The clouds of Venus have captivated Earthlings for decades. They form a dazzling mirror obscuring the surface of the planet, and in the 1950s an Israeli scientist speculated that clouds might conceal a world teeming with insect life that can withstand intense heat.
When the Russian spacecraft Venera took pictures of the surface in 1975, there were no insects to be found. Venus is a deserted inferno, a victim of a runaway greenhouse effect that has raised temperatures on Earth to more than 850 degrees Fahrenheit — hot enough to melt lead. But in the clouds, the milder atmospheres await any potential alien life forms.
In the air, life may find a way.
At least, that was one hypothesis. It could happen on Earth… so why wouldn’t it happen elsewhere? Last year, the idea that microbes might call Venus’s atmosphere home was bolstered by a study that claimed to have detected elevated levels of phosphine – an unstable gas linked to biological activity – in the surface of clouds on our sister planet. This gave rise to a theory that microbes in clouds could produce the gas.
As scientists decoded the phosphine signal, though, the possibility that it might be a sign of life in Venus’ clouds seemed less and less likely.
On Monday, the prospect of a community of microbes drifting into the clouds took another blow.
In a study published in Nature Astronomy, researchers ruled out the possibility of life – as we know it – in the clouds of Venus. Clouds are uninhabitable.
“The most extreme life on Earth would have absolutely no chance of living in the clouds of Venus,” John Hallsworth, a microbiologist at Queen’s University Belfast and lead author of the new research paper, told me.
The problem is the availability of water. When planetary scientists look for life elsewhere in the universe, they look for water because it is essential for life. Venus clouds might seem like a good place to start, but they’re not the kind of clouds we’re used to on Earth. Venus’ clouds consist mostly of liquid droplets of sulfuric acid – the substance we use to clean drains – at concentrations that Hallsworth describes as “biologically hostile.”
This detail already puts microbial life in the background, but Halsworth and his team wanted cold, hard data to assess the possibility. They studied data collected by spacecraft that dived into the atmosphere of Venus to analyze the water activity within the clouds and also turned their attention to Mars and Jupiter.
In September 2020, astronomers announced Detection of an excessive amount of phosphine gas in the upper atmosphere of Venus. The research team was baffled by this unexplained chemistry because on Earth, microbes can produce phosphine – which smells like rotting fish.
The researchers were quick to point out that their discovery was not Final The sign of life is in the clouds of Venus, but scientists and the public have been buzzing about it all. It inspired Jim Bridenstine, NASA administrator at the time, to announce that it was time to prioritize Venus. Venus fever spread through planetary science. Suddenly, the idea of searching for life on Mars seemed like a distant memory.
The possibility of life on Venus was puzzling, but scientists began to double and triple check their discoveries. After the phosphine was announced, other research groups began looking at the data, which had been obtained from the ALMA Ground Telescope. Secondary examinations appear to indicate that the phosphine signal may be erroneous or may not be as strong as originally thought.
This is where Hallsworth and his associates set out. For nearly three decades, Hallsworth has been investigating how water activity affects microbial life. His work explored the lower limits of life – or how “dry” it must be before biological activity collapses.
Knowing that Venus’ clouds are high in sulfuric acid, which reduces water’s activity, he said “alarm bells are ringing” before even reading the phosphine paper. His colleagues turned to him to ask if he knew of water activity in the clouds, hoping to paint a clearer picture of what was happening there.
But he did not look, so the team went to work. Within two weeks, Hallsworth and the group analyzed data collected by NASA’s Pioneer probe and Russia’s Venera, a swarm of spacecraft that plunged into Venus’s atmosphere in the 1970s. Data collected by the craft gave them insight into temperatures, pressures, and water stuck in clouds.
Any potential microbe floating through the clouds of Venus would find itself in a very hostile environment. About 30 to 44 miles above the surface, in the clouds, it is drier than the most extensive subtropical desert on Earth: the Sahara.
“The clouds of Venus are drier than the Sahara,” Hallsworth said, noting that the Sahara has about 0.25 water activity, while Venus’s clouds come in at only 0.004 water activity. This number for Venus is simply too extreme to support any life we know of.
“The most drought-tolerant microbes on Earth wouldn’t stand a chance on Venus,” Hallsworth said.
Not only is the water activity extremely low, but supposing that a microbe found a drop of liquid in the clouds of Venus — an oasis in the desert sky — its fate would be no better. These potential oases are actually toxic droplets of sulfuric acid. Any microorganism will find acidity too much.
