Venus clouds have captivated Earthlings for decades. They form a dazzling mirror that obscures the planet's surface and, in the 1950s, one Israeli scholar even speculated the clouds may hide a world teeming with insect life capable of enduring the extreme heat.
When Russia's Venera spacecraft took images of the surface in 1975, there were no insects to be found. Venus is a desolate hellscape, the victim of a runaway greenhouse effect that has sent temperatures on the ground soaring to well over 850 degrees Fahrenheit -- hot enough to melt lead. But in the clouds, more temperate climes await any would-be alien lifeforms.
Up in the atmosphere, life might just find a way.
At least, that was one hypothesis. It can happen on Earth... so why not elsewhere? Last year, the idea that microbes might call the atmosphere of Venus home was bolstered by a study that claimed to have discovered elevated levels of phosphine -- an unstable gas associated with biological activity -- in the cloud deck of our sister planet. That spawned the theory that microbes in the clouds could be producing the gas.
As scientists untangled the phosphine signal, though, the possibility that it was a sign of life in Venus' clouds looked less and less likely.
On Monday, the likelihood of a drifting community of microbes in the clouds took another blow.
In a study published in the journal Nature Astronomy, researchers rule out the possibility of life -- as we know it -- existing in the Venusian clouds. The clouds are uninhabitable.
"The most extreme life on Earth would stand no chance whatsoever of living in the Venus clouds," John Hallsworth, a microbiologist at Queen's University Belfast and the lead author of the new paper, told me.
The problem is the availability of water. When planetary scientists go looking for life elsewhere in the cosmos, they look for water because it's essential for life to survive. The clouds of Venus might sound like a good place to start, but they're not the type of clouds we're used to on Earth. Venus' clouds are composed predominantly of liquid droplets of sulfuric acid -- the stuff we use to clean drains -- at concentrations that Hallsworth describes as "biologically hostile."
That detail already puts microbial life on the back foot, but Hallsworth and his team wanted cold, hard data to assess the possibility. They studied data collected by spacecraft that had plunged through 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 the detection of an excessive amount of phosphine gas in Venus' upper atmosphere. The research team was perplexed by this unexplained chemistry because on Earth, phosphine -- which smells like decaying fish -- can be produced by microbes.
The researchers were quick to point out that their find wasn't a definitive sign of life in the clouds of Venus, but scientists and the public were buzzing all the same. It inspired Jim Bridenstine, NASA's administrator at the time, to declare it was "time to prioritize Venus." The Venus fever spread through planetary science. Suddenly, the idea of searching for life on Mars felt like a distant memory.
The possibility of life on Venus was tantalizing, but scientists began double- and triple-checking their discoveries. After the phosphine announcement, other research groups began looking at the data, which was acquired from the Earth-based ALMA telescope. The secondary checks seemed to suggest the phosphine signal might be in error or may not be as strong as originally believed.
This is where Hallsworth and his collaborators jumped in. For almost three decades, Hallsworth has been investigating how water activity affects microbial life. His work has explored the lower limits of life -- or just how "dry" does it have to be before biological activity breaks down.
Knowing the clouds of Venus have high sulfuric acid content, which decreases water activity, he said that "alarm bells rang" before even reading the phosphine paper. Colleagues turned to him to ask if he knew the water activity in the clouds, hoping to paint a clearer picture of what was going on up there.
But he hadn't looked, and so the team went to work. Within two weeks, Hallsworth and the group had analyzed data collected by NASA's Pioneer and Russia's Venera probes, a flock of spacecraft that plunged through Venus' atmosphere in the 1970s. The data collected by the craft gave them insight into the temperatures, pressures and water lingering in the clouds.
Any potential microbe floating through the Venus clouds would find itself in an extremely hostile environment. About 30 to 44 miles above the surface, up in the clouds, it's more dry than the Earth's most expansive subtropical desert: the Sahara.
"The Venus clouds are a whole order of magnitude more dry than the Sahara," said Hallsworth, noting that the Sahara has around 0.25 water activity, while the clouds of Venus come in at just 0.004 water activity. That figure for Venus is simply far too extreme to support any life we know of.
"The most dry-tolerant microbe on Earth wouldn't stand a chance in Venus," Hallsworth said.
Not only is the water activity extremely low, but say a microbe did find a droplet of liquid in Venus' clouds -- an oasis within the desert sky -- its fate would be no better. Those potential oases are actually poisonous droplets of sulfuric acid. Any microorganism would find the acidity far too much to bear.
