A Model for the Thermal Equilibrium of the Surface Venusian Atmosphere

It is shown that the composition of the Venusian surface atmosphere is satisfactorily described by a model for thermochemical equilibrium between gases in the C-O-S system at the level of the planet's modal radius subject to the assumption of constant SO₂, COS, and CO concentrations below the measurement heights (12-26 km). The conditions for equilibrium between CO₂, SO₂, COS, and CO at the modal radius level correspond to log f_O2, = -21.3 ± 0.2 bar, which is close to the values determined by the hematite-magnetite buffer in association with nonunit activities of the solid phases. This estimated oxygen fugacity agrees with the Kontrast experiment on the Venus 13 and 14 probes and the reflectivities of the surface in the near IR. There may be a gradient equilibrium distribution of the gases along the vertical in the thin (1-2 km) near-surface layer of the atmosphere and in the pores of the soil. Within the equilibrium layer, the oxygen fugacity is probably determined by the contents of CO₂, SO₂, and COS and decreases upwards, which leads to an increase in the stability of the more oxidized iron minerals at the surface. The thermochemical equilibrium model is unsuitable for high levels on the surface of Venus and above, where the attainment of equilibrium between most gases is unlikely. This makes it difficult to estimate correctly the redox conditions in the atmosphere and the reactivities of the gases within the framework of equilibrium chemical thermodynamic methods.