Venus’s clouds are too dry for life

Scientists announced last September that they had spotted phosphine, produced by certain microbial metabolism on Earth, in Venus’s atmosphere—a possible sign that microbes live in the planet’s thick clouds. The claim set off an ongoing debate about whether or not the detection was real, and what it meant about extraterrestrial life. But new research says that argument is moot; Venus’s clouds don’t have enough water to support life (Nature Astron. 2021, DOI: 10.1038/s41550-021-01391-3).

Venus has a similar size, composition, and gravity to Earth, but its dense carbon dioxide atmosphere, sulfuric acid clouds, and surface temperatures above 450 °C make it inhospitable to life as we know it. Conditions are more moderate a few dozen kilometers up, in Venus’s clouds, and examples on Earth of microbes that can survive in clouds and in acidic environments had made life on Venus seem plausible.

Not so, according to a group of researchers led by Queen’s University Belfast’s John E. Hallsworth, an expert on organisms that can survive extremely dry conditions. The team’s analysis of Venusian sulfuric acid cloud droplets found that their water content is two orders of magnitude below the known limit for life, set by a common household fungus here on Earth.

The researchers drew on measurements of temperature, pressure, and moisture made by probes sent to Venus in the 1970s and 80s to calculate the water activity—a value analogous to relative humidity—of droplets at different temperatures and sulfuric acid concentrations. “We can say directly: Is there enough water in the clouds for life?” study coauthor Christopher P. McKay of the NASA Ames Research Center asked at a press conference. The answer, he said, was no, “by a long shot.” McKay added that new missions to Venus recently approved by NASA are unlikely to change that conclusion.

Prebiotic chemistry expert Paul B. Rimmer of the University of Cambridge agrees that the results make it seem impossible that Earth-like organisms could survive on Venus. But he cautioned in an email that scientists’ understanding of Venus’s clouds and their chemistry remain limited. New missions should help with that, he says, but he agrees that they probably won’t make life on Venus seem more probable.

Laura Kreidberg, an exoplanet researcher at the Max Planck Institute for Astronomy, calls the study a “nail in the coffin for the life on Venus theory” in an email. She sees a valuable lesson for exoplanet researchers: to make sure the next generation of telescopes can make the measurements necessary to analyze water availability on those distant bodies.

While Hallsworth, McKay and their colleagues have ruled out life on Venus, their calculations suggest that Jupiter has clouds with enough water to support life as we know it. But they caution that water is just one of the factors that determine habitability.