WATER MEETS ROCK IN THE SHALLOW SUBSURFACE OF MARS: EXTENT AND DURATION OF CRUSTAL HYDROTHERMAL SYSTEMS

Similar to the debate of warm wet vs cool wet or cold and dry for the surface of early Mars is the question of under what conditions did the aqueous alteration of Mars occur? For example did it occur in the shallow crust in a relatively closed hydrologic system or at the surface in an open hydrologic system? Syntheses of global observations from high (19 m/pixel CRISM) to moderate (1 km/pixel OMEGA) resolution VNIR data show diverse assemblages of aqueous minerals, the products of water meeting rock (e.g. Ehlmann et al., 2013; Carter et al., 2012), that suggest up to seven formation mechanisms starting with a basaltic composition. These environments range from subsurface hydrothermal to near-surface pedogenic where the resulting mineral assemblages depend on the water:rock mass ratio (W/R), amount and source or protons (i.e. acidity), and the nature of the reactants (i.e. glass vs crystalline igneous phases). The most common environment observed from orbit is the subsurface hydrothermal-type site encompassing more than 70% of observed sites with hydrated silicates. The dominant hydrated mineral phase is smectite clay, implying a moderate pH environment and a thermal environmental <350°C. The heat source to drive hydrothermal systems in the crust could be crustal cooling following planet formation, conductive heat flow from the mantle, impact generated heat pulses, deuteric alteration and magmatism. Evidence of relatively rapid cooling of the crust after formation perhaps by hydrothermal circulation is provided by ancient topography preserved by a rigid crust. Exhumation of Noachian-aged phyllosilicate-rich terrains reveal abundant linear ridges resistant to erosion interpreted to be mineralized fraction zones in regional hydrothermal systems. I will synthesize these and new observations to assess the evidence of the size and duration of a subsurface environment for aqueous alteration on Mars.