Life on Venus

The surface is out of the question for life: David Clements of Imperial College London, who leads the phosphine follow-up, describes it as above 450 °C under nearly a hundred times Earth's sea-level pressure. The whole argument is about the clouds. Sanjay Limaye and colleagues put the lower cloud layer at 47.5 to 50.5 km, near 60 °C and one atmosphere, and Sara Seager's group describes the layers between 48 and 60 km as holding the main requirements for life: temperatures suited to covalent chemistry, sunlight for energy, and a liquid. The droplets are sulfuric acid, which laboratory teams model at 81 to 98 percent by weight, and on Earth, as Clara Sousa-Silva of the 2020 phosphine team pointed out, some microbes cope with up to about 5 percent acid.

The idea predates any knowledge of that acid. In September 1967 Harold Morowitz and Carl Sagan asked in Nature whether there could be life in the clouds, where water, carbon dioxide and sunlight seemed plentiful. They took the cloud tops to be ice crystals and the bottoms probably water droplets, and noted an ultraviolet darkening that pure ice could not explain. In 1973 Young asked in Icarus whether the clouds were sulfuric acid, and every side of today's debate treats the droplets that way. Venera 8 had already carried a small chemical test for ammonia down through the atmosphere in July 1972. It changed colour at two heights, and Mikhail Marov's account, quoted in Russian on the epizodsspace archive, says the reading was later traced to sulfuric acid, which turns the same indicator.

What keeps the question alive is a list of unexplained cloud features. William Bains, Janusz Petkowski, Paul Rimmer and Seager set several out in 2021: about 10 parts per million of oxygen in the cloud layers, large particles of unknown composition in the lower clouds, and vertical profiles of sulfur dioxide and water that models struggle to match. Another is the ultraviolet absorber, the unidentified substance that darkens the dayside clouds near 365 nm. Limaye's team argued in 2018 that particles in the lower clouds could hold enough mass to harbour microorganisms, potentially enough to be detected by optical methods. In June 2026 Polish and Czech researchers measured the spores of three acid-loving fungi and found absorption peaks between 243 and 323 nm that partly overlap the absorber's range. An August 2026 preprint by Mogul, Limaye and colleagues proposes a non-biological answer: aromatic hydrocarbons and iron compounds from altered cometary dust, which match the absorber's spectra. Each of these is a gap in the chemistry, and biology is one of several candidates for filling it.

Phosphine turned the speculation into headlines. On 14 September 2020 a team led by Jane Greaves of Cardiff University reported in Nature Astronomy a single absorption line at 266.94 GHz, the J=1-0 rotational line of phosphine (PH3), in spectra taken with the James Clerk Maxwell Telescope (JCMT) in Hawaii in June 2017 and with ALMA in Chile in March 2019, and from the JCMT spectrum inferred about 20 parts per billion. Phosphine is a reduced gas that should not persist in an oxidising atmosphere. On Earth, the Royal Astronomical Society's release said, it is made only industrially or by microbes living without oxygen; Jupiter and Saturn have it too, but there it forms deep in hot, high-pressure hydrogen atmospheres, chemistry the team's modeller did not expect on a rocky planet. William Bains led the team's search for other sources, from sunlight and lightning to volcanoes and meteorites, and found they could supply at most a ten-thousandth of the amount seen. The paper named unknown photochemistry or geochemistry as possible origins, or, by analogy with Earth, life. Greaves had expected only to rule out extreme scenarios, and called the first hints of the line a shock.

Hideo Sagawa of Kyoto Sangyo University, the team's atmospheric modeller, gave a cautious account. He told the Japanese edition of Nature Digest that he joined the international team only after the telescope observations were over, and that his radiative transfer model placed the absorbing gas at about 50 to 60 km or higher, at roughly 20 molecules in every billion. His own view, he said, was that chemical explanations should be tested and ruled out before life is discussed; the paper had ended by saying life could not be excluded, and to his surprise that was the phrase the press picked up. Going back through older infrared data from other telescopes, he found no phosphine line; he called the situation full of contradictions and said testing a result from many viewpoints is how science should proceed.

Within weeks other groups had the data open. A team led by T. Encrenaz, with Sagawa, Greaves and Sousa-Silva as co-authors, searched infrared spectra taken at NASA's Infrared Telescope Facility in March 2015 and set a 3-sigma upper limit of 5 ppb at the cloud top, four times below the millimetre value. Snellen and colleagues reprocessed the ALMA data with the published scripts and showed that the 12th-order polynomial used to flatten the spectrum could produce other strong lines out of noise; their own analysis left a feature of about 2 sigma. M. A. Thompson reached the same verdict on the JCMT spectrum with bootstrap statistics. Geronimo Villanueva's group pointed to a sulfur dioxide line only 1.3 km/s from the phosphine line, too close to separate at the resolution used. Trompet and colleagues searched solar occultation spectra from the SOIR instrument on Venus Express and reported upper limits almost two orders of magnitude below 20 ppb. In Moscow, IKI deputy director Oleg Korablev told RIA Novosti in January 2021 that his colleagues, having first allowed that phosphine might be present, ran a standard atmospheric model and were left with the impression that sulfur dioxide alone explained the data; he called the episode a good reason to explore Venus with more sensitive methods.

