Sulfur oxide profiles on Venus: Interactions among atmospheric chemistry, dynamics, and microphysics

Venus can be considered our nearest exoplanet. Several of its distinctive features are due to the sulfur chemistry occurring in its atmosphere. The vertical profile of the primary sulfur species, SO₂, reflects the combined effects of gas-phase chemistry, aerosol and cloud microphysics, and dynamics. The primary reservoir for SO₂ lies in the troposphere. Through the cloud layers, but particularly within the upper cloud, SO₂ oxidizes and reacts with water to form sulfuric acid, one of the main constituents of the cloud particles. Another, so far unidentified species is present within the upper cloud that accounts for half of the solar energy absorbed within Venus’ atmosphere. Polysulfur, S_x, is one of the leading candidates for this unidentified species. If this identification is correct, then its production is linked to the loss of SO₂ as air parcels are convected and mixed upward through the cloud layers. At higher altitudes, in the upper mesosphere, an inversion layer has been observed where SO₂ abundances again increase with increasing altitude. This inversion layer may arise from upward transport of sulfuric acid, polysulfur, and/or chlorine-sulfur compounds. It may also, at least partially, arise from transport and quenching. The current state of research on these phenomena will be reviewed and potential implications for extrasolar planet simulations and observations discussed.