Martian post-impact hydrothermal systems incorporating freezing

We simulate the evolution of post-impact hydrothermal systems within 45 km and 90 km diameter craters on Mars. We focus on the effects of freezing, which alters the permeability structure and fluid flow compared with unfrozen cases. Discharge rates, total discharge and water–rock ratios increase with permeability. Systems with permeabilities of 10−10 m2 or higher exhibit convection in the hydrosphere, allowing them to derive heat from greater depths. Surface discharges persist for ∼103–105 years under freezing surface conditions, with higher permeabilities permitting longer lifetimes. Maximum discharge rates and total discharges range from 0.1 to 10 m3 s−1 and 109 to 1012 m3, respectively, for systems with permeabilities between 10−14 and 10−12 m2. Near-surface water–rock ratios range from <1 for low permeability, frozen cases to ∼103 for high permeabilities and/or unfrozen cases. Propagation of the freezing front radially inwards focuses flow towards the center of the crater resulting in a diagnostic increase in water–rock ratios there. This process may explain the phyllosilicate assemblages observed at some crater central peaks.