, J. H. (1996). Ida and Dactyl: Spectral reflectance and color variations. Icarus, 120(1), 66-76.
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  Abstract: Galileo SSI color data between 0.4 and 1.0 μm demonstrate that both Ida and Dactyl are S-type asteroids with similar, but distinct spectra. Small but definite color variations are also observed on Ida itself and involve both the blue part of the spectrum and the depth of the 1-μm pyroxene-olivine band. Ida's surface can be classified into two color terrains: Terrain A has a shallower 1-μm absorption and a steeper visible red slope than does Terrain B. Qualitatively, the color-albedo systematics of these two terrains follow those noted for color units on Gaspra and the variations in 1-μm band depth with weathering described by Gaffey et al. (Gaffey, M. J., J. F. Bell, R. H. Brown, T. H. Burbine, J. Piatek, K. L. Reed, and D. A. Chaky 1993. Icarus 106, 573-602). Terrain A, with its slightly lower albedo, its shallower 1-μm band, and its slightly steeper visible red slope relative to Terrain B could be interpreted as the "more processed," "more mature," or the "more weathered" of the two terrains. Consistent with this interpretation is that Terrain A appears to be the ubiquitous background on most of Ida, while Terrain B is correlated with some small craters as well as with possible ejecta from the 10-km Azzurra impact structure. Because of these trends, it is less likely that differences between Terrains A and B are caused by an original compositional inhomogeneity within the body of Ida, although they do fall within the range known to occur within the Koronis family. The spectrum of Dactyl is similar to, but definitely different from, that of Terrain B on Ida. It does not conform to the pattern that obtains between the colors and albedos of Terrains A and B: the satellite's 1-μm band is deeper than that of Terrain B, but its albedo is lower, rather than higher. By itself, the deeper band depth could be interpreted, following Gaffey et al., to mean that Dactyl is a less weathered version of Terrain B on Ida, but such an interpretation is at odds with Dactyl's redder spectral slope. Thus, the explanation for the color difference between Dactyl and Ida is likely to be different from that which accounts for the differences between the two terrains on Ida. Given that Dactyl and Ida have very similar photometric properties (Helfenstein, P., J. Veverka, P. C. Thomas, D. P. Simonelli, K. Klassen, T. V. Johnson, F. Fanale, J. Granahan, A. S. McEwen, M. J. S. Belton, and C. R. Chapman 1996 Icarus 120, 48-65), thus ruling out any dramatic texture differences between the two surfaces, the most likely explanation is that the satellite has a slightly different composition (more pyroxene?) than Ida. The spectral difference is within the range reported by Binzel et al. (Binzel, R. P., S. Xu, and S. J. Bus 1993. Icarus 106, 608-611.) for members of the Koronis family, and could be caused by compositional inhomogeneities of the Koronis parent body rather than by post-breakup regolith processes. © 1996 Academic Press, Inc.
• Chapman, C. R., Veverka, J., Thomas, P. C., Klaasen, K., Belton, M. J., Harch, A., McEwen, A., Johnson, T. V., Helfenstein, P., Davies, M. E., Merline, W. J., & Denk, T. (1995). Discovery and physical properties of Dactyl, a satellite of asteroid 243 Ida. Nature, 374(6525), 783-X.
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  Abstract: OBSERVATIONS of stellar occultations by asteroids have suggested that some may have satellites1. But given the absence of any confirmatory evidence, the prevailing view has been that although such satellites probably do exist, they are likely to be rare2. Here we report the discovery3 by the Galileo spacecraft of a satellite associated with the asteroid 243 Ida. Although the satellite, Dactyl, is only 1.6 km across, it has been imaged with sufficient resolution for geological analysis. We describe the physical properties of Dactyl, with emphasis on its notably smooth shape, its crater population (which includes a crater chain) and its photometric properties. We find that, spectroscopically, Dactyl resembles both Ida and the other members of the Koronis asteroid family, implying a similar composition; small spectral differences may reflect a space weathering process that slightly alters the colours with time. We argue that Dactyl originated during the breakup of the Koronis parent body, and that satellites could be common around other asteroids (particularly members of asteroid families).
• Gaddis, L. R., McEwen, A. S., & Becker, T. L. (1995). Compositional variations on the Moon: recalibration of Galileo solid- state imaging data for the Orientale region and farside. Journal of Geophysical Research, 100(E12), 26,345-26,355.
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  Abstract: Updated radiometric calibration and systematic processing procedures for Galileo solid-state imaging data from the first (1990) Earth-Moon encounter are presented. These procedures were applied to a whole-disk imaging sequence of the Moon centered near Mare Orientale, Called Lunmap 14 (L14). Recalibrated spectra indicate that mare ponds of the limb show moderately deep 1-μm absorptions, and thus mafic mineral contents, comparable to those of other nearside basalts. Mafic mineral contents of Schiller-Schickard cryptomaria are higher than previously thought and are similar to some low-Ti nearside basalts. Many of the recalibrated spectra from South Pole/Aitken are similar to those of Schiller-Schickard cryptomaria, suggesting that many of these soils represent a mixed mare/highland lithology. -from Authors
• McEwen, A. S. (1995). SO₂-Rich Equatorial Basins and Epeirogeny of Io. Icarus, 113(2), 415-422.
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  Abstract: The most concentrated deposits of SO2 frost on Io occur within a series of large equatorial basins. About 30% of the surface is covered by SO2 outside of the basins, increasing to more than 50% within the basins. This pattern is poorly expressed in the region from longitude 240° to 360° where bright areas are frequently buried by the fallout from the large Pele-type plumes. The fourfold pattern of alternating basins and swells in Io's equatorial region is similar to the heat-flow pattern predicted from tidal heating in a thin, partially molten asthenosphere. However, the topographic pattern is offset from the predicted heat-flow pattern; thus it is unclear whether topographic highs correspond to regions of higher or lower predicted heat flow. These two possibilities imply two very different models for Io's highlands: a thermal-uplift model or a continental-crust model. In the thermal-uplift model, the regions of enhanced asthenospheric heating cause lithospheric thinning and isostatic uplift, perhaps accompanied by uplift due to penetrative magmatism or basaltic underplating. In the continental-crust model, "continents" of differentiated crust float on low-density roots, the crust and lithosphere are approximately one and the same, and basal melting controls its thickness. Although both models are plausible, the thermal-uplift model best explains the SO2 distribution. Cold trapping must be important for concentrating SO2 frost in optically thick patches; thus either cold traps are preferentially initiated over large basin areas or they are preferentially removed from the highlands. The patchy distribution and approximately 30% SO2 coverage of the highlands show that cold traps are abundant here, but not extensive; thus the SO2 must be preferentially removed and/or buried. Higher heat flows in the highlands should lead to increased volatilization of SO2 frost, and a greater frequency of relatively SO2-poor volcanism should tend to bury frost patches. This model links asthenospheric tidal heating, large-scale heat flow and topography, volcanic activity, and the global distribution of surface SO2, and it leads to several specific predictions for future observations. © 1995 Academic Press. All rights reserved.
• Carr, M. H., Kirk, R. L., McEwen, A., Veverka, J., Thomas, P., Head, J. W., & Murchie, S. (1994). The Geology of Gaspra. Icarus, 107(1), 61-71.
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  Abstract: The surface of Gaspra can be divided into several facets separated by ridges. Superimposed on the facets and ridges are two populations of craters. Type 1 craters are crisply defined and constitute a production function of impact origin. Type 2 craters are mostly irregular shallow depressions and probably of various origins. Some of the type 2 craters may have formed by impact and be remnants of a crater population that predates the type 1 population. The number of type 1 craters suggests that they started to accumulate 2 × 107 to 3 × 108 years ago. The freshest craters have a depth/diameter ratio of 1:7, as compared with 1:5 typically observed on other bodies. The craters appear to have become shallower with age at a rate of 10-6 to 10-7 m/year. The shallow depth of newly formed craters and the progressive degradation with age are ascribed to downslope movement of poorly coherent surface materials, the movement being aided by seismic shaking as a result of impact. The surface shows subtle color differences. The most prominent differences are observed around craters on ridges, where the surface has a stronger 1-μm absorption than elsewhere. The contrast is not observed around craters on the facets. The color differences are attributed to alteration of the materials brought to the surface. The unaltered material is estimated to be located at least 50 m below the surface on the facets. Only on the ridges is unaltered material at a shallow enough depth to be excavated by the observed craters. The rate of degradation of craters, the rounded form of the ridges, the depth of altered material on the facets, and the presence of old degraded craters all suggest that Gaspra is covered with a regolith a few tens to several tens of meters thick. © 1994 Academic Press. All rights reserved.
• Helfenstein, P., Veverka, J., Thomas, P. C., Simonelli, D. P., Lee, P., Klaasen, K., Johnson, T. V., Breneman, H., Head, J. W., Murchie, S., Fanale, F., Robinson, M., Clark, B., Granahan, J., Garbeil, H., McEwen, A. S., Kirk, R. L., Davies, M., Neukum, G., , Mottola, S., et al. (1994). Galileo Photometry of Asteroid 951 Gaspra. Icarus, 107(1), 37-60.
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  Abstract: Galileo images of Gaspra make it possible for the first time to determine a main-belt asteroid's photometric properties accurately by providing surface-resolved coverage over a wide range of incidence and emission angles and by extending the phase angle coverage to phases not observable from Earth. We combine Earth-based telescopic photometry over phase angles 2° ≤ α ≤ 25° with Galileo whole-disk and disk-resolved data at 33° ≤ α ≤ 51° to derive average global photometric properties in terms of Hapke's photometric model. The microscopic texture and particle phase-function behavior of Gaspra's surface are remarkably like those of other airless rocky bodies such as the Moon. The macroscopic surface roughness parameter, θ̄ = 29°, is slightly larger than that reported for typical lunar materials. The particle single scattering albedo, ώ0 = 0.36 ± 0.07, is significantly larger than for lunar materials, and the opposition surge amplitude, B0 = 1.63 ± 0.07, is correspondingly smaller. We determine a visual geometric albedo pv = 0.22 ± 0.06 for Gaspra, in close agreement with pv = 0.22 ± 0.03 estimated from Earth-based observations. Gaspra's phase integral is 0.47, and the bolometric Bond albedo is estimated to be 0.12 ± 0.03. An albedo map derived by correcting Galileo images with our average global photometric function reveals subdued albedo contrasts of ±10% or less over Gaspra's northern hemisphere. Several independent classification algorithms confirm the subtle spectral heterogeneity reported earlier (S. Mottola, M. DiMartino, M. Gonano-Beurer, H. Hoffman, and G. Neukum, 1993, Asteroids, Comets, Meteors, pp. 421-424; M. J. S. Belton et al., 1992, Science 257, 1647-1652). Whole-disk colors (0.41 ≤ λ ≤ 0.99 μm) vary systematically with longitude by about ±5%, but color differences as large as 30% occur locally. Colors vary continuously between end-member materials whose areal distribution correlates with regional topography. Infrared: violet (0.99:0.41-μm) color ratios on Gaspra are strongly correlated with local elevation, being largest at lower elevations and smaller at higher elevations. No correlation was detected between elevation and the green:violet (0.56:0.41-μm) color ratio. Bright materials with a strong 1-μm absorption occur primarily in association with craters along ridges, while darker materials with 30% weaker 1-μm signatures occur downslope. The variations of color and albedo cannot be easily explained by grain-size effects alone or by differences in photometric geometry. The trends observed are consistent with those revealed by laboratory studies of the effects of comminution, glass formation, and segregation of metal from silicate components in chondritic meteorites and also in some silicate mixtures. The relative importance of these various processes on Gaspra remains to be determined. © 1994 Academic Press. All rights reserved.