"No known microbial cell can remain intact at the high sulfuric acid concentrations within the droplets of the Venus clouds," Hallsworth said.
Microbes that found themselves in such a place would face instant death, their membranes rupturing and their insides spilling out.
Hallsworth's previous work has probed the outer limits of life for microbes on Earth. When it comes to water activity, a fungus known as Aspergillus penicillioides, which is found in old books or bound up in dust within pillows and duvets, reigns supreme on our planet. In 2017, Hallsworth's lab published a paper showing the fungus can grow and divide at as low as 0.585 water activity. That's impressive for an Earth organism.
They use that figure -- 0.585 water activity -- as the limit that a microbe can tolerate, but the water activity in Venus' clouds is two orders of magnitude lower.
Mars, too, doesn't have the water activity to facilitate life in the atmosphere -- it's too cold -- but another planet in our solar system has water activity exceeding 0.585. It's a surprising candidate for an extraterrestrial home.
The gas giant Jupiter looms over all the planets that orbit the sun. It's monstrous and, like Venus, it's famous for its clouds. When the research team analyzed water activity in Jupiter's atmosphere, they found it was at a high enough level to permit habitation by microbes.
"We absolutely hadn't expected that," said Hallsworth.
Using readings from NASA's Galileo probe, which launched in 1989 and released a probe into the gas giant's atmosphere in 1995, the team revealed somewhere beneath Jupiter's roiling, swirling face, there are conditions permissible for life.
"There is at least a layer in the clouds of Jupiter where the water requirements are met," Chris McKay, a planetary scientist at NASA's Ames Research Center and co-author on the paper, said during a press briefing.
However, the team is quick to point out this is no indication of life on the mammoth planet. The water activity merely demonstrates that microbes we know of on Earth could inhabit a distinct layer within Jupiter's atmosphere. Whether Jupiter is currently inhabited would take a "whole new study," Hallsworth said, but there are some more fundamental questions to answer first.
"We would have to demonstrate that a biologically available energy source and all the essential nutrients for life are present," he said.
The results 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's no probe data for the team to assess. However, Hallsworth notes he "would not expect any of them to have a layer of high-water activity" and they'd be much colder than Jupiter, making it much harder for life to take hold there.
And what about even further? Beyond the solar system? Studying the water activity in exoplanet atmospheres with NASA's James Webb Space Telescope, set to launch in October, could provide another way for planetary scientists to assess the habitability of distant places in the cosmos.
In a paper published in the journal Astrobiology last year, the research team that discovered the phosphine signal on Venus proposed how life may be able to survive in liquid droplets about 30 miles above the surface.
They hypothesized that microbes might reside in liquid droplets for brief periods of time, before drying out and floating toward the planet surface. Eventually, they could be lifted back up into the higher layers of the Venusian atmosphere and able to rehydrate and continue their life cycle.
The paper published Monday deals a mighty blow to that hypothesis.
If there is life on Venus, it's like nothing we've ever seen on Earth -- and it can survive without water, the life-giving stuff involved in so many aspects of terrestrial biology.
That leaves us with a lingering question: Where did the phosphine come from?
A re-analysis couldn't detect phosphine at a significantly increased level. Then, another team suggested the signal wasn't phosphine at all, but sulfur dioxide. Yet another team suggested it might be there, but it was at much lower levels than originally believed. At lower levels, phosphine could be explained by volcanic eruptions -- and Venus does contain active volcanoes.
If the signal is legitimate, though, Venus would be a whole lot more mysterious.
"If it is phosphine and it's not biology, then it means something's happening in Venus that is producing this chemical at a rate much higher than it should be," said Laura McKemmish, a planetary scientist at Australia's University of New South Wales who was not affiliated with the study.
Whatever the phosphine signal turns out to be, Venus is now a premium travel destination and is set to be visited by a fleet of interplanetary probes over the next decade.
NASA plans to send two spacecraft in the late 2020s, including one known as Davinci+, which will specifically probe the atmosphere. The European Space Agency has its own plans to visit Venus, too, with a mission known as EnVision. Both are expected to provide a more holistic view of the Venusian atmosphere and acquire further data about its composition, but they aren't expected to change ideas about the existence of Earth-like microbes in the clouds.
"It's hard to imagine that the results will change as we do further exploration," said McKay.