Vega mission results and chemical composition of Venusian clouds

Abstract

Data (sometimes contradictory) on the chemical composition and physical properties of the cloud aerosol measured by the Vega payload are analyzed to yield the most probable composition and structure of the cloud layer, mass loadings of individual species, and processes of their formation. The detection of phosphorus correlates with the cloud layer extending down to at least 33 km and is considered real. Phosphorous anhydride P 4O 6 is the main phosphorus-bearing gas and shows a mixing ratio of 2 ppmv below 25 km. Its reaction with sulfuric acid droplets transforms the latter into phosphoric acid droplets with SO 2 release. These droplets lose water when moving down to 25 km, where the total evaporation of dry phosphoric anhydride P 4O 10 should occur. The ratio of sulfur to sulfuric acid mass loading is about 0.1, which agrees with the theoretical findings of atmospheric photochemistry. Ferric chloride constitutes 1% of the column mass loading above 47 km, which coincides with the respective value at the landing site of the Pioneer Venus sounder probe. Aluminum chloride is the most probable chlorine-bearing species in the middle cloud layer. Variations of the chemical composition and structure of the clouds and striking differences between the measurements of the Vega and other probes are discussed briefly.