• McEwen, A. S., Robinson, M. S., Eliason, E. M., Lucey, P. G., Duxbury, T. C., & Spudis, P. D. (1994). Clementine observations of the Aristarchus region of the moon. Science, 266(5192), 1858-1862.
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  PMID: 17737082;Abstract: Multispectral and topographic data acquired by the Clementine spacecraft provide information on the composition and geologic history of the Aristarchus region of the moon. Altimetry profiles show the Aristarchus plateau dipping about 1° to the north-northwest and rising about 2 kilometers above the surrounding lavas of Oceanus Procellarum to the south. Dark, reddish pyroclastic glass covers the plateau to average depths of 10 to 30 meters, as determined from the estimated excavation depths of 100- to 1000-meter-diameter craters that have exposed materials below the pyroclastics. These craters and the wall of sinuous rilles also show that mare basalts underlie the pyroclastics across much of the plateau. Near-infrared images of Aristarchus crater reveal oilvine-rich materials and two kilometer-sized outcrops of anorthosite in the central peaks. The anorthosite could be either a derivative of local magnesium-suite magmatism or a remnant of the ferroan anorthosite crust that formed over the primordial magma ocean.
• Duffield, W., Heiken, G., Foley, D., & McEwen, A. (1993). Oblique synoptic images, produced from digital data, display strong evidence of a "new" caldera in southwestern Guatemala. Journal of Volcanology and Geothermal Research, 55(3-4), 217-224.
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  Abstract: The synoptic view of broad regions of the Earth's surface as displayed in Landsat and other satellite images has greatly aided in the recognition of calderas, ignimbrite plateaus and other geologic landforms. Remote-sensing images that include visual representation of depth are an even more powerful tool for geologic interpretation of landscapes, but their use has been largely restricted to the exploration of planets other than Earth. By combining Landsat images with digitized topography, we have generated regional oblique views that display compelling evidence for a previously undocumented late-Cenozoic caldera within the active volcanic zone of southwestern Guatemala. This "new" caldera, herein called Xela, is a depression about 30 km wide and 400-600 m deep, which includes the Quezaltenango basin. The caldera depression is breached only by a single river canyon. The caldera outline is broadly circular, but a locally scalloped form suggests the occurrence of multiple caldera-collapse events, or local slumping of steep caldera walls, or both. Within its northern part, Xela caldera contains a toreva block, about 500 m high and 2 km long, that may be incompletely foundered pre-caldera bedrock. Xela contains several post-caldera volcanoes, some of which are active. A Bouguer gravity low, tens of milligals in amplitude, is approximately co-located with the proposed caldera. The oblique images also display an extensive plateau that dips about 2° away from the north margin of Xela caldera. We interpret this landform to be underlain by pyroclastic outflow from Xela and nearby Atitlán calderas. Field mapping by others has documented a voluminous rhyolitic pumiceous fallout deposit immediately east of Xela caldera. We speculate that Xela caldera was the source of this deposit. If so, the age of at least part of the caldera is between about 84 ka and 126 ka, the ages of deposits that stratigraphically bracket this fallout. Most of the floor of Xela caldera is covered with Los Chocoyos pyroclastics, 84-ka deposits erupted from Atitlán caldera. Oblique images produced from digital data are unique tools that can greatly facilitate initial geologic interpretation of morphologically young volcanic (and other) terrains where field access is limited, especially because conventional visual representations commonly lack depth perspective and may cover only part of the region of interest. © 1993.
• McEwen, A. S., Gaddis, L. R., Neukum, G., Hoffman, H., Pieters, C. M., & Head, J. W. (1993). Galileo observations of post-imbrium lunar craters during the first Earth-Moon flyby. Journal of Geophysical Research, 98(E9), 17,207-17,231.
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  Abstract: Copernican-age craters are among the most conspicuous features seen on the far side and western limb of the Moon in the Galileo multispectral images acquired December 1990. Among the new morphologic observations of far-side craters are bright rays, continuous ejecta deposits, and dark rings associated with probable impact-melt veneers. These observations suggest that the mapped age assignments of several large far-side craters (Ohm, Robertson, and possibly Lowell and Lenz) need revision. New crater size-frequency measurements on Lunar Orbiter images suggest the following age reassignments: Hausen (170 km diameter), Pythagoras (120 km), and Bullialdus (61 km) from Eratosthenian to Upper Imbrian, and Carpenter (60 km) and Harpalus (39 km) from Copernican to Eratosthenian. Colors and albedos of craters (away from impact-belt veneers)) are correlated with their geologic emplacement ages as determined from counts of superposed craters; these age-color relations are used to estimate the emplacement age (time since impact event) for other Copernican-age craters. These age-color relations indicate a probable Copernican age for 27 far-side or western limb craters larger than 10 km diameter that were not previously mapped as Copernican. The apparent deficiency of Copernican craters on the far side compared with the near side in published geologic maps is not present in our data. The soil-maturity trends represent longer geologic time periods in regions with thinner regoliths, such as the maria. -from Authors
• McEwen, A. S. (1991). Photometric functions for photoclinometry and other applications. Icarus, 92(2), 298-311.
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  Abstract: Least-squared fits to the brightness profiles across a disk or "limb darkening" described by Hapke's photometric function are found for the simpler Minnaert and lunar-Lambert functions. The simpler functions are needed to reduce the number of unknown parameters in photoclinometry, especially to distinguish the brightness variations of the surface materials from that due to the resolved topography. The limb darkening varies with the Hapke parameters for macroscopic roughness (θ), the single-scattering albedo (w), and the asymmetry factor of the particle phase function (g). Both of the simpler functions generally provide good matches to the limb darkening described by Hapke's function, but the lunar-Lambert function is superior when viewing angles are high and when (θ) is less than 30°. Although a nonunique solution for the Minnaert function at high phase angles has been described for smooth surfaces, the discrepancy decreases with increasing (θ) and virtually disappears when (θ) reaches 30° to 40°. The variation in limb darkening with w and g, pronounced for smooth surfaces, is reduced or eliminated when the Hapke parameters are in the range typical of most planetary surfaces; this result simplifies the problem of photoclinometry across terrains with variable surface materials. The Minnaert or lunar-Lambert fits to published Hapke models will give photoclinometric solutions that are very similar (>1° slope discrepancy) to the Hapke-function solutions for nearly all of the bodies and terrains thus far modeled by Hapke's function. © 1991.
• Eliason, E. M., & McEwen, A. S. (1990). Adaptive box filters for removal of random noise from digital images. Photogrammetric Engineering and Remote Sensing, 56(4), 453-458.
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  Abstract: We have developed adaptive box-filtering algorithms to (1) remove random bit errors (pixel values with no relation to the image scene) and (2) smooth noisy data (pixels related to the image scene but with an additive or multiplicative component of noise). For both procedures, we use the standard deviation (σ) of those pixels within a local box surrounding each pixel, hence they are adaptive filters. A series of two or three filters with decreasing box sizes can be run to clean up extremely noisy images and to remove bit errors near sharp edges. Our second filter, for noise smoothing, is identical to the 'sigma filter' of Lee (1983a) except that we use the local (adaptive) σ rather than a fixed σ. The filter averages only those pixels within the box that have intensities within 1.0 to 2.0 σ of the central pixel. This technique effectively reduces speckle in radar images without eliminating fine details.
• Hansen, C. J., McEwen, A. S., Ingersoll, A. P., & Terrile, R. J. (1990). Surface and airborne evidence for plumes and winds on triton. Science, 250(4977), 421-424.
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  Abstract: Aeolian features on Triton that were imaged during the Voyager Mission have been grouped. The term "aeolian feature" is broadly defined as features produced by or blown by the wind, including surface and airborne materials. Observations of the latitudinal distributions of the features probably associated with current activity (known plumes, crescent streaks, fixed terminator clouds, and limb haze with overshoot) all occur from latitude -37° to latitude -62°. Likely indicators of previous activity (dark surface streaks) occur from latitude -5° to -70°, but are most abundant from -15° to -45°, generally north of currently active features. Those indicators which give information on wind direction and speed have been measured. Wind direction is a function of altitude. The predominant direction of the surface wind streaks is found to be between 40° and 80° measured clockwise from north. The average orientation of streaks in the northeast quadrant is 59°. Winds at 1- to 3-kilometer altitude are eastward, while those at >8 kilometers blow west.
• McEwen, A. S., & Lunine, J. I. (1990). Comment on "The surface of lo: A new model" by Bruce Hapke. Icarus, 84(1), 268-274.
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  Abstract: Hapke (1989, Icarus 79, 56-74) proposed that the surface of Io is dominantly basaltic with thin coatings of polysulfur oxide, S2O, ad SO,2. However, observations and models of the active volcanism indicate that volatiles such as sulfur and SO2 must be more abundant than envisioned by Hapke. © 1990.
• McEwen, A. S. (1989). Mobility of large rock avalanches: evidence from Valles Marineris, Mars. Geology, 17(12), 1111-1114.
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  Abstract: Measurements of H/L (height of drop/length of runout) vs. volume for landslides in Valles Marineris on Mars show a trend of decreasing H/L with increasing volume. This trend, which is linear on a log-log plot, is parallel to but lies above the trend for terrestrial dry rock avalanches. This result and estimates of 104 to 105 Pa yield strength suggest that the landslides were not water saturated, as suggested by previous workers. The offset between the H/L vs. volume trends shows that a typical Martian avalanche must be nearly two orders of magnitude more voluminous than a typical terrestrial avalance in order to achieve the same mobility. This offset might be explained by the effects of gravity on flows with high yield strengths. These results should prove useful to future efforts to resolve the controversy over the mechanics of long-runout avalanches. -Author
• McEwen, A. S., & Malin, M. C. (1989). Dynamics of Mount St. Helens' 1980 pyroclastic flows, rockslide-avalanche, lahars, and blast. Journal of Volcanology and Geothermal Research, 37(3-4), 205-231.
