Chemical composition of the atmosphere of Venus

Measurements onboard the Venera 11, 12 (refs 1-4) and Pioneer Venus^5,6 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.