The Likelihood of Life on Venus Just Increased Dramatically

Jane Greaves from Cardiff University in the U.K. and several co-authors, including some from MIT, just announced in an exciting new paper the discovery of phosphine gas in the clouds of Venus. Astrobiologists have long sought to detect a “biosignature” that would prove the presence of life on another world beside Earth. This finding falls short of that ambitious and elusive goal, but it may be the closest we’ve come yet.

The authors detected phosphine in the Venusian atmosphere at a concentration of about 20 parts per billion, using thorough spectral analysis and observations from two different Earth-based telescopes in 2017 and 2019.

In a paper published earlier this year, some members of the same team identified phosphine as an ideal biosignature gas to detect life on exoplanets, because it has uniquely identifiable spectral features and lacks “abiotic” false positives—meaning that no non-biological explanations are known that could account for measurable concentrations in a planet’s atmosphere. In that earlier paper, however, they pointed out that phosphine could be difficult to detect on another world, given the low amounts that would be likely to exist, its reactivity with other chemical compounds, and its vulnerability to being broken apart by ultraviolet irradiation.

It’s all the more surprising, then, that phosphine was detected in the Venusian clouds, where high amounts of UV irradiation and at least some radicals are present, which would destroy (oxidize) the gas rather rapidly. That means there must be something continuously replenishing the phosphine.

What, though? On Earth, this colorless gas, which is toxic to humans and other mammals, is produced by microbes living under oxygen-free conditions. The authors took a Sherlock Holmes-type approach to searching for other explanations for the phosphine detected on Venus: try the most obvious possibilities first. They tested for various chemical reaction pathways in the atmosphere and on the surface, including reactions induced by light, and couldn’t explain the high concentrations that were observed. Lightning, which has been reported to occur on Venus, couldn’t explain it either. They even considered the possibility that the phosphine was delivered by asteroids or comets, but again, the calculated values were way too low. Besides, radar images of the Venusian surface didn’t show a recent major impact. That left biology in the clouds of Venus, or some unknown geochemical or photochemical process, as the only explanations.

The possible presence of a Venusian aerial biosphere has been suggested by several authors, including me, and most recently, by Sara Seager and co-authors, some of whom are also authors on this new paper. Environmental conditions in the clouds of Venus are extremely harsh, but may not have been at earlier times in the planet’s history, meaning that microbial life might have had time to adapt.

There is one other intriguing detail about the new finding. Phosphine was detected near the equatorial and temperate latitudes, but not in the polar area. As the authors point out, this would be consistent with a biological explanation, because the atmospheric circulation patterns in the mid-latitudes would offer the most stable environment for life.

Dirk Schulze-Makuch | | READ MORE

Dirk Schulze-Makuch is a Professor at the Technical University Berlin, Germany, and an Adjunct Professor at Arizona State University and Washington State University. He has published eight books and nearly 200 scientific papers related to astrobiology and planetary habitability. His latest books are The Cosmic Zoo: Complex Life on Many Worlds and the 3rd edition of Life in the Universe: Expectations and Constraints.

Website