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  Abstract: A computer model for the movement of gravity flows calculates the velocities and simulated flow paths over digital topographic models. The nonuniform flow movements are determined from initial conditions, gravitational accelerations, and resistance to motion (tr) described by the general equation τr = α0 + α1ν + α2ν2, where ν is velocity. Although empirical, the terms α0, α1 and α2 may be related to Coulomb, viscous, and turbulent resistance, respectively. The energy-line model is used by setting α0 proportional to a coefficient of friction and setting α1 and α2 to zero. Use of the terms α0 and α1 results in a Bingham-like model. The models were tested against the reported velocities and distributions of the pyroclastic flows, rockslide-avalanche, lahars, and blast of the 1980 eruptions of Mount St. Helens, Washington. The energy-line model, which has been widely used for this type of effort, generally predicts velocities that are too high, resulting in flow paths that are not sufficiently responsive to the topography. Use of the α1 or α2 term usually results in better matches to the observed velocities and flow paths. An August 7th pyroclastic flow was modeled in detail for comparison with the velocities acquired from a timed sequence of photographs. Both the energy-line model and a Bingham model based on measured rheologic properties result in model velocities that are much too high. The Reynolds and Bingham numbers indicate that local turbulence was likely, which is consistent with the presence of plane-parallel and cross-bedded deposits on the steep northern flanks of Mount St. Helens. Addition of turbulent resistance to the Bingham model results in a much better match to the measured velocities. A similar model best matches the distribution of deposits from the more voluminous May 18th pyroclastic flows. Our best result for the rockslide-avalanche in the North Fork Toutle River is a Bingham model with a viscosity of 3 × 104 Pa s and a yield strength of 104 Pa. For the lahars, viscous models gives the best results, but model viscosities range from 103-104 Pa s near the flanks of the volcano and in relatively dry creeks and canyons to 101-102 Pa s where the lahars entered river channels and were significantly diluted. For the blast, each of the three basic types of resistance models compares reasonably well with the observed distribution and velocities, provided the resistance is small. However, the expected resistances from both boundary-layer turbulence and air drag are substantial; in order to account for the blast's travel distance of 20-30 km, some process in addition to the initial momentum and gravity must have operated, such as continued decompression far from the vent. © 1989.
• Smith, B. A., Soderblom, L. A., Banfield, D., Barnet, C., Basilevksy, A. T., Beebe, R. F., Bollinger, K., Boyce, J. M., Brahic, A., Briggs, G. A., Brown, R. H., Chyba, C., Collins, S. A., Colvin, T., II, A. C., Crisp, D., Croft, S. K., Cruikshank, D., Cuzzi, J. N., , Danielson, G. E., et al. (1989). Voyager 2 at Neptune: Imaging science results. Science, 246(4936), 1422-1449.
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  Abstract: Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.
• Gaskell, R. W., Synnott, S. P., McEwen, A. S., & Schaber, G. C. (1988). Large-scale topography of Io: implications for internal structure and heat transfer.. Geophysical Research Letters, 15(6), 581-584.
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  Abstract: New stereographic techniques were applied to Voyager 1 images to determine the shape and large-scale topopgraphy of Io. The best- fit triaxial figure has semiaxes of 1830.0, 1818.7 and 1815.3 km with random errors of about 200 m. Systematic navigation and optical system errors are comparable. The resulting figure is consistent with a differentiated satellite in hydrostatic equilibrium. The deviation from homogeneity could be explained by the presence of an iron core with a radius of about 800 km. If the large-scale topography is positively correlated with heat flow, then the global pattern of basins and swells suggests tidal dissipation in a molten or partialy molten asthenosphere rather than in the mantle. Areas higher than 1 km may consist of low-density crustal materials. Media Regio, the largest such region, is centered near the equator at longitude 70o. -from Authors
• McEwen, A. S. (1988). Global color and albedo variations on Io. Icarus, 73(3), 385-426.
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  Abstract: Three multispectral mosaics of Io have been produced from Voyager imaging data: a global mosaic from each of the Voyager 1 and Voyager 2 data sets and a high-resolution mosaic of the region surrounding the volcano Ra Patera. The mosaics are maps of normal albedo and color in accurate geometric map formats. Io's photometric behavior, mapped with a two-image technique, is spatially variable, especially in the bright white areas. The disk-integrated color and albedo of the satellite have been remarkably constant over recent decades, despite the volcanic activity and the many differences between Voyager 1 and 2 images (acquired just 4 months apart). This constancy is most likely due to the consistent occurrence of large Pele-type plumes with relatively dark, red deposits in the region from long 240 to 360°. A transient brightening southeast of Pele during the Voyager 1 encounter was probably due to real changes in surface and/or atmospheric materials, rather than to photometric behavior. The intrinsic spectral variability of Io, as seen in a series of two-dimensional histograms of the multispectral mosaics, consists of continuous variation among three major spectral end members. The data were mapped into five spectral units to compare them with laboratory measurements of candidate surface materials and to show the planimetric distributions. Unit 1 is best fit by the spectral reflectance of ordinary elemental sulfur, and it is closely associated with the Peletype plume deposits. Unit 2 is strongly confined to the polar caps above about latitude ±50°, but its composition is unknown. Unit 5 is probably SO2 with relatively minor contamination; it is concentrated in the equatorial region and near the long-lived Prometheus-type plumes. Units 3 and 4 are gradational between units 1 and 5. In addition to SO2 and elemental sulfur, other plausible components of the surface are polysulfur oxides, FeCl2, Na2S, and NaHS. © 1988.
• McEwen, A. S. (1988). Topics in planetary geology. Ph.D. thesis. Array.
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  Abstract: This dissertation consists of three parts. Part I describes the global color and albedo variations on the surface of Jupiter's moon Io. The temporal variability of Io's surface over short time scales is marked, yet the long-term hemispherical patterns have been remarkably constant. Part II is an analysis of the global distribution, abundance, and stability of sulfur dioxide on Io. Mixing models with three end-members indicate ~30% coverage of Io's total optical surface by SO2. Part III is a computer modeling study of the kinetics of sediment gravity flows and application to the lahars, avalanche, pyroclastic flows, and blast surge of Mount St. Helens, Washington. The best models for the lahars and avalanche result from the use of a Bingham model, whereas the pyroclastic flows require the addition of turbulent resistance mechanisms. Decompression of the blast surge must have contributed, along with the force of gravity, to driving the 20-30 km travel distance of the cloud against the resistance from turbulence against the ground and air drag. -from Author
• McEwen, A. S., Johnson, T. V., Matson, D. L., & Soderblom, L. A. (1988). The global distribution, abundance, and stability of SO₂ on Io. Icarus, 75(3), 450-478.
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  Abstract: Sulfur dioxide distribution and abundances, bolometric hemispheric albedos, and passive surface temperatures on Io are modeled and mapped globally from Voyager multispectral mosaics, Earth-based spectra, and photometric descriptions. Photometric models indicate global average values for regolith porosity of 75-95% and macroscopic roughness with a mean slope angle of ∼30°. Abundances of SO2 suggested by observations at uv-visible wavelengths and at 4.08 μm are partially reconciled by intimate-mixing models; 30-50% SO2 coverage of the integral disk is indicated. Three major spectral end members, with continuous mixing, are recognized from the Voyager multispectral mosaics; one of these end members is identified as SO2. Intimate-mixing models with the three spectral end members are used to produce abundance maps for the optical surface; ∼30% of Io's total optical surface consists of SO2. The SO2 is concentrated in the bright equatorial band is relatively deficient in the region of Pele-type volcanic eruptions (long 240°-360°°) and the polar regions. Temperatures are computed to vary over a 40°K range, at the same illumination angle, according to variations in surface bolometric hemispheric albedo. The brightest (and locally coldest) areas correspond to areas rich in SO2 and are concentrated in an equatorial band (±30° lat), but many small cold patches occur elsewhere. These cold patches have radiative equilibrium temperatures ≤120°K at the subsolar point, resulting in SO2 saturation vapor pressures ≤10-8 bar. Midlatitude areas and the region of Pele-type plume eruptions are generally warmer (due to lower albedos). These results for surface temperatures and SO2 abundances and distribution support the regional coldtrapping model for the surface and atmospheric SO2 presented by F. P. Fanale, W. B. Banerdt, L. S. Elson, T. V. Johnson, and R. W. Zurek (1982, In Satellites of Jupiter (D. Morrison, Ed.), pp. 756-781, Univ. of Arizona Press, Tucson), although the region of Pele-type volcanic eruptions may be better characterized by the regolith coldtrapping/volcanic-venting model of D. L. Matson and D. B. Nash (1983, J. Geophys. Res. 88, 4771-4783). The bright equatorial band is especially effective at slowing the formation of polar caps of SO2, both by reducing the sublimation rate near the subsolar point and by coldtrapping the SO2 in the equatorial region, so that competing processes of sputtering and volcanic resurfacing may prevent the formation of polar SO2 caps. © 1988.
• Mcewen, A. S. (1986). Tidal reorientation and the fracturing of Jupiter's moon Europa. Nature, 321(6065), 49-51.
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  Abstract: The most striking characteristic of Europa is the network of long linear albedo markings over the surface, suggestive of global-scale tectonic processes. Various explanations for the fractures have been proposed: Freezing and expansion of an early liquid water ocean1, planetary expansion due to dehydration of hydrated silicates2, localization by weak points in the crust generated by impacts3, and a combination of stresses due to planetary volume change and tidal distortions from orbital recession and orbital eccentricity4,5. Calculations by Yoder6 and Greenberg and Weidenschilling7 have shown that Europa may rotate slightly more rapidly than the synchronous rate, with a rotation period (reorientation through 360°) ranging from 20 to >103 yr if a liquid mantle is present, or up to 1010 yr if the satellite is essentially solid7. Helfen-stein and Parmentier8 modelled the stresses due to nonsynchronous rotation, and concluded that this could explain the long fractures in part of the anti-jovian hemisphere. In this note, I present a global map of lineaments with long arc lengths (>20° or 550 km), and compare the lineament orientations to the tensile stress trajectories due to tidal distortions (changes in the lengths of three principal semiaxes) and to nonsynchronous rotation (longitudinal reorientation of two of the principal semiaxes). An excellent orthogonal fit to the lineaments is achieved by the stresses due to nonsynchronous rotation with the axis radial to Jupiter located 25° east of its present position. This fit suggests that nonsynchronous rotation occurred at some time in Europa's history. © 1986 Nature Publishing Group.
