Sulfites and sulfates formed by weathering of early martian carbonates in a sulfur dioxide-bearing atmosphere.

Despite ample evidence that extensive water-rock interactions occurred under a CO2-dominated atmosphere on early Mars, carbonate minerals are relatively rare at the surface. One possibility to explain this scarcity is that carbonates were initially abundant, but were later destroyed when atmospheric conditions changed, particularly as a result of volcanism releasing large volumes of sulfur dioxide SO2 into the atmosphere. However, despite some early theoretical and experimental results, no study has investigated the stability of the most common carbonates (Ca, Mg and Fe) in the presence of abundant SO2 gas. Here we present the results of experiments demonstrating that carbonates are systematically unstable when exposed to 0.8 bar of SO2 in moderately oxidizing (SO2 + H2O) or strongly oxidizing (SO2 + H2O + H2O2) environments. In both environments, the reaction end products are systematically sulfates, except for calcium carbonate, which predominantly transforms into calcium sulfite (hannebachite) in moderately oxidizing conditions. Based on these results, carbonates formed early in martian history would have been rapidly decomposed and replaced by sulfates (and sulfites if calcium was abundant enough) in the presence of abundant SO2 released by major volcanic events such as those associated with the build-up of the Tharsis rise.