Hydrothermal processes have been suggested to explain a number of recent observations for Mars, including hydrous mineral assemblages and D/H ratios of water extracted from SNC meteorites, the removal of CO2 from the atmosphere and its sequestering in the crust as carbonates, and the presence of nontronite clays in Martian dust and iron oxide-rich spectral units on the floors of some chasmata. Numerous examples have been noted of Martian channels formed by discharges of subsurface water, and hydrothermal processes are a suggested mechanism for aquifer recharge needed to sustain long term erosion in sapping channels.

Simple channel systems associated with potential heat sources, such as impact craters or shallow igneous intrusives and volcanic constructs, provide a present focus in targeting sites for hydrothermal prospecting during upcoming mission opportunities. But reliable site evaluations will also require information about surface composition. A preferred method is high spatial resolution, mid- or near-IR spectroscopy, tools of choice for mineralogical mapping on Earth. This discussion will focus on available geomorphic evidence and site selection criteria for hydrothermal sites on Mars, with comparisons of potential analog systems on Earth. The following geological contexts appear to offer the best targets for ancient hydrothermal systems on Mars: impact structures, rifted basins, areas flanking volcanic constructs and shallow intrusives, caldera floor deposits, and features formed by volcano-ground ice interactions in subglacial and periglacial environments.

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