Signs of the gas phosphine in Venus’s atmosphere have faded a bit — but they’re still there, according to a new data analysis.
In September, an international team of astronomers made headlines when they reported finding phosphine — a potential marker of life — in the planet’s atmosphere1. Several studies questioning their observations and conclusions quickly followed. Now the same team has re-analysed part of its data, citing a processing error in the data it originally used. The researchers confirmed the phosphine signal, but say that it’s fainter than before.
The work is an important step forward in resolving the most exciting Venus debate in decades. “I’ve waited all my life for this,” says Sanjay Limaye, a planetary scientist at the University of Wisconsin in Madison, for whom the debate has reinvigorated the field.
The re-analysis, based on radio telescope observations at the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, concludes that average phosphine levels across Venus are about 1 part per billion — approximately one-seventh of the earlier estimate. Unlike in their original report, the scientists now describe their discovery of phosphine on Venus as “tentative”2.
It is the team’s first public response to the criticisms that have been levied against them in the past two months. “The scientific process is working,” says Bob Grimm, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who is not involved with any of the phosphine studies. Researchers tend to follow up big claims with big efforts to gather evidence and either prove or disprove them.
Taking another look
In its September report, the team used data from ALMA and the James Clerk Maxwell Telescope (JCMT) in Hawaii to make its discovery. Jane Greaves, the team’s leader and an astronomer at Cardiff University, in the UK, said she and her colleagues re-did the work because they learned that the original ALMA data contained a spurious signal that could have affected the results. ALMA posted the corrected data on 16 November, and Greaves’ team ran a fresh analysis and posted it ahead of peer review that night on the preprint server arxiv.org. “We’ve been working like crazy,” she told a meeting of the Venus Exploration Analysis Group, a community forum for NASA, on 17 November.
According to Greaves and her colleagues, the ALMA data show the spectral signature of phosphine, a molecule made of one phosphorous and three hydrogen atoms. They say no other compound can explain the data. Finding phosphine on Venus would be tantalizing because microbes produce the gas on Earth. If the signal is real and indeed due to phosphine, it’s possible that microbes living in and drifting among the planet’s clouds could be producing the gas4,3 — but it’s also possible there might be a non-living source for phosphine that scientists have yet to identify. To determine whether either of these scenarios is true, researchers first need to confirm phosphine’s presence.
In one critique of the original study, researchers suggested that the signal reported as phosphine might really be coming from sulphur dioxide, a gas that is common in Venus’s clouds but is not produced by life there5. Greaves and her team fired back in their latest report that that can’t be the case, based on how the phosphine fingerprint appears in data collected by the second telescope they used, the JCMT. Other critiques have focused on the difficulty of extracting a phosphine signal out of complicated data.
The re-analysis found phosphine concentrations in Venus’s atmosphere occasionally peaking at 5 parts per billion. That means levels of the gas may wax and wane over time at different places on the planet, said Greaves — a situation similar to methane spikes appearing on Mars.
One other new strand of evidence supports phosphine on Venus. Inspired by Greaves’ report, a team led by Rakesh Mogul, a biochemist at California State Polytechnic University in Pomona, California, recently dug through decades-old data from NASA’s 1978 Pioneer-Venus mission. This spacecraft dropped a probe through the planet’s atmosphere that measured the clouds’ chemistry as it fell. It detected a phosphorous compound that could be phosphine or another phosphorous-based molecule6. But “we believe the simplest gas that fits the data is phosphine,” Mogul said at the meeting on 17 November.
Work still ahead
Where the phosphine comes from remains a mystery. Even at the 1 part-per-billion level, there’s too much of it to be explained by volcanic eruptions at the planet’s surface or by lightning strikes in the atmosphere, several scientists said at the meeting. But phosphorous-based compounds might be produced by geological processes and then transform into other chemicals, such as phosphine, as they rise into the clouds, said Mogul.
The only spacecraft currently orbiting Venus, Japan’s Akatsuki, does not carry instruments that could help settle the debate. The Indian Space Research Organization is planning a Venus mission that would launch in 2025 and could potentially carry instruments capable of looking for phosphine. In the meantime, Greaves and other researchers are applying for more time on Earth-based telescopes, including ALMA.
Researchers are investigating many other aspects of Venus than just phosphine, says David Grinspoon, an astrobiologist at the Planetary Science Institute who is based in Washington, DC. “There are 1,001 reasons to go back to Venus, and if the phosphine ‘goes away’ through further observations and analysis, there will still be 1,000 reasons to go.”