The formation of peroxonitrites and oxygen on Mars

When Martian regolith samples are moistened with water, a strong oxidant and oxygen gas are released. The many possible oxidants which have been suggested as present, including superoxide (O2(-)) hydrogen peroxide (H2O2(-)), and manganese (IV) oxide (MnO2), as well as peroxonitrite (ONOO(-)), can be distinguished by their ability to account for the numerous observations of the Viking Labelled Release (LR) and Gas Exchange (GEX) experiments. The magnitudes, kinetics, and heat labilities of the LR experiments and the magnitudes and heat labilities of the GEX experiments have been quantitatively replicated with peroxonitrites. Further, it is to be expected from information about the photochemistry of nitrogen in the atmosphere, the calculated fluxes of the NO(x) to the surface, the known reactions of NO(x) with carbonates to produce nitrates and photolysis studies of nitrates, that peroxonitrites are present on Mars. However, considerable uncertainty has existed about the form in which oxygen is stored in photolyzed nitrate crystals. Models have ranged from micropores of O2 to chemically bound oxygen as peroxonitrite (O2NOO(-)), as well as peroxonitrite. Studies of the solution chemistry and solid state photochemistry were completed, which show that much of the oxygen evolved on wetting comes from thermal breakdown of peroxonitrite after it is dissolved. A chemical model is developed for the photochemical processes occurring during UV irradiation of solid nitrates which resolves the several contradictory models discussed in the literature in recent years.