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Modeling Hydrothermal Activity Associated with Martian Impact Craters: An Overview

Introduction: Impact events locally increase the temperature of a planetary crust, initiating hydrother-mal activity if water or ice is present. Impact-induced hydrothermal activity is responsible for mineralogical-ly and morphologically modifying many terrestrial craters [e.g., 1], and has been suggested for Martian craters [2, 3]. Present-day subsurface ice has been inferred at high latitudes (poleward of 60°) based on the detection of hydrogen by the Gamma Ray Spectrometer (GRS) onboard Mars Odyssey [4] (and confirmed by the Phoenix lander), and indirectly by the presence of fresh craters with fluidized ejecta blankets [e.g., 5] and rootless cones [6] at lower latitudes. Thus, a present-day impact may still generate hydrothermal activity. Modeling history: The first modeling effort of impact-induced hydrothermal systems on Mars [2] focused on impact melt sheets and suggested that (i) hydrothermal circulation of steam in Martian melt sheets may have produced iron-rich alteration clays, ferric hydroxides, and near-surface accumulations of salts, (ii) the ability of vapor-dominated hydrothermal systems of concentrate sulfate relative to chloride is consistent with the high sulfate to chloride ratio found in the Martian soil by the Viking landers, and (iii) a major fraction of the Martian soil may consist of the erosion products of hydrothermally altered impact melt sheets. Further analytical modeling suggested that the formation of large impact craters on Mars (>65 km diameter) may have resulted in the creation of ice-covered impact crater lakes, which would not freeze for thousands of years, even under present climatic conditions [3]. The first numerical effort to model Martian impact induced hydrothermal systems [7] used the finite-difference computer modeling code HYDROTHERM [8] to explore system mechanics, estimate lifetimes, and predict expected mineralogies. This work predicted cessation of any significant hydrothermal activity at a simple (~7 km) Martian crater in under 10,000 years. However, their simulations were limited to 50,000-100,000 years. Abramov and Kring (2005) study: Building on the work by [7], hydrothermal activity was simulated for up to several million years, allowing estimates of system lifetimes for larger craters [9]. Crater lakes and the latent heat of fusion are were explicitly included in the model. The crater topography was improved based on observations of lunar craters, and was preserved