MARS AS SEEN BY MRO/HIRISE: THE PRESENT IS THE KEY TO THE RECENT PAST

Ron Greeley co-chaired the Science Definition team for MRO and was a leader in planetary aeolian processes. With the high resolution, high signal:noise, and repeat-image capability of MRO/HiRISE, we have been documenting present-day surface activity on Mars. This activity includes processes related to sublimation of seasonal frost (spots, fans, etc.), polar avalanches, impacts, wind, gullies, and steep slopes. By understanding present-day surface activity, we can better say which Martian landforms can form under present conditions and which required different conditions in the past or as-yet unobserved rare events. New results indicate that some suggestions of recent climate change on Mars may need revision. One such suggestion is that the enlargement of pits in the south polar residual cap indicates present-day global warming. However, models of continuous sublimation and redeposition of the CO₂ predict a suite of landforms that have been observed today (Byrne, 2009, AREPS 37, 535). Another suggestion is that mid-latitude gullies formed by melting snow or shallow ice during a recent period of high obliquity, but HiRISE data shows rapid and widespread gully expansion and channel incision (Dundas et al., 2012, Icarus 220, 124). Likewise, suggestions that a higher atmospheric density was needed to explain the presence of sand dunes have been countered by observations of widespread moving dunes (e.g., Bridges et al., 2012, Geology 40, 31). These observations do not rule out significantly different past climates but do clarify their effects on the landscape. Aeolian bedforms not active today include transverse aeolian ridges. Activity has not been seen in mid-latitude icy lobate aprons, which appear partially sublimated and are likely remnants of recent past climates. Ground ice excavated by new craters is found closer to the equator than expected for current atmospheric conditions, but consistent with likely conditions from the recent past (Byrne et al., 2009, Science 325, 1674). Formation of this clean subsurface ice (also found by the Phoenix mission) may require thin films of salty water. Another form of activity that might be linked to present-day water and recent climate change is the Recurring Slope Lineae (McEwen et al., 2011, Science 333, 740), but it is not yet clear how these may have varied in the recent past.