Phosphine Detected in Venus’s Atmosphere May Have Just Been Sulfur

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The potential discovery of phosphine in Venus’s atmosphere last year made headlines around the world. On Earth, phosphine is produced by living things. Any detection of it inside another planet’s atmosphere would be a strong potential indicator of life. One reason folks got excited about the possibility is that Venus’s upper atmosphere is a much friendlier place for life to exist than its lower cloud layers or the lead-melting temperatures on the ground. The conditions approximately 50km above the planet’s surface are reportedly the most Earthlike in the solar system, with a pressure of approximately 1 atm and temperatures ranging from 0 to 50C. Could life have evolved within those cloud layers, or even migrated from the surface to the atmosphere as Venus’s climate changed? The detection of phosphine suggested that it might have.

A new analysis of the initial data, however, finds that the Cardiff researchers who first reported the detection of phosphine may have mistakenly been picking up sulfur dioxide instead. The authors of this new paper, to be published in Nature, note that the original paper claimed 20ppb (parts per billion) of PH3 were detected in the Martian atmosphere. After some reassessment of their initial findings, the original Cardiff team asserted that the phosphine signal still remained, but at a much lower concentration of 1ppb, not 20pbb. Even one part per billion would still be interesting because phosphine is not thought to persist for any length of time in Venus’ atmosphere under any conditions, but it’s obviously a much weaker signal than initially thought.

The surface of Venus captured by a Soviet Venera probe.

Now, however, a further examination of the data argues that even that 1ppb is a measurement error.

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

Meadows, lead author Andrew Lincowski, and the other researchers affiliated with this project created a radiative transfer model of the planet’s atmosphere and re-examined the data. Their paper suggests that the initial report erred by attempting to use ALMA (Atacama Large Millimeter Array) to estimate the amount of SO2 in Venus’s atmosphere. The telescope may have missed as much as 90-95 percent of the sulfur dioxide actually present, greatly increasing the chance that the specific signal attributed to phosphine at 266.94GHz is actually being caused by sulfur dioxide instead. The initial findings were attributed to phosphine because the amount of sulfur dioxide in the atmospheric layer where the phosphine was detected was thought to be low.

This new research also claims that the signal was detected far higher in the Martian atmosphere than previously thought. Venus has a thick, dense atmosphere, dense enough to prevent smaller meteors from reaching the ground. If the phosphine signal was coming from the troposphere, there was a chance the upper layers of the atmosphere were shielding whatever life forms might be present. According to this team, however, the signal was actually being picked up in the mesosphere. Venus is much closer to the sun than Earth is, and the increased solar radiation at the top of its clouds would tear phosphine molecules apart almost as quickly as they could form. The chance of detecting phosphine in the upper levels of the atmosphere is very small, even if it’s produced by living things below.

The authors do not claim to have disproven the initial phosphine report and they call for other research teams to continue investigating Venus for any hint of phosphine. Even if the signal turns out to be a false positive, this process of claims and counter-claims is almost certainly how scientists will eventually prove we have discovered life on a different planet. Any fossil or purported living creature found within the sands of Mars or beneath the ice sheets of Europe will undergo extensive analysis to prove that it’s extraterrestrial in origin and not evidence of sample contamination from an Earth-based source.

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