Central Pit Craters: Comparisons between Ganymede and Mars and Implications for Formation Models

Details Central Pit Craters: Comparisons between Ganymede and Mars and Implications for Formation Models Central Pit Craters: Comparisons between Ganymede and Mars and Implications for Formation Models

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.5903b&link_type=abstract

American Astronomical Society, DPS meeting #40, #59.03; Bulletin of the American Astronomical Society, Vol. 40, p.506

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Impact craters containing central pits are common on icy bodies but rare on volatile-poor objects. Three formation models have been proposed for central pit craters: release of impact-generated gases during crater formation, central peak collapse, and excavation into subsurface liquid layers. We have conducted a survey of central pit craters on Ganymede and Mars to better constrain the possible formation mechanisms. Our findings reveal that (1) central pits are common over all terrains on both bodies, displaying no statistical variation in latitude, longitude, or geologic unit; (2) the diameter where the frequency of pit craters peaks is 3 times larger on Ganymede than Mars, probably reflecting gravity differences; (3) pit craters display a wide range of preservation, indicating that conditions favoring their formation have existed over most of the history of these bodies; (4) fresh Martian pit craters are usually associated with multiple layer ejecta morphologies; (5) pits tend to be larger relative to their parent crater on Ganymede than on Mars, perhaps due to Ganymede's higher ice concentration; (6) Ganymede pits occur directly on the crater floor while Martian pits occur either on the floor or atop a central peak; (7) Ganymede pit craters often occur on updomed crater floors due to ice relaxation; Martian floor pit craters do not show this updoming, indicating that target ice concentrations of 20% are sufficient for pit formation; and (8) central peaks occur in the same crater diameter range as central pits. The results suggest that collapse of central peaks is not the likely mechanism producing central pits. Subsurface liquid layers could be responsible but such layers would have to exist at depth over most of Mars and Ganymede. Vaporization of subsurface volatiles remains a viable mechanism. Funding from NASA OPRP Award #NNG05G116G, MFRP Award #NNG05GM14G, and MDAP Award #NNX08AL11G is acknowledged.

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