Chemical composition of the atmosphere of Venus

Abstract

Measurements onboard the Venera 11, 12 (refs 1–4) and Pioneer Venus5,6 spacecrafts stimulated us to study the chemical composition of the subcloud atmosphere of Venus in terms of the thermochemical equilibrium calculations, comparison of typical mixing and chemical times and a rule of height-independent element mixing ratio in the absence of condensation7. The photochemistry of the atmosphere down to 50 km was calculated using transport effects and number densities of CO2, H2O, HCl, SO2 and CO at the lower boundary and rate coefficients of 102 reactions. These reactions include catalytical cycles of COCl and COCl2 which accelerate O2 destruction and CO2 formation. Altitude profiles of 27 components agree well with those measured in the upper and middle atmosphere. H2O and SO2 mixing ratios are very similar and sharply decrease at 60 km due to SO2 photolysis and sulphuric acid formation. Calculations show that sulphuric acid and sulphates are the main components of the second and third modes of particle size distribution in the upper and middle cloud layers. The lower cloud layer may consist of AlCl3 and FeCl3.