Venus: composition and structure of the visible clouds.

It is proposed that the visible cloud deck on Venus is composed of droplets of sulfuric acid. These are formed by the very rapid photooxidation of carbonyl sulfide in the upper atmosphere. The clouds are best described as an extensive haze since the predicted particulate scale height probably exceeds the gas scale height within the layer. The predicted mixing ratio for water is 10(-6) (lower limit), and for both carbonyl sulfide and sulfur dioxide it is 10(-7) (upper limit); these are in good agreement with observations. Gaps in the layer are not possible unless the planetary scale dynamics produce cloud turnover times of less than a few days. Under these conditions the water mixing ratio could approach 10(-4) and the formation of a thin hydrochloric acid haze at high altitude above the main cloud is possible.