Observations of ice-exposing impacts in the Martian mid-latitudes

Details Observations of ice-exposing impacts in the Martian mid-latitudes Observations of ice-exposing impacts in the Martian mid-latitudes

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23a1694d&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P23A-1694

Physics

[5400] Planetary Sciences: Solid Surface Planets, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5422] Planetary Sciences: Solid Surface Planets / Ices

Scientific paper

The Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) have observed the sites of hundreds of new impacts on the Martian surface. Impacts in the northern mid-latitudes have excavated ice [1], allowing a new approach to studying the Martian cryosphere via active surface processes. HiRISE has systematically monitored these sites to characterize their morphology and changes over time. We will discuss the morphology, distribution and implications of the icy craters observed in this campaign. At the time of writing, ice has been excavated at thirteen impact sites at middle to high northern latitudes. (Detection of impacts is strongly biased towards dusty areas, where large dark blast zones form during impact; due to this bias, impacts are rarely detected in the southern mid-latitudes.) The crater observations indicate that the latitudinal and depth distribution of ice-exposing impacts is in general agreement with models of ice stability [e.g. 2], with ice visible in most craters above 43° N and absent below. A handful of small craters at high latitude do not show visible ice. While the lower-latitude icy impact sites may be closer to the equator than the current stability boundary [1], this boundary is expected to vary over time and extant ice could be a remnant of previous climatic conditions, particularly in exposures associated with lobate aprons. The ice, initially bright, is observed to fade over time, eventually matching the surrounding regolith over a period of months to years. Ice remains visible over a period of months, while as much as millimeters of sublimation occurs [3,4]; the ice must be clean rather than pore-filling. While some clean ice might be produced by melting and refreezing during impact, little or no melting is expected in the smaller craters [5], suggesting that the ice originally had a low regolith content. Such excess ice was observed by the Phoenix lander [6], and our observations suggest that it is widespread. Instances of possible brightening or expansion of ice patches are also observed, perhaps due to eolian activity.

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