Geochemical and Geomorphological Effects of Post-impact Hydrothermal

Post-impact hydrothermal (PIH) systems subjected to mars-like surface temperatures (-53 °C) and reasonable surface permeabilities (10 to 10 m) produce flow patterns with spatially diagnostic, surface-exposed water-to-rock (W/R) ratios of ~1 to 1000, channel-carving surface discharge rates of ~1 to 10 m/s and lake-forming total discharges of ~10 to 10 m. A bolide 3.9 km in diameter traveling at 7 km/s generates a 45 km crater and delivers enough energy to heat subsurface water, and drive hydrothermal circulation (figure 1). This PIH circulation can lead to surface discharge of water, and chemical alteration – both are potentially detectable [1,2,3]. Our models differ from previous efforts [4,5,6,7,8] in that we incorporate freezing. Model: We simulate the evolution of PIH systems using MAGHNUM (cf. [12]). MAGHNUM solves the time-dependent transport of water and heat through a porous medium. It incorporates phase transitions between ice, water and vapor. Given a particular crater size and associated heat sources, two principal dichotomies control PIH behavior: (1) frozen vs unfrozen surface and (2) conductive vs convective heat and fluid transfer. Results: Discharge rates, total discharge and W/R ratios increase with permeability [13] (figures 2, 3 and 4). Systems with higher permeabilities (10 m) allow convection: rendering them capable of mining heat from the central uplift before the surface freezes. Convective systems subjected to surface temperatures below freezing are particularly interesting because an impermeable freezing front – the surface ice/water interface that marches from mid-floor to the central peak – forces flow towards the center of the crater. This effect prolongs modest near-surface temperatures (0 to 100 ° C for 50 kyr) and yields W/R ratios > 1000. For systems with lower permeability, the upper 200 m of rock at the crater’s center experience fluid temperatures between 0 and 100 °C for 3500 yrs and W/R ratios of ~ 10. These results imply that different surface permeabilities should produce spatially diagnostic mineral alteration patterns (figure 3). This may explain mineral assemblages and fluvial features associated with central peaks of craters [3]. Discharge rates and total discharges are capable of producing gullies, ponds and lakes, but not alluvial fans (table 1).