• Moore, J. M., McEwen, A. S., Albin, E. F., & Greeley, R. (1986). Topographic evidence for shield volcanism on Io. Icarus, 67(1), 181-183.
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  Abstract: A shield volcano has been identified on Io, based on photoclinometrically determined slope values and planimetric presentation. The slope values (typically 10°) and 2.5 km height of the volcano imply that it is composed of a material with the mechanical properties of basaltic rock. The height of the volcano may also indicate a minimum value of ∼40 km for the thickness of the local lithosphere. © 1986.
• McEwen, A. S., Matson, D. L., Johnson, T. V., & Soderblom, L. A. (1985). Volcanic hot spots on Io: correlation with low- albedo calderas.. Journal of Geophysical Research, 90(B14), 12,345-12,379.
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  Abstract: Comparison of Voyager imaging data with Voyager 1 infrared observations (IRIS) and with ground-based telescopic infrared data demonstrates a strong correlation between hot spots and low-albedo features (reflectivity = or
• McEwen, A. S., & Soderblom, L. A. (1983). Two classes of volcanic plumes on Io. Icarus, 55(2), 191-217.
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  Abstract: Comparison of Voyager 1 and Voyager 2 images of the south polar region of Io has revealed that a major volcanic eruption occured there during the period between the two spacecraft encounters. An annular deposit ∼1400 km in diameter formed around the Aten Patera caldera (311°W, 48°S), the floor of which changed from orange to red-black. The characteristics of this eruption are remarkably similar to those described earlier for an eruption centered on Surt caldera (338°W, 45°N) that occured during the same period, also at high latitude, but in the north. Both volcanic centers were evidently inactive during the Voyager 1 and 2 encounters but were active sometime between the two. The geometric and colorimetric characteristics, as well as scale of the two annular deposits, are virtually identical; both resemble the surface features formed by the eruption of Pele (255°W, 18°S). These three very large plume eruptions suggest a class of eruption distinct from that of six smaller plumes observed to be continously active by both Voyagers 1 and 2. The smaller plumes, of which Prometheus is the type example, are longer-lived, deposit bright, whitish material, erupt at velocities of ∼0.5 km sec-1, and are concentrated at low latitudes in an equatorial belt around the satellite. The very large Pele-type plumes, on the other hand, are relatively short-lived, deposit darker red materials, erupt at ∼1.0 km sec-1, and (rather than restricted to a latitudinal band) are restricted in longitude from 240° to 360°W. Both direct thermal infrared temperature measurements and the implied color temperatures for quenched liquid sulfur suggest that hot spot temperatures of ∼650°K are associated with the large plumes and temperatures 650°K by hot or even molten silicates in the black sulfur zone seems the best explanation for the large plume class. The large Pele-type plumes are apparently concentrated in a region of the satellite in which a thinner sulfur-rich crust overlies the tidally heated silicate lithosphere, so the black sulfur zone may be fairly shallow in this region. The Prometheus-type plumes are possibly confined to the equatorial belt by some process that concentrates SO2 fluid in the equatorial crust. © 1983.

Proceedings Publications

• Daubar, I., Garcia, R., Stott, A., Fernando, B., Collins, G., Dundas, C., W{\'ojcicka}, N., Zenh{\"ausern}, G., McEwen, A., St{\"ahler}, S., Golombek, M., Charalambous, C., Giardini, D., Lognonn{\'e}, P., & Banerdt, W. (2024, jul). Seismically Detected Cratering on Mars: Enhanced Recent Impact Flux?. In Mars Interior and Geophysics After InSight, 3060.
• Dundas, C., Daubar, I., McEwen, A., & Bray, V. (2024, mar). Controls on the Formation of Secondary Markings Around New Impacts on Mars. In 55th Lunar and Planetary Science Conference, 3040.
• Landis, M., Byrne, S., Daubar, I., Dundas, C., Hayne, P., Herkenhoff, K., McEwen, A., Rangarajan, V., Russell, P., Shore, G., Sutton, S., Tornabene, L., & Wilcoski, A. (2024, jul). ``Polar'' Opposites: Distinct Recent Activity at Mars' Polar Ice Deposits Suggests Divergent Histories. In LPI Contributions, 3007.
• Lognonn{\'e}, P., Banerdt, W., Bill, C., Charalambous, C., Collins, G., Daubar, I., Drilleau, M., Kim, D., Fernando, B., Froment, M., Garcia, R., Karakostas, F., Kawamura, T., Larmat, C., McEwen, A., Panning, M., Posiolova, L., Malin, M., Miljikovic, K., , Rolland, L., et al. (2024, jul). Multi-Messenger Impact Seismology with SEIS-Recorded Seismic and Acoustics Signals and High-Resolution Impact Imaging. In Mars Interior and Geophysics After InSight, 3060.
• McEwen}, A., Byrne, S., & Team, {. (2024, jul). The Future of High-Resolution Orbital Imaging of Mars. In LPI Contributions, 3007.
• McEwen}, A., Byrne, S., & Team, {. (2024, mar). The Future of High-Resolution Orbital Imaging of Mars. In 55th Lunar and Planetary Science Conference, 3040.
• Mills, M., Bickel, V., McEwen, A., & Valantinas, A. (2024, mar). A Global Dataset of Pitted Cones on Mars Using a Deep Learning Identification Approach. In 55th Lunar and Planetary Science Conference, 3040.
• Mills, M., McEwen, A., Bickel, V., & Valantinas, A. (2024, jul). Pitted Cones on the Vastitas Borealis Formation. In 8th International Conference on Mars Polar Science and Exploration, 3064.
• Dundas, C., Mellon, M., Posiolova, L., Miljkovic, K., Collins, G., Tornabene, L., Rangarajan, V., Golombek, M., Warner, N., Daubar, I., Byrne, S., McEwen, A., Seelos, K., Viola, D., Bramson, A., & Speth, G. (2023, mar). The Limits of Ice on Mars: Ice Exposed by a Large New Impact Crater at 35\textordmasculineN. In LPI Contributions, 2806.
• McEwen, A. S., Haapala, A. F., Keszthelyi, L. P., & Mandt, K. E. (2023, jan). Outstanding Questions and Future Observations of Io. In Astrophysics and Space Science Library, 468.
• McEwen}, A., Byrne, S., Hanson, C., & Team, {. (2023, mar). The High-Resolution Imaging Science Experiment (HiRISE) in the MRO Extended Science Phases (2009-2022). In LPI Contributions, 2806.
• Mills, M., McEwen, A., & Hughes, A. (2023, mar). Spatial Distribution of Pitted Cones in Utopia Planitia Follow Basin Contours. In LPI Contributions, 2806.
• Turtle, E., McEwen, A., Patterson, G., Ernst, C., Elder, C., Tucker, O., Torres, I., Thomas, N., Sutton, S., Stickle, A., Soderblom, J., Slack, K., Seifert, H., Robbins, G., Quick, L., Prockter, L., Pommerol, A., Phillips, C., Nimmo, F., , Niewola, J., et al. (2023, mar). The Europa Imaging System (EIS) Flight Instruments in Spacecraft and Environmental Testing for Europa Clipper. In LPI Contributions, 2806.
• Valantinas, A., Thomas, N., Pommerol, A., Hauber, E., Bickel, V., Schorghofer, N., McEwen, A., Almeida, M., Read, M., Rangarajan, V., Munaretto, G., Pajola, M., Re, G., & Cremonese, G. (2023, mar). Tropical Frost on Martian Volcanoes. In LPI Contributions, 2806.
• Daubar, I., Dundas, C., McEwen, A., Gao, A., Wexler, D., Piqueux, S., Collins, G., Miljkovi{\'c}, K., Neidhart, T., Eschenfelder, J., Bart, G., Wagstaff, K., Doran, G., Posiolova, L., Speth, G., Susko, D., Werynski, A., & Malin, M. (2022, mar). New Craters on Mars: Results from a Complete Catalog of 1,203 Recent Impacts. In 53rd Lunar and Planetary Science Conference, 2678.
• Dicecca, A., Tornabene, L., Neish, C., Thomas, N., Cremonese, G., & McEwen, A. (2022, dec). Mapping Fe-Mg Phyllosilicates and adjacent stratigraphy in Her Desher Vallis and Nirgal Vallis in northwest Noachis Terra, Mars. In AAS/Division for Planetary Sciences Meeting Abstracts, 54.
• Hansen, C., Cesar, C., Conway, S., Pommerol, A., Portyankina, G., Thomas, N., McEwen, A., & Perry, J. (2022, mar). Searching for Seasonal Jets on Mars in CaSSIS and HiRISE Images Over Multiple Mars Years. In 53rd Lunar and Planetary Science Conference, 2678.
• McEwen, A., Byrne, S., & Hansen, C. (2022, sep). Results from MRO's High Resolution Imaging Science Experiment (HiRISE), 2006-2022. In European Planetary Science Congress.
• McEwen, A., Carter, L., DellaGiustina, D., Kestay, L., Denevi, B., Haapala, A., Westlake, J., Park, R., Khurana, K., Thomas, N., Wurz, P., & Helbert, J. (2022, sep). A NASA New Frontiers Mission Concept to Io. In European Planetary Science Congress.
• Mcewen, A., & Robinson, M. (2022, jul). Monitoring Human Activity on the Moon and Mars. In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Mcewen, A., & Thomas, N. (2022, jul). Future Exploration of Io. In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Mcewen, A., Block, K., Schaller, C., Baugh, N., Hansen, C., Byrne, S., Ebben, T., Bergstrom, J., & Delamere, W. A. (2022, jul). The Future of the High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter (MRO). In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Mills, M., McEwen, A., & Hughes, A. (2022, mar). Modelling Upwelling Martian Fluids and Resurfacing in Utopia Planitia. In 53rd Lunar and Planetary Science Conference, 2678.
• Munaretto, G., Pajola, M., Lucchetti, A., Cremonese, G., Simioni, E., Re, C., Bertoli, S., Tornabene, L., McEwen, A., Becerra, P., Rangarajan, V., Valantinas, A., Pommerol, A., Thomas, N., & Portyankina, G. (2022, mar). Multiband Photometry of Martian Recurring Slope Lineae (RSL) and Dust-Removed Features at Horowitz Crater, Mars from TGO/CaSSIS Colour Observations. In 53rd Lunar and Planetary Science Conference, 2678.
