Four years ago, the unexpected discovery of phosphine gas in the clouds of Venus sparked controversy and skepticism. Subsequent observations failed to confirm these initial findings, leading to a heated debate within the scientific community. Now, the same research team has returned with new data, providing stronger evidence of phosphine’s presence and potentially upending our understanding of where life might exist.
The initial discovery, made in 2020, suggested that phosphine—a gas on Earth associated with life processes—was present in Venus’ atmosphere. This was surprising given Venus’ harsh conditions, including surface temperatures capable of melting lead and clouds of corrosive sulfuric acid. However, the discovery faced significant criticism as follow-up observations struggled to replicate the results.
Fast forward to July 17, at a Royal Astronomical Society meeting in Hull, England, the original team presented new findings that reinforce their initial claim. These observations were made using the James Clerk Maxwell Telescope in Hawaii, which had been upgraded with a new receiver. This upgrade, along with three extensive observation campaigns, provided 140 times more data than the original detection.
“There’s also a lot more of the data itself,” said Dave Clements, a reader in astrophysics at Imperial College London. “And what we’ve got so far indicates that we once again have phosphine detections.”
In addition to phosphine, the team, including Clements, presented evidence of another gas, ammonia. This finding could be even more significant. “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,” Clements explained.
NASA/JPL-Caltech
On Earth, phosphine is a toxic gas typically produced by decaying organic matter or bacteria, while ammonia is a naturally occurring gas, also mostly produced by bacteria decomposing plant and animal waste. The detection of these gases on Venus is unexpected because, unlike gas giants like Saturn where hydrogen-based compounds dominate, rocky planets like Venus should not have significant quantities of these gases. “By all normal expectations, they shouldn’t be there,” Clements noted. “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.”
The original phosphine findings faced challenges, with some studies suggesting that the detected gas was ordinary sulfur dioxide. Data from other instruments—such as the Venus Express spacecraft, the NASA Infrared Telescope Facility, and the now-defunct SOFIA airborne observatory—failed to replicate the phosphine findings. However, Clements said that new data from the Atacama Large Millimeter/submillimeter Array (ALMA) rules out sulfur dioxide as a contaminant. He explained that the discrepancy in previous observations could be 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, 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. Further analysis of that data by Clements’ team revealed weak traces of the molecule, reinforcing the theory.
NASA/JPL
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.”