The Oxidation State of the Lower Atmosphere and Surface of Venus

We present a comprehensive study of the redox state of the lower atmosphere and surface of Venus. This study constrains the CO concentration and oxygen fugacity at the surface of Venus. It incorporates: (1) gas phase thermochemical equilibrium and kinetic calculations to model the chemistry of the near-surface atmosphere, (2) a reanalysis of the thermodynamics of the CONTRAST experiment on the Venera 13 and 14 landers, (3) carefully selected thermodynamic data to model the stability of magnetite and hematite on the surface of Venus, and (4) the Venera 9 and 10 lander spectral reflectance data presented by Pieterset al.(1986,Science234, 1379-1383). The results of our work predict that: (1) the CO concentration at 0 km (735 K) is in the range of 3-20 parts per million by volume, (2) the oxygen fugacity (fO₂) at 0 km is in the range of 10^-21.7to 10^-20.0bars, (3) the fO₂of the atmosphere at 0 km is indistinguishable, within the uncertainties of the thermodynamic data, from the magnetite-hematite phase boundary, (4) gas phase thermochemical equilibrium is reached only, if at all, in the lowest levels of the atmosphere below about 0.7 km (730 K), (5) a disequilibrium region which is more oxidizing than predicted by thermochemical equilibrium exists at higher elevations, and (6) hematite forms at higher elevations due to the more oxidizing conditions in the disequilibrium region. Finally, we suggest experimental, observational, and theoretical studies which can be used to test our predictions and to provide a foundation for the design of experiments on future spacecraft lander missions to Venus.