• Sutton, S., Chojnacki, M., McEwen, A., Kirk, R., Dundas, C., Schaefer, E., Conway, S., Diniega, S., Portyankina, G., Landis, M., Baugh, N., Heyd, R., Byrne, S., Tornabene, L., Ojha, L., & Hamilton, C. (2022, mar). Revealing Active Mars with HiRISE Digital Terrain Models and Orthoimages. In 53rd Lunar and Planetary Science Conference, 2678.
• Turtle, E., & McEwen, A. (2022, sep). The Europa Imaging System (EIS): High-Resolution, 3-D Insight into Europa's Geology, Ice Shell, and Potential for Current Activity. In European Planetary Science Congress.
• Turtle, E., & Mcewen, A. (2022, jul). Exploring an Ocean World with the Europa Imaging System (EIS) on Europa Clipper. In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Barnes, J. W., Hayes, A. G., Soderblom, J. M., MacKenzie, S. M., Hofgartner, J. D., Lorenz, R. D., Turtle, E. P., Radebaugh, J., Burr, D., Lora, J., Neumann, G., Vance, S., Lopes, R., Nixon, C., Corlies, P., Regoli, L., Sciamma-O'Brien, E., Schindhelm, R., Rodriguez, S., , Coll, P., et al. (2021, may). New Frontiers Titan Orbiter. In Bulletin of the American Astronomical Society, 53.
• Beauchamp, P., Moore, J., Spilker, L., Hendrix, A., Hurford, T., McEwen, A., Cable, M., Rymer, A., Schmidt, B., Bowman, J., Spilker, T., Edgington, S., & Sayanagi, K. (2021, may). Technologies for Ocean Worlds. In Bulletin of the American Astronomical Society, 53.
• Cable, M., MacKenzie, S., Neveu, M., Hoehler, T. M., Hendrix, A. R., Eigenbrode, J., Postberg, F., Porco, C., Glein, C. R., Spilker, L., Hofmann, A. E., McEwen, A., Waite, J. H., Wurz, P., Helbert, J., Anbar, A. D., Vera, J., & N{\'u\~nez}, J. (2021, may). The Case for a Return to Enceladus. In Bulletin of the American Astronomical Society, 53.
• Daubar, I., Beyer, R. A., Hamilton, V., McEwen, A., Bardabelias, N., Brooks, S. M., Byrne, P. K., Byrne, S., Calef, I., Castillo-Rogez, J., Diniega, S., Gulick, V. C., Hamilton, C. W., Jha, D., Keresztur, A., Nunn, C., Schenk, P., & Sutton, S. S. (2021, may). Extended Missions in Planetary Science: Impacts to Science and the Workforce. In Bulletin of the American Astronomical Society, 53.
• Denk}, T., McEwen, A., Helbert, J., & Team, {. (2021, mar). Survey of Jovian Irregular Moons with IVO (Io Volcano Observer). In 52nd Lunar and Planetary Science Conference.
• Diniega, S., Bramson, A., Buratti, B., Buhler, P., Burr, D., Chojnacki, M., Conway, S., Daubar, I., Dinwiddie, C. L., Galofre, A. G., Jackson, B., McEwen, A., Lap{\^otre}, M., Levy, J., McKeown, L., Piqueux, S., Portyankina, A., Rafkin, S., Silvestro, S., , Smith, I., et al. (2021, may). Mars as a ``natural laboratory'' for studying surface activity on a range of planetary bodies. In Bulletin of the American Astronomical Society, 53.
• Dundas, C., Byrne, S., Chojnacki, M., Diniega, S., Daubar, I., Hamilton, C., Hansen, C., McEwen, A., Portyankina, G., & Sizemore, H. (2021, may). Current Activity on the Martian Surface: A Key Subject for Future Exploration. In Bulletin of the American Astronomical Society, 53.
• Golombek, M., Williams, N., Wooster, P., McEwen, A., Putzig, N., Bramson, A., Head, J., Heldmann, J., Marinova, M., & Beaty, D. (2021, mar). SpaceX Starship Landing Sites on Mars. In 52nd Lunar and Planetary Science Conference.
• Hendrix, A., Hurford, T. A., Barge, L. M., Bland, M. T., Bowman, J. S., Brinckerhoff, W., Buratti, B., Cable, M., Castillo-Rogez, J., Collins, G., Cooper, J. F., Diniega, S., German, C., Hayes, A., Hoehler, T., Hosseini, S., Howett, C., McEwen, A., Neish, C., , Neveu, M., et al. (2021, may). Ocean Worlds: A Roadmap for Science and Exploration. In Bulletin of the American Astronomical Society, 53.
• Hendrix, A., Hurford, T., Barge, L. M., Bland, M. T., Bowman, J. S., Brinckerhoff, W., Buratti, B., Cable, M., Castillo-Rogez, J., Collins, G., Cooper, J. F., Diniega, S., German, C., Hayes, A., Hoehler, T., Hosseini, S., Howett, C., McEwen, A., Neish, C., , Neveu, M., et al. (2021, may). Ocean Worlds: Science Goals for the Next Decade. In Bulletin of the American Astronomical Society, 53.
• Keane, J., Ahern, A. A., Bagenal, F., Barr, M., Basu, K. o., Becerra, P., Bertrand, T., Beyer, R. A., Bierson, C. J., Bland, M. T., Breuer, D., Davies, A. G., Kleer, K., Pater, I., DellaGiustina, D. N., Denk, T., Echevarria, A., Elder, C. M., Feaga, L. M., , Grava, C., et al. (2021, may). Recommendations for Addressing Priority Io Science in the Next Decade. In Bulletin of the American Astronomical Society, 53.
• Keane, J., Ahern, A. A., Bagenal, F., Barr, M., Basu, K. o., Becerra, P., Bertrand, T., Beyer, R. A., Bierson, C. J., Bland, M. T., Breuer, D., Davies, A. G., Kleer, K., Pater, I., DellaGiustina, D. N., Denk, T., Echevarria, A., Elder, C. M., Feaga, L. M., , Grava, C., et al. (2021, may). The Science Case for Io Exploration. In Bulletin of the American Astronomical Society, 53.
• Keszthelyi, L., & McEwen, A. (2021, mar). Measuring Ionian Lava Temperatures via VNIR Color Imaging. In 52nd Lunar and Planetary Science Conference.
• Landis, M., Dundas, C., Hayne, P., McEwen, A., Daubar, I., Byrne, S., Sutton, S., Britton, A., & Herkenhoff, K. (2021, aug). Two Dated Small Impacts on the South Polar Layered Deposits, Mars, and Implications for Near-Surface Properties. In 12th Planetary Crater Consortium Meeting, 12.
• McEwen, A., & Thomas, N. (2021, jan). The Io Volcano Observer (IVO): Investigating the Interior of a Tidally Heated World. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• McEwen, A., Keszthelyi, L., Mandt, K., & Team, I. (2021, mar). The Io Volcano Observer (IVO). In 52nd Lunar and Planetary Science Conference.
• Mills, M., & McEwen, A. (2021, jun). Preliminary Regional Geologic Map of Utopia Planitia in the Potential Tianwen-1 Landing Region. In 2021 Annual Meeting of Planetary Geologic Mappers, 2610.
• Moore, J., Spilker, L., Bowman, J., Cable, M., Edgington, S., Hendrix, A., Hofstadter, M., Hurford, T., Mandt, K., McEwen, A., Paty, C., Quick, L., Rymer, A., Sayanagi, K., Schmidt, B., & Spilker, T. (2021, may). Exploration Strategy for the Outer Planets 2023-2032: Goals and Priorities. In Bulletin of the American Astronomical Society, 53.
• Munaretto, G., Pajola, M., Lucchetti, A., Cremonese, G., Simioni, E., Re, C., Bertoli, S., Tornabene, L., McEwen, A. S., Becerra, P., Rangarajan, V. G., Valantinas, A., Pommerol, A., Thomas, N., & Portyankina, G. (2021, sep). Multiband photometry of Martian Recurring Slope Lineae (RSL) and dust-removed features at Horowitz crater, Mars from TGO/CaSSIS color observations. In European Planetary Science Congress.
• Munaretto, G., Pajola, M., Lucchetti, A., Cremonese, G., Simioni, E., Re, C., Bertoli, S., Tornabene, L., McEwen, A., Becerra, P., Rangarajan, V., Valantinas, A., Portyankina, G., & Thomas, N. (2021, mar). Multiband Photometry of RSL and Dust-Devil Tracks from CaSSIS Color Images. In 52nd Lunar and Planetary Science Conference.
• Murchie, S., Arvidson, R. E., Bishop, J. L., Calvin, W. M., Carter, J., Christian, J., Clark, R. N., Dundas, C. M., Ehlmann, B. L., Fox, V. K., Fraeman, A. A., Goudge, T. A., Horgan, B. H., Hughes, M. N., Leask, E. K., McEwen, A. S., Mustard, J. F., Parente, M., Powell, K. E., , Seelos, F. P., et al. (2021, may). Maximizing the Science and Resource Mapping Potential of Orbital VSWIR Spectral Measurements of Mars. In Bulletin of the American Astronomical Society, 53.
• Pommier, A., McEwen, A., & Keszthelyi, L. (2021, mar). Investigating the Interior of Terrestrial Planets and Moons Using Electrical Laboratory Measurements. In 52nd Lunar and Planetary Science Conference.
• Sayanagi, K., Wong, M., Becker, T., Brooks, S., Brueshaber, S. R., Dahl, E. K., Pater, I., Ebert, R. W., El, M. M., Fletcher, L. N., McEwen, A., Molyneux, P. M., Nenon, Q., Orton, G. S., Paranicas, C., Showalter, M. R., Spilker, L. J., Tiscareno, M., Westlake, J. H., , Moore, L., et al. (2021, may). Priority Questions for Jupiter System Science in the 2020s and Opportunities for Europa Clipper. In Bulletin of the American Astronomical Society, 53.
• Smith, I., Calvin, W., Smith, D., Hansen, C., Diniega, S., McEwen, A., Thomas, N., Banfield, D., Titus, T., Becerra, P., Kahre, M., Forget, F., Hecht, M., Byrne, S., Hvidberg, C., Hayne, P., Head, I., Mellon, M., Horgan, B., , Mustard, J., et al. (2021, may). Solar-System-Wide Significance of Mars Polar Science. In Bulletin of the American Astronomical Society, 53.
