Subsurface formation of oxidants on Mars and implications for the preservation of organic biosignatures

Hydrogen peroxide can form through the interaction of pyrite and anoxic water. The oxidation of pyrite results in the precipitation of sulfates and iron oxides, high redox potentials (~ 1000 mV) and acidic pH (3–4). The oxidative potential of the resultant solution may be responsible for the oxidation of organic compounds, as observed in the subsurface of the Rio Tinto Mars analog. On Mars subsurface migration of groundwater interacting with volcanogenic massive pyrite deposits would have mobilized acidic and oxidizing fluids through large portions of the crust, resulting in the widespread deposition of sulfates and iron oxides. This groundwater could have leached substantial volumes of aquifer material and crustal rocks, thereby erasing any organic compounds possibly down to depths of hundreds of meters. Therefore, the preservation of organic biosignatures must have been severely constrained in the portions of the ancient Martian crust that were exposed to aqueous processes, calling for a redefinition of the future targets in the search for biomolecular traces of life on Mars.