Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31g..10i&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31G-10
Other
[0774] Cryosphere / Dynamics, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5422] Planetary Sciences: Solid Surface Planets / Ices, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
For the last decade the team of Dr. Elisabetta (Betty) Pierazzo (LPL+PSI) study physical and mechanical processes involved in impact melting of Martian permafrost. The idea is that on Mars large enough impact craters would start substantial hydrothermal activity underneath the crater for thousands of years (possibly for >1 Myr, if a crater is larger than about 200 km in diameter). Numerical efforts to predict the extent and time scale of hydrothermal activity in Martian impact craters have mostly relied on numerical simulations of impact cratering into uniform or layered ice-rock targets. We conduct a case modeling study of impact melting of permafrost on Mars to investigate the general thermal state of the rock layers modified in the formation of hyper-velocity impact craters. We model the formation of a mid-size crater, about 30 km in diameter, formed on target consisting of a mixture of large particles of H2O-ice and rock (something like ice lenses in rock fractures) and fine mix equilibrated in temperature with an ice/water content variable with depth. The model results indicate that for craters larger than about 30 km in diameter the onset of post-impact hydrothermal circulation is characterized by two stages: first, the formation of a mostly dry, hot central uplift, followed by water beginning to flow in and circulate through the initially dry and hot uplifted crustal rocks. The post-impact thermal field in the periphery of the crater is dependent on crater size: in mid-size craters, 30-50 km in diameter, crater walls are not strongly heated in the impact event, and even though ice present in the rock may initially be heated enough to melt, overall temperatures in the rock remain below melting, undermining the development of a crater-wide hydrothermal circulation. We speculate that salt deposition from supercritical water may occur immediately after impact in some locations before the normal water circulation starts. In larger craters, crater walls are heated well above the melting point of ice, thus facilitating the onset of an extended hydrothermal circulation. Based on these conclusions, we expect that large Martian impact craters like Lyot (D ~240 km), Gusev (D ~ 150 km) and Holden (D ~ 150 km) should have initially dry, fractured and hot central uplifts and warm, water-bearing walls and rims. In these large craters the primary phase of hydrothermal circulation would be the penetration of water into the hot central uplift. Rock temperatures above melting of ice around the crater walls support the hypothesis that in this region near surface ice could have melted during the impact, favoring mobilization and ponding of liquid ground water in the immediate topographic low: the crater.
Ivanov Boris
Melosh Henry Jay
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