Possible oxidant sources in the atmosphere and surface of Mars

Summary Photolysis of H₂O in the atmosphere near the surface is a copious source of OH, HO₂, and probably superoxides, some of which are likely to condense on the surface and migrate through the pores. The processes have been modeled in detail for their atmospheric interest. The models successfully account for the rarity of CO and O₂, the notable variability of ozone, and the escape flux of hydrogen. Though only qualitative estimates can be made of surface deposition rates and lifetimes, the suggested amounts are in the range inferred by Viking. The OH rapidly destroys any organic molecules that are present as vapors.Analogous reactions involving adsorbed water have been studied by Huguenin. These processes can be driven by the much larger photon fluxes at longer ultraviolet wavelengths. The suggested explanations, and many of the experiments, make it likely that peroxides, superoxides, and adsorbed OH are all present. Both kinds of process, and their combinations, seem in principle able to explain the absence of all organic molecules and the variety of observed oxidants. Since they operate planetwide, there is a strong suggestion that the observed conditions are typical. Oases of higher than average humidity may in fact be even more hostile than the average region, because water under Martian surface conditions is anything but benign.Laboratory simulation of the atmospheric processes must pay careful attention to scaling.Curiously, similar OH densities occur at the Earth's surface, The notable differences are food for thought, and ideas about the origin of life may be particularly affected.Based on an invited paper at the Second International Colloquium on Mars, Pasadena, January 15–18, 1979