In 1978, NASA launched the Pioneer Venus mission, consisting of an orbiter and a group of four small probes that were jettisoned into the Venusian atmosphere. Within the atmosphere were signs of deuterium—a heavy isotope of hydrogen that can result from the breakdown of water. Scientists wondered whether Venus might have once been home to larger amounts of water, and whether some had actually stuck around in larger quantities in the atmosphere.
Fast forward to 2020 and the possible detection of traces of phosphine in Venus’s atmosphere. Those scientists contemplated a scenario for how a potential water cycle in Venus’s sulfuric acid-heavy clouds could allow microbes on Venus to exist in droplets at high altitudes and produce spores that could be hydrated and keep a reproductive life cycle going. Though the planet’s surface is hell, its clouds are stable and more temperate.
Well, the new paper means that’s pretty unlikely. The study focuses on “water activity,” or the amount of water that’s available to microorganisms to use, measured on a scale of 0 to 1. For this study, the research team sought to measure water activity in the clouds by calculating the atmosphere’s relative humidity (the amount of water that has saturated the air at a given temperature). Scientists have used Aspergillus penicillioides, a fungus capable of living in some of the driest conditions imaginable, as a baseline for understanding how much water scarcity an organism can endure while still being able to run metabolic functions and reproduce. The answer is a water activity score of 0.585—effectively the “life limit” of biological activity as we know it.
Using atmospheric data collected from past Venus missions and using newer models for how to assess water activity, Hallsworth and his team calculated the water activity of Venus’s clouds 68–42 kilometers in altitude, where life-tolerant temperatures range from -40°C to 130°C. They found that water activity is, at best, 0.004. “The most dry tolerant microbe on Earth wouldn’t stand a chance on Venus,” says Hallsworth.
The researchers also point out that even if the water activity itself was higher, Venus’s atmosphere is saturated with hostile elements that would likely hinder cellular systems from running properly (for example, sulfuric acid dehydrates cells).
Other planets fared better. The team also calculated water activity in Martian clouds to be 0.537 (comparable to Earth’s stratosphere, and just a smidge below the “life limit” for life on Earth), and in Jupiter’s clouds it happens to be at least 0.585 in locations where the temperature is between -10°C and 40°C. “We can’t say that Jupiter’s clouds are habitable,” says Christopher McKay, a NASA scientist and a coauthor of the study. “We can say they are not limited by water activity.”
The findings will need to be confirmed with more study, but the authors are pretty confident that won’t change, even with two new NASA missions and one new ESA mission headed to Venus near the end of the decade.
Of course, there are some caveats. “We have to base our discussions on life on other worlds on what we know about life on Earth, because we have a basis for that,” says McKay. “But some part of me hopes that when we do find life elsewhere, it is really very, very different,” with biochemistry that works beyond the limits of what we’ve seen here on Earth.
And while current life on Venus might seem improbable with these new findings, it doesn’t mean Venus was always barren. There’s a whole hidden history to the planet that scientists want to investigate.