Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news about fascinating discoveries, scientific developments and more.
Four years ago, the unexpected discovery in the clouds of Venus of a gas that on Earth indicates life — phosphine — was controversial. Later observations rejected the gas because it did not match the findings.
Now, the same team behind that discovery has returned with more observations, first presented July 17 at a meeting of the Royal Astronomical Society in Hull, England. They will eventually form the basis for one or more scientific studies, and that work has already begun.
The data, the researchers say, provide even stronger evidence that phosphine is present in the clouds of Venus, our nearest planetary neighbor. The planet, sometimes called Earth’s evil twin, is similar in size to our own but has surface temperatures that could melt lead and clouds made up of corrosive sulfuric acid.
The work has benefited from a new receiver installed on one of the instruments used for the observations, the James Clerk Maxwell Telescope in Hawaii, giving the team greater confidence in its findings. “There’s also a lot more of the data itself,” said Dave Clements, a senior lecturer in astrophysics at Imperial College London.
“We’ve had three observation campaigns and in just one run we got 140 times more data than we did in the original detection,” he said. “And what we have so far indicates that we have phosphine detections again.”
Another team, which included Clements, presented evidence for the presence of another gas: ammonia.
“That may be more important than the discovery of phosphine,” he added. “We’re still a long way from saying this, but if there’s life on Venus that produces phosphine, we have no idea why it’s doing it. But if there’s life on Venus that produces ammonia, we have an idea why it would want to breathe ammonia.”
Sign of life?
On Earth, phosphine is a foul-smelling, poisonous gas produced by decaying organic matter or bacteria. Ammonia is a pungent gas that occurs naturally in the environment and is also produced primarily by bacteria at the end of the decomposition process of plant and animal waste.
“Phosphine has been detected in Saturn’s atmosphere, but that’s not unexpected because Saturn is a gas giant,” Clements said. “There’s an awful lot of hydrogen in the atmosphere, so all the hydrogen-based compounds, like phosphine or ammonia, are what dominate there.”
However, rocky planets like Earth, Venus, and Mars have atmospheres where oxygen is the chemically dominant. This is because they did not have enough mass to hold the hydrogen they had when they originally formed, and that hydrogen escaped.
Finding these gases on Venus is therefore unexpected. “By all normal expectations, they shouldn’t be there,” Clements said. “Phosphine and ammonia have both been proposed as biomarkers, including on exoplanets. So finding them in the atmosphere of Venus is interesting on that basis as well. When we published the phosphine findings in 2020, it was understandably a surprise.”
Later studies disputed the results, suggesting that the phosphine was actually just sulfur dioxide. Data from instruments other than those used by Clements’ team — including the Venus Express spacecraft, the NASA Infrared Telescope Facility and the now-defunct SOFIA observatory — also failed to replicate the phosphine findings.
But Clements said his new data, from the Atacama Large Millimeter/submillimeter Array, or ALMA, rule out sulfur dioxide as a contaminant and that the lack of phosphine in other observations is due to timing. “It turns out that all of our observations that detected phosphine were taken when the atmosphere of Venus was changing from night to day,” he said, “and all of the observations that didn’t detect phosphine were taken when the atmosphere was changing from day to night.”
During the day, ultraviolet light from the sun can break apart molecules in Venus’ upper atmosphere. “All the phosphine is baked out, and that’s why you don’t see it,” Clements said, adding that the only exception was the Stratospheric Observatory for Infrared Astronomy, which made observations at night. But further analysis of that data by Clements’ team revealed faint traces of the molecule, bolstering the theory.
Clements also pointed to unrelated research from a group led by Rakesh Mogul, a professor of chemistry and biochemistry at California State Polytechnic University in Pomona. Mogul reanalyzed old data from NASA’s Pioneer Venus Large Probe, which entered the planet’s atmosphere in 1978.
“It showed phosphine in the clouds of Venus at about the part per million level, which is exactly what we’ve been detecting for the most part,” Clements said. “So it’s starting to add up, but we still don’t know what’s producing it.”
Using the Pioneer Venus Large Probe data, the Mogul-led team published a “compelling case for phosphine deep in the cloud layer[of Venus]” in 2021, Mogul confirmed in an email. “To date, our analyses are uncontested in the literature,” said Mogul, who was not involved in Clements’ team’s research. “This is in stark contrast to the telescopic observations, which remain controversial.”