• Tornabene, L., Becerra, P., Cesar, C., Conway, S., Cremonese, G., Lucchetti, A., Munaretto, G., McEwen, A., Pajola, M., Patel, M., Perry, J., Pommerol, A., Rangarajan, V., Seelos, F., Thomas, N., & Wray, J. (2021, mar). Potential Detection of Exposed Martian Water Ice with Multispectral Images from the 2016 Exomars TGO Colour and Stereo Surface Imaging System (CaSSIS). In 52nd Lunar and Planetary Science Conference.
• Valantinas}, A., Becerra, P., Tornabene, L., Pommerol, A., Hauber, E., McEwen, A., Munaretto, G., Rangarajan, V., Schorghofer, N., Thomas, N., Cremonese, G., & Team, {. (2021, mar). Colour and Multi-Angular Observations of Martian Slope Streaks. In 52nd Lunar and Planetary Science Conference.
• Vorburger, A., Thomas, N., McEwen, A., Wurz, P., Mandt, K., Horst, S., & Davies, A. (2021, jan). Monte-Carlo model of Io's atmosphere for the IVO mission. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• Wurz, P., Vorburger, A., McEwen, A., Mandt, K., Davies, A., H{\"orst}, S., & Thomas, N. (2021, apr). Modelling of Io's Atmosphere for the IVO Mission. In EGU General Assembly Conference Abstracts.
• Wurz, P., Vorburger, A., McEwen, A., Mandt, K., Davies, A., H{\"orst}, S., & Thomas, N. (2021, mar). Measurement of Io's Atmosphere During the IVO Mission. In 52nd Lunar and Planetary Science Conference.
• Bray, V., Artemieva, N., Atwood-Stone, C. .., McElwaine, J., McEwen, A., Richardson, J., Neish, C., & Martin-Wells, K. .. (2020, mar). Ejected Impact Melt Flows on the Moon: Observations and Models. In Lunar and Planetary Science Conference.
• Chojnacki, M., McEwen, A., Byrne, S., Hansen, C., Daubar, I., Beyer, R., & McArthur, G. (2020, mar). HiWish: The High Resolution Imaging Science Experiment (HiRISE) Suggestion Tool. In Lunar and Planetary Science Conference.
• Conway, S., Pommerol, A., Raack, J., Philippe, M., Mcewen, A., Thomas, N., & Cremonese, G. (2020, may). Gullies on Mars and seasonal ices visualised using the Colour and Stereo Surface Imaging System (CaSSIS). In EGU General Assembly Conference Abstracts.
• Daubar, I., Gao, A., Wexler, D., Eschenfelder, J., Neidhart, T., Collins, G., Miljkovic, K., Dundas, C., McEwen, A., Piqueux, S., Malin, M., & Posiolova, L. (2020, oct). New Craters on Mars: An Updated Catalog. In AAS/Division for Planetary Sciences Meeting Abstracts, 52.
• Daubar}, I., Lognonn{\'e}, P., Teanby, N., Clinton, J., Malin, M., McEwen, A., Miljkovi{\'c}, K., Banks, M., Collins, G., & Group, {. (2020, mar). Searching for Impacts with Insight: A New Approach. In Lunar and Planetary Science Conference.
• Denk}, T., McEwen, A., Helbert, J., & Team, {. I. (2020, sep). Survey of Irregular Jovian Moons with IVO. In European Planetary Science Congress.
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• {McEwen}, A., {Daubar}, I., {Ivanov}, B., {Oberst}, J., {Malhotra}, R., {JeongAhn}, Y., , S. (2015, mar). "{Current Impact Rate on Earth, Moon, and Mars}". In Lunar and Planetary Science Conference, 46.
• {McEwen}, A., {Turtle}, E., , N. (2015, apr). "{The Io Volcano Observer (IVO) for NASA Discovery 2015}". In EGU General Assembly Conference Abstracts, 17.
• {Milazzo}, M., {Herkenhoff}, K., {Becker}, K., {Russell}, P., {Delamere}, A., , A. (2015, jun). "{MRO/HiRISE Radiometric Calibration Update}". In Second Planetary Data Workshop, 1846.
• {Milazzo}, M., {Herkenhoff}, K., {Becker}, K., {Russell}, P., {Delamere}, A., , A. (2015, mar). "{MRO/HiRISE Radiometric Calibration Update}". In Lunar and Planetary Science Conference, 46.
• {Schaefer}, E., , A. (2015, jun). "{How Long is that Polygon?: A Centerline Algorithm}". In Second Planetary Data Workshop, 1846.
• {Schaefer}, E., {McEwen}, A., {Mattson}, S., , L. (2015, mar). "{Quantifying the Behavior of Recurring Slope Lineae (RSL)}". In Lunar and Planetary Science Conference, 46.
• {Sutton}, S., {Heyd}, R., {Fennema}, A., {McEwen}, A., {Kirk}, R., {Howington-Kraus}, E., , A. (2015, jun). "{HiRISE Digital Terrain Models: Updates and Advances}". In Second Planetary Data Workshop, 1846.
• {Tornabene}, L., {Osinski}, G., {Barlow}, N., {Bray}, V., {Caudill}, C., {D'Aoust}, B., {Ding}, N., {Hopkins}, R., {Nuhn}, A., {Mayne}, A., , A. (2015, sep). "{Meter- to Decameter-Scale Characteristics of Central Uplifts Revealed by the Mars Reconnaissance Orbiter}". In Bridging the Gap III: Impact Cratering In Nature, Experiments, and Modeling, 1861.
• {Viola}, D., {McEwen}, A., , C. (2015, oct). "{Mid-Latitude Ice as a Target for Human Exploration, Astrobiology, and In-Situ Resource Utilization}". In First Landing Site/Exploration Zone Workshop for Human Missions to the Surface of Mars, 1879.
• {Viola}, D., {McEwen}, A., {Dundas}, C., , S. (2015, mar). "{Inferring the Subsurface Structure of Double Layer Ejecta Craters from Overlying Secondary Craters}". In Lunar and Planetary Science Conference, 46.
• {Beaty}, D., {Rummel}, J., {Jones}, M., {Bakermans}, C., {Barlow}, N., {Boston}, P., {Chevrier}, V., {Clark}, B., {de Vera}, J., {Gough}, R., {Hallsworth}, J., {Head}, J., {Hipkin}, V., {Kieft}, T., {McEwen}, A., {Mellon}, M., {Mikucki}, J., {Nicholson}, W., {Omelon}, C., , {Peterson}, R., et al. (2014, jul). "{An Updated Planetary Protection Special Regions on Mars Analysis by the MEPAG Special Regions-Science Analysis Group 2}". In Eighth International Conference on Mars, 1791.
• {D'Aoust}, B., {Tornabene}, L., {Osinski}, G., , A. (2014, jul). "{Morphological and Structural Mapping of Massive-Fractured Bedrock in Crater Central Uplifts on Mars}". In Eighth International Conference on Mars, 1791.
• {Daubar}, I., {McEwen}, A., {Byrne}, S., {Kreslavsky}, M., {Saper}, L., , M. (2014, jul). "{New Dated Impacts on Mars and an Updated Current Cratering Rate}". In Eighth International Conference on Mars, 1791.
• {Dundas}, C., {Byrne}, S., , A. (2014, jul). "{Clean Ground Ice on Mars: Evidence from Spacecraft, Fresh Craters, and Thermokarst}". In Eighth International Conference on Mars, 1791.
• {Dundas}, C., {Diniega}, S., {McEwen}, A., {Hansen}, C., , S. (2014, jul). "{Eight Years of Gully Activity on Mars}". In Eighth International Conference on Mars, 1791.
• {Hansen}, C., {Byrne}, S., {Bourke}, M., {Bridges}, N., {Diniega}, S., {Dundas}, C., {McEwen}, A., {Mellon}, M., {Pommerol}, A., {Portyankina}, G., , N. (2014, jul). "{New Advances in Understanding Northern Seasonal Processes on Mars}". In Eighth International Conference on Mars, 1791.
• {Keszthelyi}, L., {Jaeger}, W., {Dundas}, C., , A. (2014, jul). "{A New Paradigm for, and Questions About, Volcanism on Mars}". In Eighth International Conference on Mars, 1791.
• {Mass{'e}}, M., {Conway}, S., {Gargani}, J., {Patel}, M., {McEwen}, A., {Vincendon}, M., {Poulet}, F., {Jouannic}, G., , L. (2014, jul). "{Geomorphological Impact of Transient Liquid Water Formation on Mars}". In Eighth International Conference on Mars, 1791.
• {Mattson}, S., {McEwen}, A., {Kirk}, R., {Howington-Kraus}, E., {Chojnacki}, M., {Runyon}, K., {Cremonese}, G., , C. (2014, may). "{Martian Landscapes in Motion}". In EGU General Assembly Conference Abstracts, 16.
• {McEwen}, A. (2014, jul). "{Active Modern Mars: What is Happening on the Surface Today?}". In Eighth International Conference on Mars, 1791.
• {McEwen}, A., {Bridges}, N., {Byrne}, S., {Chevrier}, V., {Chojnacki}, M., {Conway}, S., {Cull}, S., {Dundas}, C., {Gulick}, V., {Hansen}, C., {Masse}, M., {Mattson}, S., {Murchie}, S., {Ojha}, L., {Paige}, D., {Pommerol}, A., {Schaefer}, E., {Thomas}, N., {Toigo}, A., , {Viola}, D., et al. (2014, jul). "{Recurring Slope Lineae on Mars}". In Eighth International Conference on Mars, 1791.
• {McEwen}, A., {Byrne}, S., {Chevrier}, V., {Chojnacki}, M., {Dundas}, C., {Masse}, M., {Mattson}, S., {Ojha}, L., {Pommerol}, A., {Toigo}, A., , J. (2014, may). "{Recurring Slope Lineae and Future Exploration of Mars}". In EGU General Assembly Conference Abstracts, 16.
• {McEwen}, A., {Sandel}, B., , N. (2014, may). "{Lyman Alpha Camera for Io's SO2 atmosphere and Europa's water plumes}". In EGU General Assembly Conference Abstracts, 16.
• {Pommerol}, A., {Thomas}, N., {Jost}, B., {Beck}, P., {Okubo}, C., {McEwen}, A., {Mass{'e}}, M., , M. (2014, jul). "{Spectro-Photometry of Mars Soil Analogs}". In Eighth International Conference on Mars, 1791.
• {Schaefer}, E., {McEwen}, A., {Mattson}, S., , L. (2014, mar). "{Quantifying Recurring Slope Lineae in Space and Time}". In Lunar and Planetary Science Conference, 45.
