Atmospheric chemistry of nitrogen on Mars: A link between oxidants in the soil and chlorine cycling

NO and NO2, collectively denoted NOx, are generated in the Earth’s atmosphere mainly from the photochemistry of biogenic nitrogen-containing gases whereas on Mars they form through the oxidation of products derived from ionospheric dissociation of N2. NOx disappears from both atmospheres when acids form from further oxidation and deposit to the surface. We show that peroxynitric acid (PNA), HO2NO2, dominates the removal of NOx from the atmosphere of Mars in contrast to nitric acid, HNO3, in the Earth’s atmosphere because PNA is stabile against thermal decomposition at the colder temperatures of Mars. Following the dry deposition of PNA, peroxynitrate or peroxynitrite minerals are possible, while a proportion of PNA may also decompose and recycle NOx back to the atmosphere. Deposition of PNA must have been delivering nitrogen to the martian surface since at least ~3 Ga (during the Amazonian) when Mars has been predominantly cold and dry. In aqueous solution, peroxynitrates or peroxynitrites decompose and release O2. Thus, their inevitable formation from atmospheric chemistry could explain the release of O2 when water was added to the martian soil in the Viking Lander (VL) Gas Exchange experiment. Furthermore, PNA could interact with chlorides to liberate chlorine volatiles, enabling cyclic oxidation of chlorine compounds into perchlorate even in the absence of volcanic chlorine volatiles.