Inhaling microbes?
Ammonia on Venus would be an even more surprising discovery. The findings, presented at the Hull lectures by Jane Greaves, professor of astronomy at Cardiff University in the United Kingdom, form the basis of a separate scientific paper, using data from the Green Bank Telescope in West Virginia.
Venus’ clouds are made up of droplets, Clements said, but they’re not water droplets. They have water in them, but also so much dissolved sulfur dioxide that they become extremely concentrated sulfuric acid — a highly corrosive substance that can be deadly to humans in severe exposure. “It’s so concentrated that, as far as we know, it wouldn’t be compatible with any life that we know of on Earth, including extremophile bacteria, which like very acidic environments,” he said, referring to organisms that are able to survive in extreme environmental conditions.
However, the ammonia in these acid droplets can act as a buffer against the acidity, reducing it to levels low enough that some known bacteria on Earth can survive in them, Clements adds.
“The exciting thing about this would be if it were some kind of microbial life making the ammonia, because that would be a nice way of regulating its own environment,” Greaves said during the Royal Astronomical Society talks. “It would make its environment much less acidic and much more survivable, to the point where it’s only as acidic as some of the most extreme places on Earth — so not entirely crazy.”
The role of ammonia, in other words, is easier to explain than phosphine. “We understand why ammonia can be useful to life,” Clements said. “We don’t understand how ammonia is produced, just as we don’t understand how phosphine is produced, but if ammonia is there, it would have a functional purpose that we can understand.”
Greaves cautioned, however, that even the presence of both phosphine and ammonia would not be evidence of microbial life on Venus, because so much information is missing about the state of the planet. “There are lots of other processes that could be going on, and we just don’t have the ground truth to say whether that process is possible or not,” she said, citing the hard evidence that can only come from direct observations of the planet’s atmosphere.
One way to conduct such observations would be to convince the European Space Agency to turn on some of the instruments aboard the Jupiter Icy Moons Explorer —– a probe headed to the Jupiter system —– when it flies by Venus sometime next year. But even better data would come from DAVINCI, an orbiter and atmospheric probe that NASA plans to launch to Venus in the early 2030s.
Cautious optimism
From a scientific perspective, the new data on phosphine and ammonia are intriguing but warrant cautious optimism, said Javier Martin-Torres, a professor of planetary sciences at the University of Aberdeen in the United Kingdom. He led a study published in 2021 that cast doubt on the phosphine findings and argued that life in Venus’ clouds is not possible.
“Our paper highlighted the harsh and seemingly inhospitable conditions in the atmosphere of Venus,” Martín-Torres said in an email. “The discovery of ammonia, which could neutralize the sulfuric acid clouds, and phosphine, a potential biosignature, challenges our understanding and suggests that more complex chemical processes may be at play. It is crucial that we approach these findings with careful, rigorous scientific investigation.”
The findings open up new avenues for research, he added, but it’s essential to approach them with a healthy dose of skepticism. While detecting phosphine and ammonia in Venus’ clouds is exciting, it’s just the beginning of a longer journey to unravel the mysteries of that planet’s atmosphere, he said.
Scientists’ current understanding of Venus’ atmospheric chemistry can’t explain the presence of phosphine, said Dr. Kate Pattle, a lecturer in the physics and astronomy department at University College London. “It’s important to note that the team behind the phosphine measurements are not claiming to have found life on Venus,” Pattle said in an email. “If phosphine is really present on Venus, it could indicate life, or it could indicate that there is atmospheric chemistry on Venus that we don’t yet understand.”
The discovery of ammonia would be exciting if confirmed, Pattle added, because ammonia and sulfuric acid couldn’t coexist without some process — volcanic, biological or something not yet considered — driving the production of ammonia itself.
She stressed that both results are only preliminary and require independent confirmation, but they make future missions to Venus, such as the Jupiter Icy Moons Explorer and DAVINCI, interesting, she concluded.
“These missions can provide answers to the questions raised by recent observations,” Pattle said, “and will certainly give us fascinating new insights into the atmosphere of our nearest neighbor and its potential to host life.”
For more CNN news and newsletters, create an account at CNN.com