Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars

Observations from in situ experiments and planetary orbiters have shown that the sedimentary rocks found at Meridiani Planum, Mars were formed in the presence of acidic surface waters(1-4). The water was thought to be brought to the surface by groundwater upwelling(5,6), and may represent the last vestiges of the widespread occurrence of liquid water on Mars. However, it is unclear why the surface waters were acidic. Here we use geochemical calculations, constrained by chemical and mineralogical data from the Mars Exploration Rover Opportunity(7-10), to show that Fe oxidation and the precipitation of oxidized iron (Fe3+) minerals generate excess acid with respect to the amount of base anions available in the rocks present in outcrop. We suggest that subsurface waters of near-neutral pH and rich in Fe2+ were rapidly acidified as iron was oxidized on exposure to O-2 or photo-oxidized by ultraviolet radiation at the martian surface. Temporal variation in surface acidity would have been controlled by the availability of liquid water, and as such, low-pH fluids could be a natural consequence of the aridification of the martian surface. Finally, because iron oxidation at Meridiani would have generated large amounts of gaseous H-2, ultimately derived from the reduction of H2O, we conclude that surface geochemical processes would have affected the redox state of the early martian atmosphere.