A MODEL OF THERMOCHEMICAL EQUILIBRIUM IN THE NEAR-SURFACE ATMOSPHERE OF VENUS

Providing constant concentration of SO2, COS and CO below altitudes of measurements (12-26 km), the chemical composition of the near-surface atmosphere of Venus appears to be in agreement with a model of thermo chemical gaseous equilibrium in the C-O-S system at the condition of t he modal planetary radius. The thermochemical equilibrium between CO2, SO2, COS and CO at the level of modal radius indicated an oxygen fuga city logf(O2)==-21.3 +/- 0.2 bar. That value appears to be quite near with the condition of hematite-magnetite buffer providing non-unity ac tivities of those solid phases. Besides, that value of oxygen fugacity is consistent with the result of the ''Contrast'' experiment on the V enera-13,14 landers as well as with the reflectance properties of soil at the Venera-9,10 landing sites. The presented model of thermochemic al equilibrium predicts concentration gradients for gases in the thin (1-2 km) layer in the near-surface atmosphere, Oxygen fugacity within the equilibrium layer seems to be governed by CO2, SO2 and COS and dec reases with increase of elevation. That situation should result in the growth of hematite stability from lowlands to more elevated levels. T he,model of thermochemical equilibrium between gases could not apply f or the Venus highlands. Lack of the thermochemical equilibrium between gases al highland condition does not allow to estimate oxygen fugacit y in the frame of methods of equilibrium chemical thermodynamics.