The Oxidation State of the Lower Atmosphere and Surface of Venus

and the chemical weathering of primary minerals to secondary minerals (i.e., soil) on the surface. Physical properWe present a comprehensive study of the redox state of the lower atmosphere and surface of Venus. This study constrains ties such as the spectral reflectance, dielectric constant, the CO concentration and oxygen fugacity at the surface of electrical conductivity, magnetic susceptibility of rock and Venus. It incorporates: (1) gas phase thermochemical equilibsoil, and the loss of water from Venus over geologic time rium and kinetic calculations to model the chemistry of the (via oxidation of the surface and hydrogen escape to space) near-surface atmosphere, (2) a reanalysis of the thermodynamare also influenced by the redox state of the atmosphere. ics of the CONTRAST experiment on the Venera 13 and 14 The relative concentrations of oxidized versus reduced carlanders, (3) carefully selected thermodynamic data to model bon, sulfur, and hydrogen gases, resulting from the net the stability of magnetite and hematite on the surface of Venus, thermochemical reactions and (4) the Venera 9 and 10 lander spectral reflectance data presented by Pieters et al. (1986, Science 234, 1379–1383). The results of our work predict that: (1) the CO concentration 2CO 1 O2 5 2CO2 (1) 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 CH4 1 2O2 5 CO2 1 2H2O (2) of 10 to 10 bars, (3) the fO2 of the atmosphere at 0 km OCS 1 O2 5 SO2 1 CO (3) is indistinguishable, within the uncertainties of the thermodynamic data, from the magnetite–hematite phase boundary, (4) H2S 1 1.5O2 5 SO2 1 H2O (4) gas phase thermochemical equilibrium is reached only, if at all, in the lowest levels of the atmosphere below about 0.7 km S2 1 2O2 5 2SO2 (5) (730 K), (5) a disequilibrium region which is more oxidizing 2H2 1 O2 5 2H2O, (6) 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. are dependent upon the oxygen fugacity (fO2) of the nearFinally, we suggest experimental, observational, and theoretical surface atmosphere. Sufficiently low oxygen fugacities, fastudies which can be used to test our predictions and to provide vor CO, CH4, OCS, H2S, S2, and H2 at the expense of a foundation for the design of experiments on future spacecraft CO2, SO2, and H2O, while higher fO2 leads to the opposite lander missions to Venus.  1997 Academic Press situation (e.g., see Fig. 1). Likewise, the stability of Fe21and Fe31-bearing minerals and the Fe21/Fe31 ratio of the surface (which are related INTRODUCTION to the ability of the surface to act as an oxygen sink during water loss) are also influenced by the oxygen fugacity. This The oxidation (redox) state of Venus’ near-surface atmosphere controls the abundances of minor and trace gases can be seen from the exemplary gas-solid reactions