Limited recharge of the southern highlands aquifer on early Mars

To determine plausible groundwater recharge fluxes on early Mars, we developed analytic and numerical solutions for an unconfined steady-state aquifer beneath the southern highlands. We showed that the aquifer's mean hydraulic conductivity, K, is the primary constraint on the plausible magnitude of the mean steady recharge, r. By using geologic constraints, a mean hydraulic conductivity of K similar to 10(-7) m/s, and varying shoreline elevations and recharge distributions, the mean recharge must be of the order of 10(-2) mm/yr. Recharge for other values of K can be estimated as r similar to 10(-5) K. Our recharge value is near the low end of previous estimates and significantly below published precipitation estimates. This suggests that, in a steady hydrologic cycle, most precipitation forms runoff as opposed to infiltrating into the subsurface. Alternatively, high rates of runoff production combined with a sufficiently slow transient aquifer response to recharge may limit major groundwater upwelling prior to the cessation of climatic excursions causing precipitation.