Large Martian regolith water content implied by rampart crater population

Details Large Martian regolith water content implied by rampart crater population Large Martian regolith water content implied by rampart crater population

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p42a0574s&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #P42A-0574

Physics

5410 Composition, 5420 Impact Phenomena (Includes Cratering), 5460 Physical Properties Of Materials, 6225 Mars

Scientific paper

We estimate the global regolith water content using a new model for rampart crater formation (Stewart et al.~LPSC 2001). The Martian surface has a high fraction (probably significantly >20%) of craters with so-called fluidized ejecta blankets, characterized by the appearance of ground-hugging flow terminating in one or more continuous distal ramparts. While rampart craters have long held the promise of revealing information about the water content of the Martian regolith, the lack of a comprehensive physical model for the formation of fluidized ejecta blankets has hindered quantitative studies. We have developed a model for rampart crater formation based on ice shock data obtained at Martian temperatures and numerical simulations of impacts onto ice-rock mixtures under Martian conditions. We find that significant quantities of liquid water may be produced by an impact event and that the excavation process is modified by the presence of interstitial ice. As a result, single or multiple rampart ejecta blankets do not require the presence of pre-existing water in the liquid phase. A few to several volume percent of shock-produced liquid water may be incorporated into the continuous ejecta blanket for average impact conditions and reasonable regolith pore space assumptions, e.g. 15~vol% ice-filled near-surface pores. For a given diameter rampart crater, we calculate the associated minimum regolith ice content. Using the Viking-based rampart crater database by Barlow and Bradley (1990), the observed rampart crater population ( ~20% of all craters) implies a minimum regolith ice content of order 0.1~m global layer equivalent. The Mars Orbiter Laser Altimeter (MOLA) data suggest that a much larger fraction of craters, especially in the northern plains, may have rampart ejecta features. To derive the implied global regolith ice content, we correct for the impact flux rate over the past ~3~Ga using a number density for 1-10~km diameter craters, the peak rampart crater size range, of about 10-4~km-2 (Hartmann 1999). Assuming that about 1/3 of all craters sample subsurface ice, the estimated total regolith ice budget is equivalent to a few 100's~m thick global layer. These results support the hypotheses that the total Martian water budget is relatively large (e.g. Leshin 2000) and that the regolith may presently contain significant amounts of near-surface ice (e.g.Malin & Edgett 2000). More detailed studies combining simulations with the MOLA database should provide tighter constraints on regional regolith ice content.

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