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Earlier this year, Kevin Zahnle, a planetary scientist at NASA’s Ames Research Center, was one of the first people to read a science paper that would become the biggest space news of 2020. The study, on which Zahnle had been asked to provide comments while he was studying for publication in the journal Nature Astronomy made two startling claims. First, that the authors had identified signs of a gas called phosphine in the atmosphere of Venus. Second, they suggested that this gas could be a sign of life on the uninviting and hot planet.
When he read the paper, Zahnle was skeptical. “I can assure you that this reviewer explicitly warned the authors that they were fooling themselves,” he says. The planetary scientist feared that the phosphine signal was not what the study authors thought. But his main concerns were precisely what made the research paper so important, the reasoning that the presence of phosphine indicated life. On Earth, phosphine is found in small quantities, but how it is created is a matter of debate.
Since the 1980s, scientists have theorized that phosphine is created by microbes in oxygen-free environments, such as sewage sludge, but it is not a widely accepted signature of life. Zahnle is not convinced that the presence of phosphine on Venus should be interpreted as a possible sign of life – he thinks it is more likely that if phosphine is present on the planet, it is probably created by some still unknown geological phenomenon. The Nature Astronomy The authors of articles do not definitively rule out geological processes as a source of phosphine, but conclude that a living entity is the most likely source.
Despite Zahnle’s skepticism, the document was published. “It is not unusual to have strong opinions expressed during the peer review,” says Paul Byrne, an associate professor of planetary sciences at North Carolina State University who was not involved in the study. “But I sincerely hope that reviewer clearly communicated to the team why he believed those researchers were fooling themselves and that the team defended their methods and conclusions appropriately.”
Jane Greaves of Cardiff University, lead author of the study, does not remember this specific language used in the review process. It is possible, he says, that the comments were made to the publisher and not disclosed to his team. All reviewers on paper, for which there are usually between two and four for Nature Astronomy, were anonymous until publication. “All of them made their comments very clearly and agreed that the document was suitable for publication in the final comment stage,” he says.
Despite his reservations about his conclusions, Zahnle was thrilled to see the study published. “Publishing accelerates confirmation or refutation, which becomes the science that stands after the pursuit of glory has been forgotten,” he says.
Now, a month and a half after the initial paper was published, new papers are coming out suggesting the phosphine may not be there at all. In one study, still to go through the peer review process, researchers led by Ignas Snellen of the Leiden Observatory looked at the data used in the initial research. They analyzed it differently and found no evidence for the phosphine.
Part of the reason data analysis was so difficult was that the Atacama Large Millimeter Array (ALMA), a series of telescopes, is used to looking at the cold, vast cold clouds in interstellar space and not the nearby Venus, the third brightest natural object in the sky. To make sense of the collected data, to calibrate it and to reduce noise and disturbances, a lot of mathematical stunts are required. Greaves and his team fit the data using a 12th order polynomial, a mathematical expression with 12 variables. According to Snellan and his team, using this polynomial actually introduced spurious results. From the way they analyzed it, no phosphine was found.
So which method to trust? “No method is necessarily better than another, or at least not inherently more reliable,” says Byrne. The important thing, he says, is to make sure each method is tested and maintained to the same standard.
The team behind the original Venus study spotted signs of phosphine from data collected by the James Clerk Maxwell Telescope (JCMT), then followed it up with a closer look using ALMA. “Any argument to argue that there is no phosphine in Venus’ atmosphere must also explain the JCMT detection,” says Byrne. Snellan’s article does not explain it, but another does.
On October 27, a team led by Geronima Villanueva, planetary astronomer at NASA Goddard Space Flight Center, posted a document to the arXiv preprint server. The document, which has yet to be examined, has been sent to Nature Astronomythe Matters Arising section, designed for comments or responses to research published in the journal.
In this commentary article, Villanueva and her team argue that the phosphine signal was mixed with sulfur dioxide, a gas abundant in Venus that produces a signal close to that of phosphine. The researchers say the methods used by Greaves and his team, analyzing both JCMT and ALMA data, cannot definitively distinguish between sulfur dioxide and phosphine.
“Analyzes [of the comment article] is robust and measured, and I don’t think many would question their methodology or their conclusions, “says Brad Gibson, head of physics and astrophysics at the University of Hull. Initially, the paper explicitly asked Greaves and his team to withdraw their study, which Gibson and others have objected to, but this recommendation was later removed.
Some of the data used in the initial document is now being reworked. According to the researchers working on the initial study, something potentially went wrong during the processing of the data, before handing it over to Greaves and his team.
“The European network of ALMA regional centers, which originally calibrated the data provided to Greaves, is now examining it in detail and reworking it,” says an ALMA spokesperson. Greaves and his team said it would be unfair to comment on any paper examining their findings until the revised data has been published. There is still no way to know how reprocessing this data will affect the team’s phosphine detection. But Villanueva and her team refer to the error in their document and say removing it affects the results.
High-profile papers like this get more scrutiny than an average scientific paper, but scrutiny itself isn’t a bad thing. Another astronomer confirmed to WIRED that he has just submitted a paper addressing issues with the original study, but he cannot comment until his own has been peer-reviewed. “The more control, the better,” says Byrne. “If this detection is real, subsequent observations with different tools, from different teams, is the best way to guarantee it.”
The best way to determine if the phosphine is on Venus or not is to go there. “To pilot a mission to the orbit of Venus, or better yet, an orbiter and an aerial platform that would search for that gas and simultaneously characterize the atmosphere of Venus,” says Byrne. “We will not completely solve this question from Earth.”
We have been there, but a long time ago. In the 1980s, the Russian Vega mission detected a chemical that contained phosphorus in the clouds of Venus. However, the tools were unable to determine if it was phosphine. In 1978, NASA launched probes into the atmosphere of Venus as part of the Pioneer mission. Rakesh Mogul, a professor of biological chemistry at California State Polytechnic University, examined the data from this mission, using samples taken between 50 and 60 km above the surface. He and his team found evidence of phosphine in 40-year data, although their claims have not been peer reviewed.
But even if we confirm the presence of phosphine, it doesn’t mean life on Venus. “If the phosphine is confirmed beyond doubt to be present in Venus, it is very unlikely that it is of biotic origin,” says Byrne. Other astronomers who, like Byrne, were not involved in the study agree. Phosphine is not a gas they usually look for when they spot signs of life. That said, any life in such a hot, acidic atmosphere is unlikely to resemble the life we have on Earth.
If it’s not life, figuring out how the phosphine got there could be an exciting task. In a laboratory, heating phosphoric acid to over 200 degrees Celsius can produce phosphine. On Venus, the hottest planet in the solar system, it would be easy, Zahnle says. All it would require would be phosphoric acid, which could be produced from phosphorus trioxide, a molecule that would be stable in Venus’ atmosphere, falling like rain. Exactly how phosphorus trioxide would be produced is a big, but exciting question to answer. “The abiotic phosphine cycle would be a very powerful way to interrogate Venus,” says Zahnle.
Publishing, checking, collecting multiple results and then returning to the drawing board are all part of the scientific method. This is the incremental nature of science, which doesn’t work in eureka moments. Instead, researchers do their best to interpret the data they have, until new papers come out with new methods and are examined themselves.
“Each method must be thoroughly evaluated by peer review and by the wider community to ensure it holds up,” says Byrne. “Those who don’t, we can discard.” It’s still too early to dismiss the work of Greaves and his team, but it may also be too early to celebrate it.
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