Formation of Methane and (Per)Chlorates on Mars

Methane, perchlorates, chlorates, and methyl chlorides have all been detected on Mars. The origin of these species has never been adequately explained. In this paper, we irradiated mixtures of CO2, HCl, and a mineral catalystanatase, rutile, montmorillonite, and the Nakhla meteoritewith soft UV radiation for up to 3500 h and observed the formation of perchlorates, chlorates, methyl chlorides, and methane in a single experiment. Additionally, the methanogenesis for anatase was observed at −196 °C. Further, we propose that while methane is decomposed relatively quickly and therefore attains a steady-state concentration (0.41 ± 0.16 ppbv), the chlorinated compounds are much more stable and therefore would have accumulated throughout the Martian history. We estimate that this mechanism would be sufficient in the course of Martian history to accumulate perchlorate in the soil in 0.5 wt % in 5–50 cm depth, which is in accordance with the observed perchlorate content on Mars. This predicted perchlorate gradient may be observed with the Insight rover. Further, if microbes are present on Mars, they will likely inhabit depths below the perchlorate (i.e., 5–50 cm). This chemistry likely still continues on Mars to a certain extent, and any future exploration by rovers or planetary models should account for this process during their analyses