The signs of phosphine gas in Venus̵
In September, an international team of astronomers made headlines when it announced that it was finding phosphine – a potential marker of life – in the planet’s atmosphere.1. Several studies quickly followed, calling into question their observations and conclusions. The same team is now re-analyzing some of its data, citing a processing error in the data it originally used. Researchers have confirmed the phosphine signal, but say it is weaker than before.
The work is an important step forward in resolving the most exciting debate on Venus in decades. “I’ve been waiting for this my whole life,” said Sanjay Limaye, a planetary scientist at the University of Wisconsin-Madison, for whom the debate has revived the field.
A re-analysis based on observations by radiotelescope at the Large Millimeter / Submillimeter Atacama (ALMA) in Chile concludes that average phosphine levels in Venus are about 1 part per billion – approximately one-seventh of the earlier estimate. Unlike their original report, scientists now describe their discovery of Venus’s phosphine as “preliminary.”2.
This is the team’s first public response to the criticism leveled against them in the last two months. “The scientific process works,” said Bob Grimm, a planetary scientist at the Southwestern Research Institute in Boulder, Colorado, who is not involved in any of the phosphine research. Researchers tend to follow big claims with great effort to gather evidence and prove or disprove it.
Take another look
In their September report, the team used data from ALMA and the James Clerk Maxwell Telescope (JCMT) in Hawaii to make their discovery. Jane Greaves, team leader and astronomer at Cardiff University in the UK, said she and her colleagues did the job again because they learned that the original ALMA data contained a false signal that could affect the results. ALMA released the revised data on November 16, and the Greaves team performed a new analysis and published it before a peer review tonight on the arxiv.org prepress server. “We worked like crazy,” she said at a meeting of the Venus Research Group, a community forum for NASA, on November 17.
According to Greaves and her colleagues, the ALMA data showed the spectral signature of phosphine, a molecule made up of one phosphorus and three hydrogen atoms. They say no other compound can explain the data. Finding phosphine on Venus would be painful because microbes produce gas on Earth. If the signal is real and really due to phosphine, it is possible for microbes living in the planet’s clouds to float and float, producing gas4,,3 – but it is also possible that there is an inanimate source of phosphine that scientists have yet to identify. To determine if any of these scenarios are true, researchers must first confirm the presence of phosphine.
In a critique of the original study, researchers suggested that the signal, reported as phosphine, may indeed come from sulfur dioxide, a gas that is common in the clouds of Venus but is not produced by life there.5. Greaves and her team responded in their latest report that this could not be the case, based on how the phosphine fingerprint appears in the data collected by the second telescope they used, JCMT Other Criticisms Focus on Difficulty to Extract phosphine signal from complex data.
The re-analysis found that the concentrations of phosphine in the atmosphere of Venus from time to time reach 5 parts per billion. That means gas levels could swell and drop over time in different parts of the planet, Greaves said, a situation similar to methane peaks appearing on Mars.
Another new chain of evidence supports Venus’s phosphine. Inspired by Greaves’ report, a team led by Rakesh Muggle, a biochemist at California State Polytechnic University in Pomona, California, recently excavated decades of data from NASA’s 1978 Pioneer-Venus mission. This spacecraft launched a probe through the planet’s atmosphere, which measured the chemistry of the clouds as it fell. He discovered a phosphorus compound, which could be phosphine or another phosphorus-based molecule6. But “we believe that the simplest gas that matches the data is phosphine,” Mogol said at a Nov. 17 meeting.
Where phosphine comes from remains a mystery. Even at level 1 part of a billion, there is too much of it to be explained by volcanic eruptions on the planet’s surface or by thunderstorms in the atmosphere, several scientists said at the meeting. But phosphorus-based compounds can be obtained by geological processes and then transformed into other chemicals, such as phosphine, as they rise in the clouds, Muggle said.
The only spacecraft currently orbiting Venus, Japan’s Akatsuki, does not have the tools to help settle the debate. The Indian Space Research Organization is planning a mission to Venus, which will begin in 2025 and could potentially carry instruments capable of searching for phosphine. Meanwhile, Greaves and other researchers are applying for more time on ground-based telescopes, including ALMA.
Researchers are exploring many other aspects of Venus besides just phosphine, says David Greenspoon, an astrobiologist at the Washington-based Planetary Science Institute. “There are 1,001 reasons to go back to Venus, and if the phosphine is gone through further observations and analysis, there will still be 1,000 reasons to go.”