Venus' atmosphere is 96.5% CO2 and 3.5% N2 with trace abundances of SO2, OCS, H2O, HCl, HF, and HBr, as well as their photochemical and lightning-induced products. The global clouds are composed at least partly of concentrated sulfuric acid. The surface pressure is 90 atm and surface temperatures exceed 700 K. Atmospheric chemistry transitions from ion chemistry through photochemistry to thermal equilibrium chemistry with heterogeneous chemistry likely throughout the atmosphere. Three major chemical cycles have been identified: the carbon dioxide, sulfur oxidation, and polysulfur cycles (e.g., Mills et al, AGU, 2007). The carbon dioxide cycle includes CO2 photolysis, transport of a significant fraction of CO and O to the night side, production of O2, and conversion of CO and O2 to CO2, possibly via chlorine catalyzed pathways. The sulfur oxidation cycle comprises transport upward of OCS, SO2, and H2O, oxidation to H2SO4, condensation to form the global 30-km thick cloud layers, and sulfuric acid rain. The polysulfur cycle involves the upward transport of OCS and SO2, disproportionation and production of Sx (x=2-8), and downward transport of Sx to react with CO and SO3. There is solid evidence for the carbon dioxide and sulfur oxidation cycles; the polysulfur cycle is more speculative but plausible. Alternatively, sulfur chemistry on Venus has been conceptually divided into fast and slow atmospheric cycles and a geologic cycle (e.g., Marcq et al, SSR, 2018, Krasnopolsky, Icarus, 2013, 2016). Recent work (Parkinson et al, PSS, 2015) suggests the ternary SO2-H2O-H2SO4 system may bifurcate depending on the relative abundances of H2O and SO2. Despite this general understanding, five decades of spacecraft, and 200 years of observation, numerous significant unresolved issues remain. One is the means by which CO2 is stabilized over geologic time - models predict O2 abundances a factor of ten larger than the observational upper limit (e.g., Mills and Allen, PSS, 2007). Another is the lack of consistency among models of the chemistry and microphysics in different regions, especially in the cloud layers, where the mixing ratios of many important trace species change by orders of magnitude within several vertical scale heights, and at the surface (Marcq et al, SSR, 2018). A third is the mechanism(s) creating an inversion layer in SO2 abundances in the mesosphere. This talk presents an overview of our current understanding and key unresolved issues.

Atmospheric chemistry on Venus: An overview of unresolved issues

255th National Meeting (2018)

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