“…These data reveal a wealth of plausible physical and chemical processes that are poorly constrained. These include, but are not limited to, the depletion of sulfur dioxide and water in the clouds ( 6 , 8 ), an unknown ultraviolet (UV) absorber at the top of the clouds ( 1 , 9 ), discrete size distributions of aerosol particles ( 10 ), redox disequilibrium chemistry ( 5 , 11 , 12 ), and the possible presence of a cloud layer (47 to 52 km), tentatively indicated by VEGA-2 x-ray data, where phosphorus is at least as abundant as sulfur ( 13 , 14 ). Of import, current models of atmospheric chemistry on Venus are unable to explain a number of observations, such as the behavior of carbonyl sulfide (OCS) and sulfuric acid (H 2 SO 4 ) vapor below the clouds, the behavior of water vapor and sulfur dioxide (SO 2 ) in and above the clouds, and the lack of O 2 above the clouds [see Bierson and Zhang ( 5 ) for a helpful summary of these four outstanding problems].…”