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Signs of phosphine on Venus can’t be easily explained without life. That’s why some astronomers believe the compound isn’t there.

NASA/JPL-Caltech

Potential signs of life on Venus are fading as astronomers downgrade their original claims

The announcement in September took the world by storm: Researchers using two radio telescopes found signs that the clouds of Venus were harboring phosphine, a toxic compound that on Earth is only made in significant quantities by microbes and chemists. The unexpectedly high levels detected on Venus could point to a floating microbial biosphere, the researchers suggested in a paper published in Nature Astronomy. But almost immediately, other astronomers began to criticize the results, with four independent studies pointing out questionable methods or failing to reproduce the results.

Now, after reanalyzing their data, the original proponents are downgrading their claims. Even the most favorable interpretation of their data now suggests phosphine levels are at least seven times lower than first reported, making it a much more tentative finding, the authors reported in a preprint posted on 17 November to arXiv. But the team still believes the gas is there, with the possibility that local pockets rise to higher levels, said Jane Greaves, an astronomer at Cardiff University who led the work, in a talk today to NASA’s Venus Exploration Analysis Group (VEXAG). “We have again a phosphine line.”

The observations used to support the initial paper were taken by the James Clerk Maxwell Telescope (JCMT) in Hawaii in 2017, and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile in 2019. The telescopes are sensitive to the cold radiation emitted by Venus’s atmosphere, and Greaves and her colleagues concluded that phosphine was responsible for one of the so-called absorption lines in the spectrum—dips where atmospheric chemicals block some of the radiation. But the ALMA data in particular were unusually noisy—confounding effects include absorption lines from Earth’s own atmosphere—and the researchers had to use a large number of variables to model and remove the noise. Critics pointed out that such an aggressive fix made the discovery of a false positive more likely

Scientists working at ALMA have since discovered calibration errors that help explain the noise in the Venus data. After reanalyzing the data, Greaves said her team still finds an absorption line for phosphine, but at far lower levels of 1 part per billion (ppb). That’s closer to, but still above, levels that might be explained by natural processes, such as volcanic eruptions or lightning strikes, Greaves said.

The more modest claims line up better with a study published last month in Astronomy & Astrophysics, led by Therese Encrenaz, an astronomer at the Paris Observatory. Her team looked for signs of phosphine in the thermal infrared in observations collected by NASA’s Infrared Telescope Facility in Hawaii in 2015. During that run, phosphine should have popped out if it had been present at levels above 5 ppb. “It’s easy to see there’s no phosphine line,” Encrenaz said in an interview last month.

Another criticism: Phosphine is probably not the only way to explain the absorption lines seen by the JCMT and ALMA. In one critique, submitted to Nature Astronomy, Geronimo Villanueva, a planetary astronomer at NASA’s Goddard Space Flight Center, and colleagues point out that the dip in the JCMT spectrum could plausibly be explained by an overlapping absorption line from sulfur dioxide (SO2), the gas that makes up most venusian clouds. They say some 100 ppb of SO2 could explain all of the JCMT phosphine signal—not unreasonable, given observations by the Venus Express orbiter that SO2 concentrations can spike to 1000 ppb. It’s a point conceded by the Greaves team in its reanalysis. “We emphasize that there could be a contribution from SO2,” they write. But the width of the absorption line in the ALMA data suggests that the feature isn’t “solely SO2,” they write.

Another issue is the location of the phosphine within the atmosphere. ALMA should only be sensitive to absorptions from substances at altitudes above 70 kilometers (km), Encrenaz said. But the Nature Astronomy paper suggested the signal originated some 55 km above the surface, in warmer cloud layers that would be more habitable for potential life. “This is very difficult to conceive,” Encrenaz said. Greaves and her co-authors argue in their reanalysis that ALMA is unable to capture the full width—and therefore depth—of the signal. “There is no empirical evidence that [phosphine] lies only above 70 km.”

There’s much more work to do. Scientists have only just gotten a look at the corrected ALMA data, says Colin Wilson, a planetary scientist at the University of Oxford and co-author of Villanueva’s critique. “It’s still too early to tell where the Venus phosphine roller coaster will end up.” Perhaps the only true resolution will come from further observations, better tailored to their target, Greaves said at VEXAG. Such a campaign could come next year at ALMA, which is expected to restart in March 2021.

Wherever the finding ends up, the resources poured into chasing it will likely uncover something intriguing, Wilson says. “Whether or not we find phosphine, we’re likely to find something new.”