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

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High in the atmosphere of Venus, a swirl of sulfur-containing clouds blankets the planet. Dark spots in these clouds correspond with areas of high ultraviolet light absorption, but researchers have yet to confirm the chemical culprit responsible for this activity. 

In a recent study, scientists using sophisticated computational chemistry techniques have identified a new pathway for how sulfur particles can form in the Venusian atmosphere. The results clarify the planet’s source of sulfur-rich clouds, hint at the identity of the long-mysterious ultraviolet-absorbing compound, and offer a note of caution for efforts to geoengineer a solution to climate change by injecting sulfur into Earth’s atmosphere.

“Scientists have been trying to understand the source of these ultraviolet absorbing emissions on Venus for a while,” says Joseph S. Francisco, an atmospheric chemist at the University of Pennsylvania and a corresponding author on the new work, published in Nature Communications. “To date, none of these emissions correspond to anything that we know here on Earth. That’s sparked a lot of interest and excitement because that means there is some new chemistry to discover.”

“We know that the atmosphere of Venus has abundant SO₂ (sulphur dioxide) and sulfuric acid particles,” says James Lyons, senior scientist at the Planetary Science Institute and another corresponding author on the paper. “We expect that ultraviolet destruction of SO₂ produces sulfur particles. They are built up from atomic S (sulfur) to S₂ (disulfur), then S₄ and finally S₈. But how is this process initiated, that is, how does S₂ form?” 

Earlier efforts had hypothesized one possibility: that S₂ formed 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.

Francisco and Penn postdoc Tarek Trabelsi, however, looked to identify a different pathway to form S₂ that could more accurately model what was known about sulfur chemistry in Venus’ atmosphere. Laboratory experiments involving sulfur, chlorine, and oxygen can be difficult and even hazardous, making computational methods an appealing alternative. Working with colleagues from Spain, they developed state-of-the-art computational models that factored in the wave lengths of light that would cause sulfur to compounds to break apart and the rates at which they would do so. Lyons and other colleagues incorporated those calculations into atmospheric models of Venus.

Together, their work revealed a new and faster pathway to form S_2, involving a reaction between sulfur monoxide (SO) and disulfur monoxide (S₂O), with interactions with chlorine compounds as an intermediate step.

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

Francisco adds that the findings can also help in weighing what to avoid when it comes to chemistry on Earth. While some scientists and engineers have proposed adding sulfur compounds like sulfur dioxide and trioxide into the Earth’s atmosphere to reflect light and reduce the impacts of climate change, Francisco notes that many unknowns exist about how such an intervention would play out.  

“Knowing what chemistry takes place on other planets helps guide us in understanding new chemistry that might occur on Earth and what we don’t want to happen in Earth’s atmosphere,” he says. 

Joseph S. Francisco is President’s Distinguished Professor in the Department of Earth and Environmental Science and Department of Chemistry in the University of Pennsylvania School of Arts & Sciences.

Francisco and Lyons’ coauthors on the work were Penn’s Tarek Trabelsi; Antonio Francés-Monerris, Javier Carmona-García, and Daniel Roca-Sanjuán of the Universitat de València, Spain; and Alfonso Saiz-Lopez of the Institute of Physical Chemistry Rocasolano in Madrid. Francés-Monerris was lead author and Francisco, Francés-Monerris, Lyons, and Roca-Sanjuán were co-corresponding authors.

The study was supported in part by Generalitat Valenciana and the European Social Fund, the Ministerio de Ciencia e Innovación, and “la Caixa” Foundation.

Adapted from a press release by Alan Fischer, Planetary Science Institute, fischer@psi.edu or 520-382-0411.