New analysis shows how sulfur clouds can form in Venus' atmosphere

Scientists using sophisticated computational chemistry techniques have identified a new pathway for how sulfur particles can form in the atmosphere of Venus. These results may help to understand the long sought-after identity of the mysterious ultraviolet absorber on Venus.

"We know that the atmosphere of Venus has abundant SO₂ and sulfuric acid particles. We expect that ultraviolet destruction of SO₂ produces sulfur particles. They are built up from atomic S (sulfur) to S₂, then S₄ and finally S₈. But how is this process initiated, that is, how does S₂ form?" said Planetary Science Institute Senior Scientist James Lyons, an author on the Nature Communications paper "Photochemical and thermochemical pathways to S₂ and polysulfur formation in the atmosphere of Venus."

One possibility is to form S₂ from two sulfur atoms, that is, reaction of S and S. Molecules of S₂ and S₂ can then combine to form S₄, and so on. Sulfur particles can form either by condensation of S₈ or by condensation of S₂, S₄ and other allotropes—different physical forms in which an element can exist—which then rearrange to form condensed S₈.

"Sulfur particles, and the yellow sulfur we more commonly encounter, are made up of mostly S₈, which has a ring structure. The ring structure makes S₈ more stable against destruction by UV light than the other allotropes. To form S₈, we can either start with two S atoms and make S₂, or we can produce S₂ by another pathway, which is what we've done in the paper," said Lyons.

"We found a new pathway for S₂ formation, the reaction of sulfur monoxide (SO) and disulfur monoxide (S₂O), which is much faster than combining two S atoms to make S₂," Lyons said.

"For the first time, we are using computational chemistry techniques to determine which reactions are most important, rather than waiting for laboratory measurements to be done or using highly inaccurate estimates of the rate of unstudied reactions. This is a new and very much needed approach for studying the atmosphere of Venus," Lyons said. "People are reluctant to go in the lab to measure rate constants for molecules made up of S, chlorine (Cl), and oxygen (O)—these are difficult and sometimes dangerous compounds to work with. Computational methods are the best—and really only—alternative.

Computational methods were used to compute the rate constants and to determine the expected reaction products. These are state-of-the-art computational models (what we call ab initio models). These ab initio calculations were done by the authors from Spain and from the University of Pennsylvania.

"This research illustrates another pathway to S₂ and sulfur particle formation. Sulfur chemistry is dominant in Venus' atmosphere, and very likely plays a key role in the formation of the enigmatic UV absorber. More generally, this work opens the doors to using molecular ab initio techniques to disentangle the complex chemistry of Venus," Lyons said.