• {Thomas}, N., {Cremonese}, G., {Banaszkiewicz}, M., {Bridges}, J., {Byrne}, S., {da Deppo}, V., {Debei}, S., {El-Maarry}, M., {Hauber}, E., {Hansen}, C., {Ivanov}, A., {Kestay}, L., {Kirk}, R., {Kuzmin}, R., {Mangold}, N., {Marinangeli}, L., {Markiewicz}, W., {Massironi}, M., {McEwen}, A., , {Okubo}, C., et al. (2014, jul). "{The Colour and Stereo Surface Imaging System (CaSSIS) for ESA's Trace Gas Orbiter.}". In Eighth International Conference on Mars, 1791.
• {Viola}, D., {McEwen}, A., {Dundas}, C., , S. (2014, jul). "{Expanded Craters in Arcadia Planitia: Evidence for $gt$20 Myr Old Subsurface Ice}". In Eighth International Conference on Mars, 1791.
• Ding, N., Bray, V., McEwen, A., Mattson, S., Chojnakci, M., Tornabene, L., & Okumbo, C. (2013, August). Mapping the Ritchey Crater Central Uplift, Mars. In Large Meteorite Impacts and Planetary Evolution V Proceedings.
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  LPI Contribution No. 1737
• Nuhn, A., Tornabene, L., Osinski, G., & McEwen, A. (2013, August). Mapping of Layered Bedrock in Martian Craters: Insights in to Central Uplift Formation. In Large Meteorite Impacts and Planetary Evolution V Proceedings.
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  LPI Contribution No. 1737
• Tornabere, L., Osinski, G., & McEwen, A. (2013, August). Meter- to Decameter-Scale Morphology of Melt Rocks, Breccias, Bedrock and Structures in Central Uplifts Revealed by the Mars Reconnaissance Orbiter. In Large Meteorite Impacts and Planetary Evolution V Proceedings.
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  LPI Contribution No. 1737

Poster Presentations

• Chojnacki, M., McEwen, A., Dundas, C., Mattson, S., Ojha, L., Byrne, S., & Wray, J. (2014, March). Geologic Context of Recurring Slope Lineae in Coprates Chasma. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/17-03/21; LPI Contribution No. 1777
• Daubar, I., Geissler, P., McEwen, A., Dundas, C., Byrne, S., Russell, P., & Bart, G. (2014, March). Changes in New Impact Blast Zones over Three Martian Years. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/17-03-21; LPI Contribution No. 1777
• Davies, A., Matson, D., McEwen, A., & Keszthelyi, L. (2014, Fall). Opportunities for Monitoring Io's Volcanic Activity in the Visible and Infrared From JUICE - It's All About (Eruption) Style. DPS meeting #44American Astronomical Society.
• Dundas, C., Diniega, S., & McEwen, A. (2014, March). Long-Term Monitoring of Martian Gully Activity with HiRISE. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/71-03/21; LPI Contribution No. 1777
• Hansen, C., Diniega, S., Bridges, N., Byrne, S., Dundas, C., & McEwen, A. (2014, March). Wind and Dry Ice: Agents of Change on Mars' North Polar Erg. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/17-03/21; LPI Contribution No. 1777
• Ivanov, B., Melosh, H., & McEwen, A. (2014, March). New Small Craters in High Resolution HiRISE Images – IV. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/17-03/21; LPI Contribution No. 1777
• Keske, A., McEwen, A., Hamilton, C., & Daubar, I. (2014, March). Distinguishing Volcanic and Fluvial Activity in Mangala Valles, Mars via Geomorphic Mapping. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/17-03/21; LPI Contribution No. 1777
• Masse, M., Beck, P., Conway, S., Gargani, J., McEwen, A., Schmitt, B., Patel, M., Jouannic, G., Ojha, L., & Pommerol, A. (2014, March). Laboratory Simulation of Martian Recurring Slope Lineae (RSL): Origin and Detectability of Liquid Brines. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/71-03/21; LPI Contribution No. 1777
• Mattson, S., Kilgallon, A., Byrne, S., McEwen, A., Herkenhoff, K., Okubo, C., Putzig, N., & Russell, P. (2014, March). Meter-Scale Pits in Mars' North Polar Layered Deposits. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/17-03/21LPI Contribution No. 1777
• Pommerol, A., Thomas, N., Jost, B., Beck, P., Okubo, C., McEwen, A., Masse, M., & El-Maarry, M. (2014, March). Visible Spectro-Photometry of Dry, Wet and Frozen Mars Soil Analogs. 45th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/17-03/21; LPI Contribution No. 1777
• Atwood-Stone, C., & McEwen, A. (2013, March). Measuring Dynamic Angle of Repose in Low Gravity Environments Using Martian Sand Dunes. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Bierhaus, E., McEwen, A., Wade, D., & Ivanov, B. (2013, March). A Fortuitous Impact Experiment at Mars. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Bray, V., Artemieva, N., Neish, C., McEwen, A., & McElwaine, J. (2013, March). Impact Melt Entrained in the Ballistic Ejecta of Lunar Craters. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Byrne, S., Becerra, P., Diniega, S., Dundas, C., Geissler, P., hansen, C., McEwen, A., Russell, P., & Thomas, N. (2013, Fall). Mars: Cold, windy and occasionally unstable. American Geophysical Union.
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  Abstract #P31C-05
• Daubar, I., McEwen, A., & Byrne, S. (2013, March). How Accurately can we Dates Recent Climate Change on Mars?. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Davies, A., Keszthelyi, L., & McEwen, A. (2013, Fall). Thermal Remote Sensing of Lava Lakes on Io and Earth. American Geophysical Union.
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  Abstract #P52A-07
• Delamere, W., McEwen, A., Li, J., Lisse, C., Bonev, B., DiSanti, M., Gibb, E., Villaneuva, G., Paganini, L., Mumma, M., & Williams, G. (2013, Fall). Comet C/2012 S1 (Ison). Central Bureau Electronic Telegrams.
• Ding, N., Bray, V., McEwen, A., Mattson, S., Okubo, C., & Tornabene, L. (2013, March). Mapping the Ritchey Crater Central Uplift. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Ding, N., Bray, V., McEwen, A., Mattson, S., Okubo, C., Chojnacki, M., & Tornabene, L. (2013, Fall). Study of Megabreccia in Ritchey Crater Central Uplift. Mars American Geophysical Union.
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  Abstract #P41F-1977
• Dundas, C., Byrne, S., McEwen, A., Mellon, M., Wu, M., Daubar, I., & Saper, L. (2013, Fall). Observations of Ice-Exposing Impacts on Mars over Three Mars Years. American Geophysical Union.
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  Abstract #P31C-07
• Hansen, C., Byrne, S., Bourke, M., McEwen, A., Pommerol, A., Portyankina, G., & Thomas, N. (2013, March). HiRISE Images and Investigation of Northern Spring on Mars. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Keske, A., McEwen, A., & Daubar, I. (2013, March). Distinguishing Volcanic and Fluvial Activity in Mangala Valles, Mars via Geomorphic Mapping. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Masse, M., Beck, P., Schmitt, B., Pollerol, A., McEwen, A., Chevrier, V., & Brissaud, O. (2013, April). Nature and origin of RSL: Spectroscopy and detectability of liquid brines in the near-infrared. EGU General Assembly. Vienna, Austria.
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  Dates: 04/07-04/12
• McEwen, A., Dundas, C., Mattson, S., toigo, A., Ojha, L., Wray, J., Chojnacki, M., Byrne, S., Murchie, S., & Thomas, N. (2013, Fall). Recurring Slope Lineae in Mid-Latitude and Equatorial Mars. American Geophysical Union.
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  Abstract #P31C-10
• McEwen, A., Dundas, C., Mattson, S., toigo, A., Ojha, L., Wray, J., Chojnacki, M., Byrne, S., Murchie, S., & Thomas, N. (2013, September). Recurring Slope Lineae (RSL) in Equatorial Mars. European Planetary Science Congress. London, UK.
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  Dates: 09/08-09/13
• Milkovich, S., Delamere, W., & McEwen, A. (2013, Fall). Observation of Comet ISON by the High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaisance Orbiter (MRO). American Geophysical Union.
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  Abstract #P24A-09
• Nuhn, A., Tornabene, L., Osinski, G., & McEwen, A. (2013, March). Decameter-Scale Morphologic and Structural Martian Mapping of Layered Bedrock in Crater Central Uplifts. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22LPI Contribution No. 1719
• Ojha, L., Wray, J., McEwen, A., & Murchie, S. (2013, March). Spectral Constraints on the Nature and Formation Mechanism of Recurring Slope Lineae. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Daes: 03/18-03/22; LPI Contribution No. 1719
• Pommerol, A., Thomas, N., Jost, B., Beck, P., Okubo, C., & McEwen, A. (2013, September). Photometry of Mars soils analogs and implications for the identification of wet and frozen soils from orbit. European Planetary Science Congress. London, UK.
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  Dates: 09/08-09/13
• Schaefer, E., McEwen, A., Mattson, S., & Ojha, L. (2013, Fall). Quantifying the Behavior of Recurring Slope Lineae and Similar Martian Features. American Geophysical Union.
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  Abstract #P41A-1909
• Tornabene, L., Ling, V., Osinski, G., Boyce, J., Harrison, T., & McEwen, A. (2013, March). A Revised Global Depth-Diameter Scaling Relationship for Mars Based on Pitted Impact Melt-Bearing Craters. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Turtle, E., Perry, J., McEwen, A., Barbara, J., Del, G. A., Hayes, A., West, R., Lorenz, R., Schaller, E., Lunine, J., Ray, T., Lopes, R., & Stofan, E. (2013, September). Titan's seasonal weather patterns, associated surface modification, and geological implications. European Planetary Science Congress. London, UK.
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  Dates: 09/08-09/13
• Turtle, E., Perry, J., McEwen, A., Hayes, A., Barnes, J., & West, R. (2013, Fall). ISS observations of Titan's northern lakes and evidence for a north-polar surface unit. American Geophysical Union.
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  Abstract #P53D-1897
• Viola, D., McEwen, A., Byrne, S., & Dundas, C. (2013, Fall). Mapping secondary crater fields in Arcadia Plantia, Mars: Implications for subsurface ice. American Geophysical Union.
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  Abstract #P41A-1918
• van, d., Hiesinger, H., Kruger, T., McEwen, A., & Dundas, C. (2013, March). New Evidence for Target Property Influence on Crater Size-Frequency Distribution Measurements. 44th Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/18-03/22; LPI Contribution No. 1719
• Bergstrom, J., Dissly, R., Delamere, W., McEwen, A., & Keszthelyi, L. (2012, June). High-Resolution Multi-Color Pushbroom Imagers for Mars Surface Characterization and Landing Safety. Concepts and Approaches for Mars Exploration. Houston, Texas.
