Photochemical and thermochemical pathways to S2 and polysulfur formation in the atmosphere of Venus
Antonio Francés‐Monerris, Javier Carmona‐García, Tarek Trabelsi, Alfonso Saiz‐Lopez, J. R. Lyons, Joseph S. Francisco, Daniel Roca‐Sanjuán
Abstract
Abstract Polysulfur species have been proposed to be the unknown near-UV absorber in the atmosphere of Venus. Recent work argues that photolysis of one of the (SO) 2 isomers, cis -OSSO, directly yields S 2 with a branching ratio of about 10%. If correct, this pathway dominates polysulfur formation by several orders of magnitude, and by addition reactions yields significant quantities of S 3 , S 4 , and S 8 . We report here the results of high-level ab-initio quantum-chemistry computations that demonstrate that S 2 is not a product in cis -OSSO photolysis. Instead, we establish a novel mechanism in which S 2 is formed in a two-step process. Firstly, the intermediate S 2 O is produced by the coupling between the S and Cl atmospheric chemistries (in particular, SO reaction with ClS) and in a lesser extension by O-abstraction reactions from cis -OSSO. Secondly, S 2 O reacts with SO. This modified chemistry yields S 2 and subsequent polysulfur abundances comparable to the photolytic cis -OSSO mechanism through a more plausible pathway. Ab initio quantification of the photodissociations at play fills a critical data void in current atmospheric models of Venus.