Phosphine on Venus

Summary

In September 2020 a team led by Jane Greaves reported phosphine, a gas that on Earth is made industrially or by microbes, in the clouds of Venus at about 20 parts per billion. Within months the strongest part of the claim had failed: the data from the confirming telescope had been processed wrongly, and independent groups found no significant signal.

The discovery team still reports phosphine, from reprocessed and new data. Other groups report upper limits. Whether the gas is there at all is unsettled, and so is what could make it if it is.

What is documented

The claim. On 14 September 2020 Nature Astronomy published a paper by Greaves and eighteen colleagues reporting "the apparent presence of phosphine (PH3) gas in Venus's atmosphere". It rested on a single absorption line near 1.1 millimetres (266.94 GHz), seen with the James Clerk Maxwell Telescope (JCMT) in Hawaii in 2017 and with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile in 2019. The paper inferred about 20 parts per billion (ppb) and found no known way to make that much without life: not in the atmosphere, clouds, surface or subsurface, nor from lightning, volcanoes or meteorites. It said the gas could come from unknown photochemistry or geochemistry or, by analogy with Earth, from life. The Royal Astronomical Society and ESO announced it under the headlines "Hints of life on Venus" and "Possible Marker of Life Spotted on Venus".

The processing error. Other groups reanalysed the ALMA data. Snellen and colleagues found that the 12th-order polynomial used to flatten the spectrum "leads to spurious results", and in their own reanalysis the feature near the phosphine frequency sat at about the 2σ level, below the usual threshold for significance. They concluded that the ALMA data "provide no statistical evidence for phosphine in the atmosphere of Venus." A calibration problem was also found in the ALMA data, which were recalibrated and returned to the ALMA archive, as ESO noted beneath its release on 17 November 2020. On 20 November Nature Astronomy added an editor's note cautioning readers "against using the paper's quantifications for the ALMA part of the dataset." M. A. Thompson reanalysed the JCMT spectrum and found no statistically significant detection there either.

The team's revision. Working from the recalibrated ALMA data, Greaves and colleagues still recovered phosphine, at about 5σ, but at a planet-averaged abundance of about 1 to 4 ppb, with local peaks of 5 to 10 ppb. They set out these figures in a preprint in November 2020, and in July 2021 Nature Astronomy published their reply to their critics together with an addendum to the original paper. They argued that the JCMT line could not be re-attributed to sulphur dioxide, because it was wider than sulphur dioxide features and the abundance required would be an extreme outlier.

The sulphur dioxide critique. Sulphur dioxide is common in Venus's atmosphere, and Villanueva and colleagues pointed out that it has a line only 1.3 km/s from the phosphine line, too close to separate at the resolution of the 2017 and 2019 observations. In January 2021 Andrew Lincowski, Victoria Meadows and colleagues argued from the shape of the JCMT line that most of the absorption came not from the clouds but from the mesosphere, 80 kilometres (50 miles) or more above the surface, where ultraviolet light and reactive chemicals would destroy phosphine within seconds. Ordinary sulphur dioxide, they concluded, was the more plausible explanation, and the array configuration ALMA used in 2019 had made widespread gases such as sulphur dioxide appear weaker than they were. In July 2021 Villanueva and colleagues published upper limits for phosphine above the cloud decks (above 75 kilometres) that were inconsistent with the reported detections.

Other searches. Encrenaz and colleagues searched thermal infrared spectra of Venus taken since 2012 and found no phosphine feature, setting a 3σ upper limit of 5 ppb at the cloud tops; Greaves was a co-author. In November 2022 Cordiner and colleagues reported that SOFIA, an airborne observatory, had searched for phosphine lines at 533 and 1,067 GHz and found none, setting an upper limit of 0.8 ppb between 75 and 110 kilometres.

Pioneer Venus. In 2021 Rakesh Mogul and colleagues re-examined data from the neutral mass spectrometer on the Pioneer Venus Large Probe, which entered the atmosphere on 9 December 1978. Among several chemicals they read as signs of disequilibrium in the middle clouds, they reported what they called "a potential signature of anaerobic phosphorus metabolism (phosphine)".

The new data. From 2022 the JCMT-Venus programme, coordinated by astronomers including Greaves and David Clements, has monitored the gases in Venus's atmosphere. At the UK's National Astronomy Meeting in July 2024 a team led by Clements reported phosphine again, and ammonia deeper in the clouds, from observing campaigns that began in February 2022; the first alone, he said, produced 140 times as much data as the original observations. Chemistry World reported that the results were yet to be peer reviewed. In a review first published online in January 2026, Clements wrote that the detection "has survived all challenges", and gave a preliminary JCMT-Venus abundance of about 0.3 parts per million (300 ppb) at an altitude of about 55 kilometres, which he described as broadly consistent with the Pioneer Venus mass spectrometer data. The team's own 2024 review of possible sources called it a "tentative detection" that "remains controversial".

Leading explanations

Phosphine in the clouds from an unknown source (the discovery team's position; peer-reviewed, contested). The team's case rests on the JCMT line of 2017, the reprocessed ALMA data, the JCMT-Venus observations from 2022 and the Pioneer Venus reanalysis. On sources, the team's 2024 review found that no known process, whether gas reactions, geochemistry, volcanism, photochemistry, lightning or meteorites, could explain phosphine at near the observed abundance, and that droplet or particle surface photochemistry remained the most plausible abiotic candidate.

No phosphine: processing artefacts and sulphur dioxide (independent analyses; peer-reviewed, contested). Snellen and colleagues and Thompson found no significant line in the ALMA and JCMT data; Lincowski and colleagues and Villanueva and colleagues attributed the line to sulphur dioxide; the infrared and SOFIA searches found nothing. For the discovery team, Clements holds that the detection has survived all of these challenges.

Unknown abiotic chemistry, if phosphine is present (hypotheses). Fabian Wunderlich, John Lee Grenfell and Heike Rauer found in 2023 that uncertainties in the photochemistry of phosphorus let modelled phosphine vary by about six orders of magnitude, with an abiotic upper limit of 2 ppb between 50 and 60 kilometres. Klaudia Mráziková and colleagues proposed in 2024 a photochemical route to phosphine over acidic mineral dust. Neither has been tested by observation.

Life in the clouds (untested). The 2020 paper raised it by analogy with Earth. The team's 2024 review called an aerial microbial biosphere "an even more extreme possibility" than unknown chemistry. Nothing has tested it.

Current status

Partially explained. Part of the case is settled: the ALMA confirmation of 2020 came from mis-processed data, which the authors and the journal both acknowledged, and the claim no longer stands in the form in which it was announced. The material part is not settled. Whether phosphine exists in the clouds is disputed: the discovery team continues to report it, while independent limits constrain it at the cloud tops (below 5 ppb) and above the clouds (below 0.8 ppb from SOFIA), and the height at which the disputed line forms is itself contested. If the gas is there, its source is unknown.

What would change this: peer-reviewed publication of the JCMT-Venus results, a detection of phosphine at another wavelength by an independent team, or a measurement made inside the clouds by a descent probe.

Last reviewed: September 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.