Early Acidification of Mars and the Potential Implications for Biology

A leading paleoclimate theory for Mars, based on the identification of phyllosilicate minerals in ancient terrains, posits that the first several million years of the planet’s history were dominated by neutral to alkaline pH. However, evidence is mounting for the consideration of an alternate hypothesis: that some smectites on Mars formed under acidic conditions, and that the early surface of Mars may not have been subject to circum-neutral pH conditions, at least not uniformly. Work on shergottitic liquids suggests that up to 2400 ppm of sulfur could have degassed from martian magma, supplying more than enough sulfur for the planet’s sulfate-rich sediments and sedimentary rocks, and isotopic evidence of mass independent fractionation reveals that sulfur in martian meteorites underwent atmospheric reactions. Radiative modeling of sulfur volatiles in the martian atmosphere indicates that SO2 and H2S would have acted as powerful greenhouse gases trapping heat in different wavelength-dependent atmospheric windows than CO2 and H2O, supplying the necessary heat for surface temperatures to rise above freezing. Photochemistry suggests that sulfur would have been removed from the atmosphere through the deposition of sulfur dioxide, oxidized to sulfate at the surfaceatmosphere interface. This, in turn, could have led to the early acidification of the surface, thereby explaining the paucity of carbonates on Mars. This idea is supported by 1) the recent laboratory synthesis of Fe/Mg smectite from an Adirondack basalt simulant in an acidic hydrothermal system, and 2) studies of the mineral composition of terrestrial analogs, particularly at acid salt lakes