Precipitation and aridity constraints from paleolakes on early Mars

The ancient climate of Mars remains an enigma despite the abundance of in situ and remote-sensing data revealing hydrological activity in the past. The crux of this debate-informed by geomorphic studies, chemical alteration observations, and numerical climate models-is the amount, distribution, and nature of surface water and precipitation (i.e., rain vs. snow). Here, we combined measurements of 96 open- and closed-basin lakes with simple hydrological balances to constrain catchment-averaged precipitation, P (sub T) , integrated over a given runoff episode of indeterminate length during early Mars history. We included 13 systems containing both open- and closed-basin lakes, providing fully bounded precipitation estimates (coupled systems). We show that, on average, P (sub T) was > or =4 m and < or =159 m; however, local precipitation was spatially variable and consistent with a complex, planetwide climate. Aridity indexes of open-basin lakes indicate some regions may have been at least as humid as semiarid terrestrial environments. Our results offer widely distributed, quantitative hydroclimate constraints that can be used to test paleoclimate model scenarios, working toward bridging the gap between geological observations and climate theory for early Mars.