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Four years ago, the unexpected discovery in the clouds of Venus of a gas that on Earth signifies life — phosphine — faced controversy, earning rebukes in subsequent observations that failed to match its findings.
Now, the same team behind that discovery has come back with more observations, presented for the first time on July 17 at a Royal Astronomical Society meeting in Hull, England. Eventually, they will form the basis of one or more scientific studies, and that work has already started.
The data, the researchers say, contains even stronger proof that phosphine is present in the clouds of Venus, our closest planetary neighbor. Sometimes called Earth’s evil twin, the planet is similar to ours in size but features surface temperatures that can melt lead and clouds made 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 more confidence in its findings. “There’s also a lot more of the data itself,” said Dave Clements, a reader in astrophysics at Imperial College London.
“We had three observation campaigns and in just one run, we got 140 times as much data as we did in the original detection,” he said. “And what we’ve got so far indicates that we once again have phosphine detections.”
A separate team, which Clements is also part of, presented evidence of another gas, ammonia.
“That is arguably more significant than the discovery of phosphine,” he added. “We’re a long way from saying this, but if there is life on Venus producing phosphine, we have no idea why it’s producing it. However, if there is life on Venus producing ammonia, we do have an idea why it might be wanting to breathe ammonia.”
Sign of life?
On Earth, phosphine is a foul-smelling, toxic gas produced by decaying organic matter or bacteria, while ammonia is a gas with a pungent smell that naturally occurs in the environment and is also produced mostly by bacteria at the end of the process of decomposition of plant and animal waste.
“Phosphine has been discovered in the atmosphere of Saturn, but that’s not unexpected, because Saturn is a gas giant,” Clements said. “There’s an awful lot of hydrogen in its atmosphere, so any hydrogen-based compounds, like phosphine or ammonia, are what dominate there.”
However, rocky planets such as Earth, Venus and Mars have atmospheres in which oxygen dominates the chemistry, because they didn’t have enough mass to keep the hydrogen that they had when they originally formed, and that hydrogen has 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 suggested 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, quite understandably, that was a surprise.”
Subsequent studies challenged the results, suggesting that the phosphine was actually ordinary sulfur dioxide. Data from instruments other than those used by Clements’ team — such as the Venus Express spacecraft, the NASA Infrared Telescope Facility and the now-defunct SOFIA airborne observatory — also failed to replicate the phosphine findings.
But Clements said that his new data, coming from the Atacama Large Millimeter/submillimeter Array, or ALMA, rules out that sulfur dioxide might be a contaminant and that the lack of phosphine from other observations is due to timing. “It turns out that all our observations that detected phosphine were taken as the atmosphere of Venus moved from night into day,” he said, “and all of the observations that didn’t find phosphine were taken as the atmosphere moves from day into night.”
During the day, ultraviolet light from the sun can break up molecules in the upper atmosphere of Venus. “All 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 weak traces of the molecule, reinforcing the theory.
Clements also pointed to unrelated research from a group led by Rakesh Mogul, a professor of chemistry and biochemistry at the California State Polytechnic University, Pomona. Mogul reanalyzed old data from NASA’s Pioneer Venus Large Probe, which entered the planet’s atmosphere in 1978.
“It showed phosphine inside the clouds of Venus at around the part-per-million level, which is exactly what we have largely been detecting,” Clements said. “So it’s beginning to hang together, but we still don’t know what’s producing it.”
Using the Pioneer Venus Large Probe data, the Mogul-led team published in 2021 a “compelling case for phosphine deep in the cloud layer (of Venus),” Mogul confirmed in an email. “To date, our analyses remain unchallenged in the literature,” said Mogul, who was not involved in the research of Clements’ team. “This is in sharp contrast to the telescopic observations, which remain controversial.”
Breathing microbes?
Ammonia on Venus would make for an even more surprising discovery. Presented at the talks in Hull by Jane Greaves, a professor of astronomy at Cardiff University in the United Kingdom, the findings will be the basis for a separate scientific paper, using data from the Green Bank Telescope in West Virginia.
The clouds of Venus are made of droplets, Clements said, but they’re not water droplets. There is 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 with severe exposure. “It’s so concentrated that, as far as we know, it would not be compatible with any life that we’re aware of on Earth, including extremophile bacteria, which do like very acidic environments,” he said, referring to organisms that are able to survive under extreme environmental conditions.
However, ammonia inside these droplets of acid can act as a buffer to the acidity and bring it down to a level low enough that some known earthly bacteria could survive in it, Clements added.
“The exciting thing behind this would be if it’s some kind of microbial life making the ammonia, because that would be a neat way for it to regulate its own environment,” Greaves said at the Royal Astronomical Society talks. “It would make its environment much less acidic and much more survivable, to the point it’s only as acidic as some of the most extreme places on Earth — so not completely crazy.”
The role of ammonia, in other words, is easier to explain than phosphine. “We understand why ammonia might be useful to life,” Clements said. “We don’t understand how the ammonia is produced, just like we don’t understand how the phosphine is produced, but if there is ammonia there, it would have a functional purpose that we can understand.”
However, Greaves warned, even the presence of both phosphine and ammonia wouldn’t be evidence of microbial life on Venus, because there’s so much information missing about the state of the planet. “There’s a lot of other processes that could go on, and we just don’t have any ground truth to say whether that process is possible or not,” she said, referring to the hard evidence that can only come from direct observations from within the planet’s atmosphere.
One way to perform such observations would be to persuade the European Space Agency to turn on some instruments aboard the Jupiter Icy Moons Explorer —– a probe en route 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 about phosphine and ammonia is intriguing but warrants 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 challenged the phosphine findings and postulated that life isn’t possible in the clouds of Venus.
“Our paper emphasized the harsh and seemingly inhospitable conditions in Venus’s atmosphere,” 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 might be at play. It’s crucial that we approach these findings with a careful and thorough scientific investigation.”
The findings open new avenues for research, he added, but it’s essential to treat them with a healthy dose of skepticism. While detecting phosphine and ammonia in Venus’ clouds is exciting, it is just the beginning of a longer journey to unravel the mysteries of that planet’s atmosphere, he said.
Scientists’ current understanding of the atmospheric chemistry of Venus cannot explain the presence of phosphine, said Dr. Kate Pattle, a lecturer in the department of physics and astronomy at University College London. “It’s important to note that the team behind the measurements of phosphine are not claiming to have found life on Venus,” Pattle said in an email. “If phosphine is really present on Venus, it might indicate life, or might indicate that there is Venusian atmospheric chemistry that we do not yet understand.”
The discovery of ammonia would be exciting if confirmed, Pattle added, because ammonia and sulfuric acid should not be able to coexist without some process — whether volcanic, biological or something not yet considered — driving the production of ammonia itself.
She emphasized that both of these results are only preliminary and would require independent confirmation, but they make upcoming missions to Venus such as the Jupiter Icy Moons Explorer and DAVINCI intriguing, she concluded.
”These missions may 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 capacity to harbor life.”
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