Four years ago, the unexpected discovery of phosphine, a gas in the clouds of Venus that on Earth signifies life, sparked controversy and drew criticism in subsequent observations that failed to confirm the findings.
Now, the same team behind this discovery presented more observations for the first time on July 17 at a meeting of the Royal Astronomical Society in Hull, England. Eventually, these observations will form the basis of one or more scientific studies, and that work has already begun.
The data, according to the researchers, contains even stronger evidence that phosphine is present in the clouds of Venus, our closest planetary neighbor. Sometimes called “Earth’s evil twin,” the planet is similar in size to ours but features surface temperatures that can melt lead and clouds made of corrosive sulfuric acid.
The work 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 their findings. “There’s also a lot more data in 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 more data than in the original detection,” he said. “And what we have so far indicates that, once again, we have detections of phosphine.”
A separate team, of which Clements is also a member, presented evidence of another gas, ammonia.
“This is arguably more significant than the discovery of phosphine,” he added. “We are far from saying this, but if there is life on Venus producing phosphine, we have no idea why it is producing it. However, if there is life on Venus producing ammonia, we have an idea why it might want to breathe ammonia.”
Sign of life?
On Earth, phosphine is a toxic, foul-smelling gas produced by decaying organic matter or bacteria. Ammonia is a pungent-smelling gas that occurs naturally in the environment and is also mainly produced by bacteria at the end of the decomposition process of plant and animal waste.
“Phosphine was discovered in Saturn’s atmosphere, but this is not unexpected because Saturn is a gas giant,” Clements said. “There is a huge amount of hydrogen in its atmosphere, so any hydrogen-based compounds like phosphine or ammonia are the ones that dominate there.”
However, rocky planets like Earth, Venus, and Mars have atmospheres in which oxygen dominates the chemistry, because they didn’t 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 been suggested as biomarkers, including on exoplanets. So finding them in the atmosphere of Venus is interesting for that reason too. When we published the phosphine discoveries in 2020, it was understandably a surprise.”
Subsequent studies challenged the results, suggesting that 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 and the now-defunct SOFIA airborne observatory — also failed to replicate the phosphine discoveries.
But Clements said his new data, coming from the Atacama Large Millimeter/submillimeter Array, or ALMA, rules out that sulfur dioxide could be a contaminant and that the absence of phosphine in other observations is due to timing.
“It turns out that all of our observations that detected phosphine were made when Venus’ atmosphere turned from night to day,” he said, “and all of the observations that didn’t find phosphine were made when the atmosphere turned from day to night.”
During the day, ultraviolet light from the sun can break down molecules in Venus’ upper atmosphere. “All the phosphine is burned, 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 subsequent analysis of this data by Clements’ team revealed faint 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 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 within the clouds of Venus at around the parts-per-million level, which is exactly what we have largely detected,” Clements said. “So it’s starting to come together, but we still don’t know what it’s producing.”
Using data from the Pioneer Venus Large Probe, the team led by Mogul in 2021 published a “compelling case for phosphine deep in the cloud layer (of Venus),” Mogul confirmed in an email. “To this day, our analyzes remain unchallenged in the literature,” said Mogul, who was not involved in the Clements team’s research. “This is in stark contrast to telescopic observations, which remain controversial.”
Breathing microbes?
Ammonia on Venus would be an even more surprising discovery. Presented at the Hull lectures by Jane Greaves, professor of astronomy at Cardiff University in the UK, the findings will be the basis for a separate scientific paper, using data from the Green Bank Telescope in West Virginia.
Venus’s clouds are made of droplets, Clements said, but they are 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 lethal to humans with severe exposure.
“It’s so concentrated that, as far as we know, it wouldn’t be compatible with any form of life we know of on Earth, including extremophile bacteria, which like very acidic environments,” he said, referring to organisms that are able to survive under conditions extreme environmental conditions.
However, the ammonia inside these acid droplets can act as a buffer for the acidity and reduce it to a low enough level that some known terrestrial bacteria can survive, Clements added.
“The exciting aspect of this would be if it was some kind of microbial life producing the ammonia, because it would be a clever way of regulating its own environment,” Greaves said at the Royal Astronomical Society lectures. “That would make their environment much less acidic and much more habitable, to the point where it would only be 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 that of phosphine. “We understand why ammonia can be useful for life,” Clements said. “We don’t understand how ammonia is produced, just like we don’t understand how phosphine is produced, but if there is ammonia there, it would have a function that we can understand.”
However, Greaves cautioned, even the presence of phosphine and ammonia would not be evidence of microbial life on Venus, because there is so much missing information about the state of the planet. “There are many other processes that could occur, and we simply don’t have any concrete truth to say whether this process is possible or not,” she said, referring to hard evidence that can only come from direct observations of the planet’s atmosphere.
One way to make these observations would be to persuade the European Space Agency to turn on some instruments aboard the Jupiter Icy Moons Explorer — a probe on its way to the Jupiter system — when it passes Venus next year. But even better data would come from DAVINCI, an orbiting 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 require cautious optimism, said Javier Martin-Torres, 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 is not possible in the clouds of Venus.
“Our paper emphasized the harsh and seemingly inhospitable conditions of Venus’ atmosphere,” Martín-Torres said in an email. “The discovery of ammonia, which could neutralize sulfuric acid clouds, and phosphine, a possible biosignature, challenges our understanding and suggests that more complex chemical processes may be at play. It is crucial that we address these findings with careful and thorough scientific investigation.”
The discoveries open up 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’s just the beginning of a long journey to unlock the mysteries of that planet’s atmosphere, he said.
Scientists’ current understanding of Venus’ atmospheric chemistry cannot explain the presence of phosphine, said Dr. Kate Pattle, a professor in the department of physics and astronomy at University College London. “It is important to note that the team behind the phosphine measurements is not claiming to have found life on Venus,” Pattle said in an email. “If phosphine really is present on Venus, it could indicate life, or it could indicate that there is Venusian atmospheric chemistry that we don’t yet understand.”
The ammonia discovery would be exciting if confirmed, Pattle added, because ammonia and sulfuric acid shouldn’t be able to coexist without some process — whether volcanic, biological or something not yet considered — that drives the production of the ammonia itself.
She emphasized that both results are only preliminary and would require independent confirmation, but they make future missions to Venus, such as the Jupiter Icy Moons Explorer and DAVINCI, intriguing, she concluded.
“These missions may provide answers to questions raised by recent observations,” Pattle said, “and will certainly give us fascinating new insights into our nearest neighbor’s atmosphere and its ability to support life.”
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