Atmospheric chemistry on Venus: An overview of unresolved issues

Venus' atm. is 96.5% CO_2 and 3.5% N_2 with trace abundances of SO_2, OCS, H_2O, HCl, HF, and HBr, as well as their photochem. and lightning-induced products. The global clouds are composed at least partly of concd. sulfuric acid. The surface pressure is 90 atm and surface temps. exceed 700 K. Atm. chem. transitions from ion chem. through photochem. to thermal equil. chem. with heterogeneous chem. likely throughout the atm. Three major chem. cycles have been identified: the carbon dioxide, sulfur oxidn., and polysulfur cycles. The carbon dioxide cycle includes CO_2 photolysis, transport of a significant fraction of CO and O to the night side, prodn. of O_2, and conversion of CO and O_2 to CO_2, possibly via chlorine catalyzed pathways. The sulfur oxidn. cycle comprises transport upward of OCS, SO_2, and H_2O, oxidn. to H_2SO_4, condensation to form the global 30-km thick cloud layers, and sulfuric acid rain. The polysulfur cycle involves the upward transport of OCS and SO_2, disproportionation and prodn. of S_x (x=2-8), and downward transport of S_x to react with CO and SO_3. There is solid evidence for the carbon dioxide and sulfur oxidn. cycles; the polysulfur cycle is more speculative but plausible. Alternatively, sulfur chem. on Venus has been conceptually divided into fast and slow atm. cycles and a geol. cycle. Recent work (Parkinson et al, PSS, 2015) suggests the ternary SO_2-H_2O-H_2SO_4 system may bifurcate depending on the relative abundances of H_2O and SO_2. Despite this general understanding, five decades of spacecraft, and 200 years of observation, numerous significant unresolved issues remain. One is the means by which CO_2 is stabilized over geol. time - models predict O_2 abundances a factor of ten larger than the observational upper limit. Another is the lack of consistency among models of the chem. and microphysics in different regions, esp. 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. A third is the mechanism(s) creating an inversion layer in SO_2 abundances in the mesosphere. This talk presents an overview of our current understanding and key unresolved issues