Life on Venus claim faces strongest challenge yet

Image of planet Venus, with blue and grey clouds, against a black background.

Japan’s Akatsuki orbiter captured this image of Venus’s clouds with its ultraviolet imager.Credit: ISAS, JAXA, Akatsuki; Processing: Meli thev

Two papers have dealt a fresh blow to the idea that Venus’s atmosphere might contain phosphine gas — a potential sign of life.

The claim that there is phosphine on Venus rocked planetary science last September, when researchers reported spotting the gas’s spectral signature in telescope data1. If confirmed, the discovery could mean that organisms drifting among Venusian clouds are releasing the gas. Since then, several studies have challenged — although not entirely debunked — the report.

Now, a team of scientists has published the biggest critique yet. “What we bring to the table is a comprehensive look, another way of explaining this data that isn’t phosphine,” says Victoria Meadows, an astrobiologist at the University of Washington in Seattle who helped to lead the latest studies. Both papers have been accepted for publication in Astrophysical Journal Letters and were posted on the arXiv preprint server on 26 January.

Alternative explanations

In one study, Meadows and her colleagues analysed data from one of the telescopes used to make the phosphine claim — and could not detect the gas’s spectral signature2. In the other, the scientists calculated how gases would behave in Venus’s atmosphere — and concluded that what the original team thought was phosphine is actually sulfur dioxide (SO2), a gas that is common on Venus and is not a sign of possible life3.

The latest papers pretty clearly show that there is no sign of the gas, says Ignas Snellen, an astronomer at the University of Leiden in the Netherlands who has published a different critique of the phosphine claim4. “This makes the whole debate about phosphine, and possibly life in the atmosphere of Venus, quite irrelevant.”

Jane Greaves, an astronomer at the University of Cardiff, UK, who led the team that made the original phosphine claim, says she and her colleagues are still reading through the new papers and will comment after they’ve evaluated them.

The stakes for confirming phosphine’s presence on Venus are huge. On Earth, the gas — which is made of one phosphorus atom plus three hydrogen atoms — can come from industrial sources such as fumigants, or from biological sources such as microbes. When first reporting the discovery of phosphine on Venus, Greaves and her colleagues said that its existence might mean there was life on the planet, because other origins for the gas weren't obvious.

But the claim rests on a chain of observations and deductions that other scientists have been chipping away at in recent months.

Chipping away

Greaves’s team first used the James Clerk Maxwell Telescope (JCMT) in Hawaii to observe a spectral line in Venus’s atmosphere at a frequency of 266.94 gigahertz — right around the frequency where both phosphine and SO2 absorb light. The scientists confirmed the existence of the line using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. With ALMA, they looked for other spectral lines that they would expect to see if the line came from SO2, and did not find them. This, they said, suggested that the line they had observed at 266.94 gigahertz came from phosphine.

But it turned out that the ALMA data the team had used had been processed incorrectly by the observatory. After the debate over phosphine on Venus began, managers at ALMA realized the mistake, pulled the raw data, reprocessed them and released the reworked batch in November. Greaves and her colleagues analysed the reprocessed data and concluded that they were still seeing phosphine — albeit at a much lower level than they had reported at first5.

Those reprocessed ALMA data are at the heart of one of the new studies challenging the claim. A team including Meadows and led by Alex Akins, a research technologist at the Jet Propulsion Laboratory in Pasadena, California, aimed to replicate the work of Greaves’s group by analysing the reprocessed data that had been released to the public. But the researchers didn’t observe phosphine’s spectral line. “We just weren’t able to see it,” says Akins.

It is the first analysis of the reprocessed ALMA data to be published by an independent team.

The second study explores the 266.94-gigahertz feature, as seen by the JCMT. Andrew Lincowski, an astronomer at the University of Washington, led Meadows, Akins and others in modelling the structure of Venus’s atmosphere at various altitudes. They found that the JCMT observation was best explained by the presence of SO2 more than 80 kilometres above the planet’s surface — not by phosphine at 50–60 kilometres above the surface, as Greaves’s team claimed.

Still, the case isn’t closed yet. The new studies argue against the presence of phosphine, but can’t entirely rule it out. “There’s enough wiggle room there,” says Meadows.

Ultimately, the debate can be resolved only with fresh observations of Venus, many of which are planned in the coming months and years, says Akins. “Until we see something new, it’s probably just going to keep going back and forth.”

Nature 590, 19-20 (2021)

doi: https://doi.org/10.1038/d41586-021-00249-y


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    Greaves, J. S. et al. Nature Astron. https://doi.org/10.1038/s41550-020-1174-4 (2020).

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    Akins, A. B., Lincowski, A. P., Meadows, V. S. & Steffes, P. G. Preprint at https://arxiv.org/abs/2101.09831 (2021).

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    Lincowski, A. P. et al. Preprint at https://arxiv.org/abs/2101.09837 (2021).

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    Snellen, I. A. G., Guzman-Ramirez, L., Hogerheijde, M. R., Hygate, A. P. S. & van der Tak, F. F. S. Astron. Astrophys. 644, L2 (2020).

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