Composition of the upper clouds of Venus

Recent developments have shed new light on the composition of the upper Venus clouds. An analysis of the Mariner 5 occultation data has led to improved temperature and pressure profiles. When these are combined with transit data, it is concluded that there is an optically thin cloud layer with a top at 81‐km altitude where the temperature and pressure are, respectively, 175°K and 3 mb. The inclusion of temperatures derived from the near‐infrared CO 2 bands leads to the postulate of a second cloud deck with a top at 61‐km altitude, where T = 260°K and P = 240 mb. Additional important constraints on cloud models are imposed by the measured abundances of HCl and H 2 O, by the polarization data, and by the reflection and emission spectra. It is concluded that the leading candidate for the uppermost clouds is liquid drops of HCl‐H 2 O, that there is no recommended candidate for the second cloud deck, and that H 2 O ice is at most a minor component of these cloud systems.