Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p44b..01p&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P44B-01
Other
[6022] Planetary Sciences: Comets And Small Bodies / Impact Phenomena, [6225] Planetary Sciences: Solar System Objects / Mars, [8135] Tectonophysics / Hydrothermal Systems, [8136] Tectonophysics / Impact Phenomena
Scientific paper
The idea that on Mars impact craters larger than about 50 km in diameter would release enough heat to drive substantial hydrothermal activity underneath the crater for thousands of years, and craters larger than about 200 km in diameter keeping warm for periods as long as 1--10 Myr, is at least a decade old. 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 dry targets or, at most, layered ice-rock targets. We present 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 modeled the formation of a mid-size crater, about 30 km in diameter, formed on a target with an ice/water content varying with depth, where water and rock form either a macroscopic mixture (something like ice lenses in rock fractures) or a microscopic mixture (where ice and rock are fully equilibrated in temperature). Our model results of dry and mixed rock-ice targets 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 even before the beginning of hydrothermal circulation. In large 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. 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. This may help explain recent observations of hydrologic activity within large Martian craters such as extensive valley systems in the inner crater walls observed within Lyot crater, or an early lacustrine phase, preceding the breaching of the rim by Uzboi Vallis, suggested for Holden crater.
Ivanov Boris A.
Pierazzo Elisabetta
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