Hot on the heels of two new space missions heading to Venus and a sensational — but disputed — paper suggesting the life-oriented chemical phosphine is present in the atmosphere, Venus is on everyone’s minds.
Some scientists are starting to turn their gaze toward the hellishly hot planet as a potential place to look for life in the Solar System — one more likely to bear fruit than Mars.
But there’s one small snag: Opportunities to look for alien life directly on other worlds are rare, so scientists will first try to rule candidates out. Typically, they will spend years studying various factors that affect whether or not an environment might be able to host life in the lab, or in a simulation. And doing just that, a study fresh off the press holds bad news for would-be Venusians — but also good news for hopeful Jovians.
What’s new — In a study published Monday in Nature Astronomy, a team of researchers takes a look at how accessible water is in the clouds of a planet, specifically Jupiter, Venus, and Mars. This could be a key indicator for whether life on another planet is possible — but unfortunately for Team Venus, the news is not good.
“The most dry-tolerant microbe on Earth wouldn’t stand a chance on Venus.”
“The most dry-tolerant microbe on Earth wouldn’t stand a chance on Venus,” John Hallsworth of Queen’s University Belfast said in a press briefing.
In the study, Hallsworth and his team describe a way to determine the “water activity” of a planet’s clouds — basically a measure of the relative humidity or saturation of water in that environment.
They applied this new measure to the clouds of Venus — clouds whose droplets are mostly made up of sulfuric acid — and found that the droplets had extremely low water activity levels.
“[Water activity] is another requirement of life for people to consider when they think about habitability,” Peter Gao, whose research focus is the atmospheres of other planets, including the clouds over Venus, tells Inverse. Gao is currently a postdoctoral fellow in the Department of Astronomy & Astrophysics at the University of California, Santa Cruz, and was not involved in the study.
HOW THEY DID IT — Researchers take many factors into account when evaluating how promising another world — whether in our Solar System or beyond — might be as a habitat for life. Common factors considered include whether liquid water could exist on the world and what kind of radiation it receives from its home star.
Another factor to consider is whether there is enough water in the clouds for extremely simple, primitive life to subsist there.
When calculating the water activity in the clouds for each planet, Hallsworth and colleagues looked at altitudes within the planets’ atmospheres where temperatures might be amenable to hosting life.
For Venus, this translated to altitudes of 40 to 70 kilometers, or about 25 to 44 miles, above the planet’s scorching hot surface.
Here’s a breakdown of the main steps the researchers take in the study:
- The team used data from past missions that studied the atmospheres of Venus and the other planets to calculate water activity
- As a way to check their water activity calculations for Venus, they also determined what the sulfuric acid concentrations of the droplets would be if they had the calculated water activity levels
- Comparing these calculations to direct measurements of sulfuric acid concentrations in these clouds from past studies confirmed that their numbers seemed accurate.
Importantly, Team Mars also has some reason for disappointment: The clouds over Mars also appear to have too low water activity for the most extreme Earth microbes to survive — though not nearly as low as on Venus.
Curiously, some of the cloud layers on Jupiter, on the other hand, did have water activity levels that could sustain Earth-like microbes.
WHY IT MATTERS — Researchers considering the possibility of microbial life in Venusian clouds have previously suggested that any lifeforms there would have adapted to collect and store water. This study suggests that if this is the case, then these alien lifeforms must be unlike any life found on Earth.
Now, it seems that at least for known species on Earth, no microbes would be capable of collecting enough water from an environment with as low a water activity as Venus’ clouds.
This may be a strike against the chances of life on Venus, but it’s not necessarily the end of the story. There’s still a chance that life evolved on Venus in a way to deal with the extremely low water activity.
“If there is no life, well then, that’s not surprising,” says Gao. “But if there is life — first of all, that would be world-shattering right? And yes, it’s extremely unlikely. But imagine if we did find some kind of life form in there.”
“That really tells you something about the resilience of life once it gets started,” he adds.
But one of the study’s other findings — that Jupiter’s clouds do have water activity levels that could sustain microbial life as we know it — also holds implications for future interplanetary exploration.
Perhaps it is there — in the tumultuous clouds of the gas giant — that we might find some signs of life.
WHAT’S NEXT — But before we get excited about the possibility of life in the mists of Jupiter, there are upcoming missions to Venus — two from NASA and one from the European Space Agency — that will teach us a lot about the planet.
DAVINCI+, one of the NASA missions, will investigate the evolutionary history of Venus’ atmosphere. That might tell us whether life may have been possible in Venus’ past, even if the prospects for current life on the planet don’t look too good.
Aside from the question of whether Venus is or was once habitable, studying Venus is vital to understanding planets — and the potential for alien life — throughout the universe. Astronomers have found many planets with masses and sizes similar to Earth orbiting other stars, but that doesn’t mean the planets are Earth-like — they may be much more like our more extreme neighbor.
Venus’ mass and size are practically identical to Earth’s, but the two planets are vastly different from one another. Gao points to three open questions for researchers to answer:
- Is the difference between Venus and Earth mostly due to the planets’ respective distances from the Sun?
- Do other factors govern the two planets’ distinctive fates?
- Is Venus an inevitable “end state” for Earth-sized planets?
“Are we living in a galaxy — or indeed, a universe — of diverse rocky planets?” Gao asks. Put another way:
“Are we living in a universe of Earths? Or are we living in a universe of Venuses?”
Abstract: The recent suggestion of phosphine in Venus’s atmosphere has regenerated interest in the idea of life in clouds. However, such analyses usually neglect the role of water activity, which is a measure of the relative availability of water, in habitability. Here we compute the water activity within the clouds of Venus and other Solar System planets from observations of temperature and water-vapour abundance. We find water-activity values of sulfuric acid droplets, which constitute the bulk of Venus’s clouds, of ≤0.004, two orders of magnitude below the 0.585 limit for known extremophiles. Considering other planets, ice formation on Mars imposes a water activity of ≤0.537, slightly below the habitable range, whereas conditions are biologically permissive (>0.585) at Jupiter’s clouds (although other factors such as their composition may play a role in limiting their habitability). By way of comparison, Earth’s troposphere conditions are, in general, biologically permissive, whereas the atmosphere becomes too dry for active life above the middle stratosphere. The approach used in the current study can also be applied to extrasolar planets.