Venus is now the hottest planet in the solar system in more ways than one, as scientists buzz over three newly announced missions to explore it, while debate continues over whether a biosignature exists within its clouds. But as new research today shows, Venus has an atmosphere that’s way too dry to support even the hardiest microorganisms — a finding that could restore the planet’s longstanding reputation as Earth’s most inhospitable neighbour.
“The water in the clouds of Venus is just not up to the levels required to support life, and that’s unfortunate,” Chris McKay, an astrobiologist at NASA Ames Research Centre and a co-author of the new study, said at a press conference held virtually this past Friday.
It’s unfortunate, McKay continued, because “I’m interested in the search for life on other worlds, and I would like to think that Venus is habitable.” The evidence presented in his team’s new paper, published today in Nature Astronomy, suggests otherwise. The finding is also unfortunate, said McKay, because “our conclusion is based directly on measurements” and not a “model based on assumptions,” so “it’s hard to imagine that the results will change as we do further exploration.”
The seemingly far-fetched idea that Venus might be able to sustain life received renewed interest last year, when scientists presented evidence showing trace amounts of phosphine in its atmosphere. As living organisms are one of the only known sources of this gas, it was taken as potential evidence of life. Suddenly, Venus — a scorching planet typically regarded as being an exceptionally poor candidate for habitability — was being noticed by astrobiologists. Other scientists have since poured cold water on this finding, with the new Nature paper piling on the doubt even further.
John Hallsworth, a microbiologist at Queen’s University in Belfast who led the new research, specialises in understanding how much stress organisms can endure, including the minimum amount of water that can sustain extremophile organisms on Earth. Alerted to the possible detection of life in the Venusian clouds, he wondered if there was enough water in this planet’s atmosphere to allow it. As the scientists wrote in the new paper, some prior “analyses usually neglect the role of water activity, which is a measure of the relative availability of water, in habitability.”
“It’s well known, of course, that life needs water,” noted Hallsworth during the press conference. “Living systems, including microorganisms, are composed mainly of water, and without being hydrated they cannot be active and are unable to proliferate.”
Some microorganisms are capable of surviving with limited access to water, including within water droplets in the clouds of Earth’s atmosphere. Some of these microorganisms, “where temperatures allow,” are metabolically active and capable of going through cell division, said Hallsworth. Accordingly, astrobiologists and planetary scientists are naturally focused on areas where water exists, or at least used to exist.
For the new study, the scientists calculated the amount of water activity in the Venusian atmosphere and those of other planets in our solar system. Water activity can be measured on a scale from 0 to 1, and it’s similar to how relative humidity is measured in our atmosphere. As the scientists pointed out, life on Earth requires water activity of at least 0.585 to sustain metabolism and reproduction. The scientists used this as a kind baseline for their analysis, in a figure that was derived by studying extremophiles capable of tolerating low water conditions and microorganisms capable of withstanding exposure to acid. Indeed, the clouds of Venus contain generous amounts of sulphuric acid that’s intermixed with small amounts of water.
To record water activity, the scientists analysed previously recorded measurements of air temperature, air pressure, and the availability of water vapour on Venus. The team decided to use the same approach to study other planets, namely Jupiter, Mars, and Earth.
On Venus, water activity was measured along a band located between 42 and 68 km above the surface, as the temperature within this layer falls within the bounds of habitability. Water activity on Venus was calculated at 0.004, which is more than 100 times below the known limit for life and an “unbridgeable distance from what life requires to be active,” as Hallsworth explained. Water activity within this layer was not uniform, but even the highest amounts were “too far off the scale” for any form of active life, he added.
Indeed, this extreme dryness is bad news for habitability. The sulphuric acid in the Venusian skies is largely responsible for reducing water availability on Venus, according to the researchers. For microbes to exist within the clouds of Venus, therefore, they’d have to be extremophiles of a completely unknown type that defies our understanding of biology.
For Mars, the water activity was recorded at 0.537, which is comparable to what’s observed in Earth’s stratosphere (a thin layer in our upper atmosphere). Again, this value is below the acceptable limit for life but considerably higher than the value obtained for Venus. The clouds on Mars “are not biologically permissive due to the low temperatures that are inconsistent with cellular function,” not to mention the “high ultraviolet radiation that can be lethal for atmospheric microbes,” as the scientists wrote in the study.
Surprisingly, water activity within the clouds of Jupiter, in regions where temperatures range between 10 degrees Celsius and -40 degrees Celsius, was recorded at levels above the minimum threshold of 0.585. Which, wow.
“One thing we found, which was unexpected, was that the clouds of Jupiter actually have the right temperature and water activity combination to support active life — we absolutely hadn’t expected that,” said Hallsworth. “Now, I’m not suggesting that life exists on Jupiter, and I’m not even suggesting that life could be there, because it would need the right nutrients to be there, and we can’t be sure of that.”
Life, as he pointed out, “doesn’t only need a nice temperature and water availability to be active,” but this finding, that water activity deep with Jupiter’s cloud layer is potentially amenable to life, is “profound and exciting,” Hallsworth said. It would require a “whole new study” to evaluate this further and to investigate the kinds of microbes that could theoretically survive in the Jovian clouds, he added.
The new paper potentially casts a shadow on three upcoming missions to Venus, namely NASA’s VERITAS and DAVINCI+ missions and ESA’s EnVision orbiter. But the authors of the new paper don’t see it that way, as these probes are likely to gather data that will further corroborate the new finding. McKay expects similar results, saying our new understanding of water activity on Venus is not likely to change.
Interestingly, the scientists said their new approach can also be used to study the atmospheres of exoplanets, worlds around other stars. The upcoming James Webb Space Telescope was invoked as an instrument capable of taking the required measurements. The ability to detect water activity on planets both near and far represents a powerful new ability for astrobiologists, who continue to study the limits of life.