Composition of the atmosphere of Venus below the clouds

Abstract

The composition of Venus' lower atmosphere, beneath the clouds, has been investigated through both in situ measurements and remote‐sensing observations. In 1978 the mass spectrometers and gas chromatographs aboard the Venera 11–12 landers and Pioneer Venus Large probe returned unique information on the abundances of carbon dioxide, nitrogen, noble gases, and other minor species. Radio occultations from the Pioneer and Magellan spacecraft provided measurements of the vertical profile of sulfuric acid vapor. In the 1980s and 1990s the discovery of near‐infrared emission from the night side of Venus, through narrow spectral windows in the 0.9–2.5 micron region, opened a new opportunity of probing the lower atmosphere and surface. The abundances of water vapor, halides, carbon monoxide, sulfur dioxide, and carbonyl sulfide were determined, as well as the deuterium‐to‐hydrogen ratio. The vertical profile of water vapor in the 0‐ to 45‐km‐altitude range and the CO and OCS mean vertical gradients in the 30‐ to 40‐km region have been determined from various analyses. In addition, evidence for latitudinal variations of these two species was found from Galileo and recent ground‐based nightside observations. Conflicting results have been reported on the SO2 abundance below 35 km and on the H2O profile below 20 km. The whole set of available measurements provides important constraints on the chemistry and dynamics of Venus' lower atmosphere. However, key information is still missing, including the abundances of CO and sulfur compounds near the surface, composition maps at high latitudes, and possible small‐scale and temporal variability of trace compounds.