Photochemical and thermochemical pathways to S2 and polysulfur formation in the atmosphere of Venus

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 S2 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 S3, S4, and S8. We report here the results of high-level ab-initio quantum-chemistry computations that demonstrate that S2 is not a product in cis-OSSO photolysis. Instead, we establish a novel mechanism in which S2 is formed in a two-step process. Firstly, the intermediate S2O 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, S2O reacts with SO. This modified chemistry yields S2 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.

Laboratory infrared spectra and fragmentation chemistry of sulfur allotropes

“…Moreover, S 8 was detected in Ryugu samples 21 . Sulfur allotropes may also play an important role in providing UV opacity in the Venusian atmosphere 22 , and in Venus-type exoplanets 23 . Density functional theory (DFT) calculations have suggested that among the S N allotropes, octasulfur S 8 is especially stable 24 , with astrochemical models highlighting its relevance in the dynamics of sulfur in the interstellar medium (ISM).…”

Potential Energy Curves and Ultraviolet Absorption Cross Sections of Sulfur Dimer

“…An essential part of sulfur-rich anoxic chemistry is elemental sulfur. Sulfur dimer (S 2 ) has been observed in various extraterrestrial environments ranging from interstellar clouds to our solar system bodies, such as Venus, − Jupiter’s moon Io, , and various comets in our solar system. , …”