“There is no known microbial cell that can remain intact at high concentrations of sulfuric acid within the droplets of Venus’ clouds,” Halsworth said.
Microbes that find themselves in such a place will face instant death, tearing their membranes and spilling their insides.
Hallsworth’s previous work investigated the outer limits of life for microbes on Earth. When it comes to aquatic activity, fungi known as Aspergillus penicillioides, which are found in old books or tied up in dust inside pillows and duvets, reign supreme on our planet. In 2017, Hallsworth’s lab published a paper showing that fungi can grow and divide at 0.585 water activity. This is impressive for the organism of the Earth.
They use this number – 0.585 water activity – as the limit the microbe can tolerate, but the water activity in Venus’ clouds is two times less by volume.
Mars, too, does not have water activity to facilitate life in the atmosphere – it’s very cold – but another planet in our solar system has water activity that exceeds 0.585. It’s a surprising candidate for an extraterrestrial home.
The gas giant Jupiter looms over all the planets orbiting the sun. It’s wild, and like Venus, it’s notorious for its pull. When the research team analyzed the activity of water in Jupiter’s atmosphere, they found that it was at a high enough level to allow microbes to inhabit.
“We didn’t expect that at all,” Hallsworth said.
Using readings from NASA’s Galileo probe, which launched in 1989 and launched a probe into the gas giant’s atmosphere in 1995, the team revealed that somewhere under Jupiter’s twisted face, conditions existed for life.
“There is at least a layer in Jupiter’s clouds where water requirements are being met,” Chris McKay, a planetary scientist at NASA’s Ames Research Center and co-author of the paper, said during a press conference.
However, the team was quick to point out that this is not an indication that there is life on this giant planet. The water activity only shows that the microbes we know on Earth can inhabit a distinct layer within Jupiter’s atmosphere. Halsworth said whether Jupiter is currently inhabited would require a “whole new study,” but there are some basic questions that must be answered first.
“We must demonstrate a bioavailable energy source and all the essential nutrients for life,” he said.
The findings extend even further into the solar system, too. Whether the water activity on Saturn, Neptune or Uranus is high enough for life to thrive is an open question — there is no probe data for the team to assess. However, Halsworth notes that “none of them would be expected to have a layer of high water activity” and they would be much cooler than Jupiter, making it difficult for life to take hold there.
And what about further? outside the solar system? Studying water activity in the atmospheres of exoplanets using NASA’s James Webb Space Telescope, due to launch in October, could provide another way for planetary scientists to assess habitability in the far reaches of the universe.
So you’re telling me they’re not aliens?
In a paper published in Astrobiology last year, the research team that discovered the phosphine signal on Venus suggested how life might survive in liquid droplets 30 miles above the surface.
They hypothesized that microbes might reside in liquid droplets for brief periods of time, before drying up and floating toward the planet’s surface. Eventually, they could be lifted back into the upper layers of Venus’s atmosphere and able to re-humidify and continue their life cycle.
The newspaper published on Monday dealt a heavy blow to this hypothesis.
If there is life on Venus, it’s unlike anything we’ve seen on Earth — and it can survive without water, the life-giving things involved in many aspects of terrestrial biology.
This leaves us with a lingering question: Where did phosphine come from?
Re-analysis was unable to detect phosphine at a significantly increased level. Next, another team suggested that the signal was not phosphine at all, but sulfur dioxide. Another team suggested it might be there, but it was at much lower levels than originally thought. At lower levels, phosphine can be explained by volcanic eruptions – and Venus has active volcanoes.
If the sign were legit, Venus would be even more mysterious.
Laura McKimish, a planetary scientist at Australia’s University of New South Wales who was not also affiliated with the study, said.
Whatever the phosphine signal, Venus is now a premium travel destination and a fleet of interplanetary probes is set to be visited over the next decade.
NASA plans to send two spacecraft in late 2020, including one known as Davinci+, which will specifically probe the atmosphere. The European Space Agency has its own plans to visit Venus, too, on a mission known as EnVision. Both are expected to provide a more comprehensive view of Venus’s atmosphere and obtain more data about its composition, but they are not expected to change ideas about the presence of Earth-like microbes in the clouds.
“It’s hard to imagine that the results will change as we do more exploration,” McKay said.