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  Dates: 06/12-06/14; LPI Contribution No. 1679
• Bray, V., Atwood-Stone, C., & McEwen, A. (2012, March). Lunar Crater Peak and Peak-Ring Volumes from the LROC Global Lunar DTM 100. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Byrne, S., Dundas, C., McEwen, A., Holt, J., Putzig, N., Mellon, M., & Daubar, I. (2012, June). Exploration of Mid-Latitude Ice on Mars. Concepts and Approaches for Mars Exploration. Houston, Texas.
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  Dates: 06/12-06/14; LPI Contribution No. 1679
• Daubar, I., Geissler, P., McEwen, A., Dundas, C., & Byrne, S. (2012, Fall). Repeat Observations of New Impact Sites on Mars: Changes in Blast Zones. American Geophysical Union.
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  Dates: 03/19-03/23; Abstract #P21C-1850
• Daubar, I., McEwen, A., Byrne, S., & Dundas, C. (2012, June). Ongoing Impact Events on Mars: Implications for Science and Exploration. Concepts and Approaches for Mars Exploration. Houston, Texas.
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  Dates: 06/12-06/14; LPI Contribution No. 1679
• Daubar, I., McEwen, A., Byrne, S., & Kennedy, M. (2012, March). Seasonal Variation in Current Martian Impact Rate. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Davies, A., Matson, D., McEwen, A., & Keszthelyi, L. (2012, Fall). Monitoring Io's Volcanic Activity in the Visible and Infrared from JUICE - It's All About (Eruption) Style. American Geophysical Union.
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  Abstract #P21H-07
• Dundas, C., Diniega, S., Hansen, C., Byrne, S., & McEwen, A. (2012, Fall). Monitoring Gully Activity in Martian Winter. American Geophysical Union.
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  Abstract #P24B-04
• Dundas, C., McEwen, A., Byrne, S., Diniega, S., & Hansen, C. (2012, June). Recent Gully Activity on Mars: Implications and Objectives for Future Exploration. Concepts and Approaches for Mars Exploration. Houston, Texas.
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  Dates: 06/12-06/14; LPI Contribution No. 1679
• Hansen, C., Bourke, M., McEwen, A., Mellon, M., Pommerol, A., Portyankina, G., & Thomas, N. (2012, March). Year 3 HiRISE Observations of Sublimation of the Northern Seasonal Polar Cap on Mars. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Masse, M., Beck, P., Schmitt, B., Pommerol, A., McEwen, A., Chevrier, V., & Brissaud, O. (2012, March). Nature and Origin of RSL: Spectroscopy and Detectability of Liquid Brines in the Near-Infrared. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Mattson, S., Heyd, R., Fennema, A., Kirk, R., Cook, D., Becker, K., McEwen, A., & Boyd, A. (2012, September). High-Precision Geometrically Corrected HiRISE Images. European Planetary Science Congress 2012. Madrid, Spain.
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  Dates: 09/23-09-28
• Mattson, S., McEwen, A., Ojha, L., Bridges, N., Kirk, R., Howington-Kraus, E., & Mogk, N. (2012, Fall). Mars' Active Surface: Observing Changes with Orthorectified HiRISE Images. American Geophysical Union.
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  Abstract #P21C-1849
• Mattson, S., McEwen, A., Robinson, M., Speyerer, E., & Archinal, B. (2012, September). Exploring the Moon with LROC-NAC Stereo Anaglyphs. European Planetary Science Congress 2012. Madrid, Spain.
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  Dates: 09/23-09-28
• Mattson, S., McEwen, A., Speyerer, E., & Robinsom, M. (2012, Fall). LROC NAC Stereo Anaglyphs. American Geophysical Union.
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  Abstract #ED41E-0709
• Mattson, S., Ojha, L., Ortiz, A., McEwen, A., & Burns, K. (2012, March). Regional Digital Terrain Model Production with LROC-NAC. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  LPI Contribution No. 1659
• Mattson, S., Russell, P., Byrne, S., Kirk, R., Herkenhoff, K., & McEwen, A. (2012, March). Production and Error Analysis of Polar Digital Terrain Models from HiRISE. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• McEwen, A. (2012, July). Search for Present-Day Water on Mars using MRO's High Resolution Imaging Science Experiment (HiRISE). 39th COSPAR Scientific Assembly. Mysore, India.
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  Dates: 07/14-07/22; Abstract D1.2-20-12
• McEwen, A., & Turtle, E. (2012, July). Science Rationale for the Io Volcano Observer (IVO) Mission Concept. 39th COSPAR Scientific Assembly. Mysore, India.
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  Dates: 07/14-07/22; Abstract B0.2-2-12
• McEwen, A., Byrne, S., Chevrier, V., Dundas, C., hansen, C., Masse, M., Mattson, S., Murchie, S., Ojha, L., paige, D., Schaefer, E., Thomas, N., & Wray, J. (2012, June). Future Orbital Measurements Needed to Understand Present-Day Liquid H2O on Mars. Concepts and Approaches for Mars Exploration. Houston, Texas.
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  Dates: 06/12-06/14; LPI Contribution No. 1679
• McEwen, A., Byrne, S., Dundas, C., Mattson, S., Murchie, S., Ojha, L., Schaefer, E., thomas, N., & Wray, J. (2012, September). Recurring Slope Lineae: Evidence for Present-Day Flowing Water on Mars?. European Planetary Science Congress 2012. Madrid, Spain.
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  Dates: 09/23-09-28
• McEwen, A., Dundas, C., Byrne, S., Mattson, S., Ojha, L., Schaefer, E., & Wray, J. (2012, Fall). Recurring Slope Lineae in Valles Marineris, Mars. American Geophysical Union.
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  Abstract #P21C-1857
• McEwen, A., Dundas, C., Diniega, S., Byrne, S., Bridges, N., & Hansen, C. (2012, Fall). Present-Day Surface Changes on Mars: Implications for Recent Climate Variability and Habitability. American Geophysical Union.
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  Abstract #P14A-03
• McEwen, A., Janesick, J., Elliot, S., Turtle, E., Strohbehn, K., & Adams, E. (2012, October). Radiation-Hard Camera for Jupiter. System Science International Workshop on Instrumentation for Planetary Missions. Greenbelt, Maryland.
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  Dates: 10/10-10/12; LPI Contribution No. 1683
• McEwen, A., Keszthelyi, L., & Grant, J. (2012, March). Have There Been Large, Recent (Mid-Late Amazonian) Water Floods on Mars?. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Ojha, L., McEwen, A., Dundas, C., Mattson, S., Byrne, S., Schaefer, E., & Masse, M. (2012, March). Recurring Slope Lineae on Mars: UpDatesd Global Survey Results. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Daes: 03/19-03/23; LPI Contribution No. 1659
• Paige, D., Hansen, C., McEwen, A., Komarek, T., Elliott, J., Nash, A., McElrath, T., Green, R., & Foote, M. (2012, June). Orbiting Observatory for Studying Hydrologically Active Regions on Mars. Concepts and Approaches for Mars Exploration. Houston, Texas.
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  Dates: 06/12-06/14; LPI Contribution No. 1679
• Sato, H., Denevi, B., Robinson, M., hapke, B., & McEwen, A. (2012, March). LROC Science Operation Team Photometric Parameter Maps of the Moon from LROC WAC Observations. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Schaefer, E., McEwen, A., Ojha, L., & Mattson, S. (2012, March). Comprehensive Survey of Recurring Slope Lineae in Tivat Crater, Mars. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Tornabene, L., Cuadill, C., Osinski, G., mcEwen, A., Wray, J., Mustard, J., Skok JR, ., Grant, J., & Mattson, S. (2012, May). A Crater-Exposed Bedrock Database for Mars with Applications for Determining the Composition and Structure of the Upper Crust. Third Conference on Early Mars: Geologic, Hydrologic, and Climatic Evolution and the Implications for Life. Lake Tahoe, Nevada.
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  Dates: 05/21-05/25; LPI Contribution No. 1680
• Tornabene, L., Osinski, G., McEwen, A., Boyce, J., Bray, V., Caudill, C., Grant, J., Hamilton, C., Mattson, S., & Mouginis-Mark, P. (2012, March). HiRISE Operations ; Science Team Wide-Spread Occurrence of Crater-Related Pitted Materials on Mars: Implications for the Role of Target Volatiles with Respect to the Impact Process. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Turtle, E., & McEwen, A. (2012, July). Seasonal changes in Titan's weather patterns and the evolution and implications of accompanying surface changes. 39th COSPAR Scientific Assembly. Mysore, India.
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  Dates: 07/14-07/22; Abstract B0.1-16-12
• Turtle, E., Perry, J., Barnes, J., McEwen, A., Barbara, J., Del, G. A., Hayes, A., West, R., Lorenz, R., Schaller, E., Lunine, J., Ray, T., Lopes, R., & Stofan, E. (2012, March). Evolution of Titan's Weather Patterns and Accompanying Surface Changes in the Wake of the Seasonal Shift of the Intertropical Convergence Zone. 43rd Lunar and Planetary Science Conference. The Woodlands, Texas.
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  Dates: 03/19-03/23; LPI Contribution No. 1659
• Turtle, E., Perry, J., Barnes, J., McEwen, A., Barbara, J., Del, G. A., hayes, A., West, R., Lorenz, R., Schaller, E., Lunine, J., Ray, T., Lopes, R., & Stofan, E. (2012, April). Evolution of Titan's equinoctial weather patterns and accompanying surface changes and implications thereof Titan Through Time. Unlocking Titan's Past, Present and Future, NASA Goddard Space Flight Center.
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  Dates: 04/03-04/05
• Turtle, E., Perry, J., McEwen, A., Barbara, J., Del, G. A., West, R., Barnes, J., Hayes, A., Lorenz, ., Lunine, J., Stofan, E., Schaller, E., Lopes, R., & Ray, T. (2012, Fall). Seasonal changes in Titan's weather patterns and the evolution and implications of accompanying surface changes. American Geophysical Union.
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  Abstract #P23F-04

Others

• Heldmann, J., Bramson, A. M., Byrne, S., Beyer, R., Carrato, P., Cummings, N., Golombek, M., Harrison, T., Head, J., Hodges, K., Kennedy, K., Levy, J., Lim, D. S., Marinova, M., McEwen, A., Morgan, G., Pathare, A., Putzig, N., Ruff, S., , Scheiman, J., et al. (2021). Accelerating Martian and Lunar Science through SpaceX Starship Missions.