Sulfur Dioxide Not Phosphine May Explain That Mysterious Atmospheric Signature On Venus

Artist's impression of Venus. Image Credit: buradaki/Shutterstock.com

Last September, a cautious but exciting result was reported. Venus appears to have an abundance of phosphine in its clouds. Phosphine is mostly produced by living organisms so that certainly got people talking. In good science praxis, other scientists got involved to test if the finding was correct. Questions were asked and some problems with the data were found, leaving the mystery wide open.

Now a different team has put forward evidence to solve this. It was never phosphine at all. It was the most-like but less-exciting sulfur dioxide. The new scenario is accepted for publication in The Astrophysical Journal and is currently available as a pre-print, which means it has not yet been peer-reviewed.

"Instead of phosphine in the clouds of Venus, the data are consistent with an alternative hypothesis: They were detecting sulfur dioxide," co-author Victoria Meadows, a UW professor of astronomy stated in the press release. "Sulfur dioxide is the third-most-common chemical compound in Venus' atmosphere, and it is not considered a sign of life."

Chemicals in outer space are discovered by looking at the emission of electromagnetic radiation at particular frequencies. These signatures can sometimes get confusing because our instruments are not perfect and this is why observations are followed up with other instruments to find more evidence to confirm detection.

Let’s take a step back to the first detection of this signal. Back in 2017, the original research team used the James Clerk Maxwell Telescope (JCMT) and discovered a particular radio wave emission from the clouds of Venus. There were two molecules that would fit that signature. Phosphine and sulfur dioxide. The team decided to use the Atacama Large Millimeter/submillimeter Array (ALMA) to distinguish between the two. In that dataset (which we now know had other issues) they found that sulfur dioxide was not abundant enough, so they concluded that phosphine was likely the source of the original signal.

The new work took a different approach. They modeled the atmosphere of Venus and used that to interpret what was seen and not seen in the data from the two telescopes. Their model indicates that the signals are not coming from the clouds of Venus but from 80 kilometers (50 miles) above them, in the mesosphere. Not a place where phosphine is likely to survive.

"Phosphine in the mesosphere is even more fragile than phosphine in Venus' clouds," explained Meadows. "If the JCMT signal were from phosphine in the mesosphere, then to account for the strength of the signal and the compound's sub-second lifetime at that altitude, phosphine would have to be delivered to the mesosphere at about 100 times the rate that oxygen is pumped into Earth's atmosphere by photosynthesis."

The other major find is that they think that the ALMA data most likely underestimated the amount of sulfur dioxide present in the Venus atmosphere, giving the false impression that the bulk of the JMCT signal was likely coming from phosphine.

"The antenna configuration of ALMA at the time of the 2019 observations has an undesirable side effect: The signals from gases that can be found nearly everywhere in Venus' atmosphere—like sulfur dioxide—give off weaker signals than gases distributed over a smaller scale," added co-author Alex Akins, a researcher at the Jet Propulsion Laboratory.

The original team is currently re-examining the whole data set, and we look forward to seeing what their analysis comes up with.

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