Then the ALMA data themselves changed. ALMA staff found an issue in the data used to confirm the detection, and on 17 November 2020 ESO added a note to its release saying they had been recalibrated and re-released. Greaves's team reprocessed them and still recovered the line, now at a planet-averaged 1 to 4 ppb with local peaks of 5 to 10 ppb, arguing that the higher JCMT figure could be reconciled if the gas varies over time; an April 2021 follow-up put the whole-planet ALMA signal at 5.4 sigma. In January 2021 Andrew Lincowski, Victoria Meadows and colleagues modelled the line and concluded that it forms above 80 km, where phosphine would be destroyed quickly, and that ordinary sulfur dioxide fits the JCMT feature. The discoverers answered with sulfur dioxide measurements taken at nearly the same time and put the contamination at about 10 percent of the JCMT line and about 2 percent of the ALMA line. Behind the chemistry sits a dispute about height. Sagawa's model placed the absorption near 50 to 60 km, Lincowski's above 80 km, and that choice decides whether the gas would be in the cloud layer where life is proposed or far above it.

An airborne observatory made a separate test. In November 2021 M. A. Cordiner and colleagues used three flights of SOFIA, a modified Boeing 747SP in service from 2014 to 2022, with the GREAT spectrometer tuned to phosphine lines near 533 and 1,067 GHz, higher in frequency than the line seen from the ground. They found no phosphine and set a disk-averaged upper limit of 0.8 ppb between 75 and 110 km, which they called more stringent than previous ground-based studies. Greaves's group then reprocessed the same spectra, bypassing calibration-load signals they judged non-essential, and reported in Geophysical Research Letters in 2023 a 5.7-sigma candidate suggesting about 3 ppb above the clouds. The instrument and the planet were the same, so the two results measure the effect of two processing choices on one data set. The discoverers also proposed a pattern: detections come from air moving from night into day, upper limits from air long in sunlight, as expected if sunlight destroys the gas.

If the gas is there, something has to keep making it. Truong and Lunine proposed in PNAS in 2021 that phosphides brought up by deep-mantle volcanism could supply it. Bains and colleagues replied that sustaining even 1 ppb this way would take at least 21,600 cubic kilometres of eruption a year, more than any historical rate on Earth, and a 2024 review by the claim team found that no gas reaction, surface or subsurface geochemistry, or photochemistry comes near the observed amount. The review names unknown geochemistry or photochemistry, which would mean the consensus on Venus's chemistry is significantly incomplete, and, as a more extreme possibility, an aerial biosphere. A finding of no known process marks the limits of today's models and laboratory data, and leaves unknown non-biological routes open.

Old spacecraft data were drawn in too. Within two weeks of the announcement, Rakesh Mogul, Limaye and colleagues reinterpreted mass spectra recorded at 51.3 km by the Pioneer Venus Large Probe in 1978 and assigned peaks to phosphine, hydrogen sulfide, nitrous and nitric acid and other species, with ammonia as a possible assignment. The claim team counts this as independent support. In 2025 the same group returned to the probe's data and concluded that its instruments had inadvertently swallowed cloud droplets, whose decomposition points to aerosols of roughly 20 percent ferric sulfate, 20 percent sulfuric acid and 60 percent water by weight. If that holds, the droplets are far wetter than the pure-acid picture assumes. It rests on one reinterpretation of instruments that were built to measure gases.

More data has been the discoverers' main answer. JCMT-Venus, the monitoring programme Clements leads with Greaves, was awarded 200 hours on the JCMT and uses a wider-band receiver called ʻŪʻū, which records phosphine, water (through HDO) and sulfur dioxide at once. Clements wrote that the first campaign alone, in February 2022, produced 140 times as much information as the discovery data, and that Venus is so bright at these wavelengths that reflections from the telescope and its dome create ripples larger than the absorption lines. With a Fourier method to remove them, the team recovered narrow phosphine and HDO lines, and an experimental technique revealed broad line wings that, in a preliminary analysis, imply about 0.3 parts per million near 55 km, a level he compared with the Pioneer Venus reanalysis. At the UK National Astronomy Meeting in July 2025 the team's slides concluded that phosphine was 'still found, but possibly variable in both time and across face of Venus', and asked whether there was tension with other limits. The same slides placed the narrow line high in the atmosphere, near 80 km, and looked for gas nearer the clouds in the pressure-broadened wings. By July 2026 four campaigns were complete and a fifth had been approved for that month. This page found no refereed JCMT-Venus phosphine paper by 4 November 2026, and Clements himself wrote that acceptance of the result is still far from universal.

The ammonia case began as a hypothesis: in December 2021 Bains, Petkowski, Rimmer and Seager proposed in PNAS that ammonia dissolving in some droplets would neutralise them into semi-solid salt slurries with a pH near 1, and that, if the source were biological, the most energy-efficient reaction for making ammonia would also release oxygen, accounting for the oxygen in the clouds. On 17 July 2024, at the National Astronomy Meeting in Hull, Greaves presented a tentative ammonia signal in Green Bank Telescope data, and Clements told CNN it was 'arguably more significant than the discovery of phosphine', because a microbe would have a reason to make it. Greaves cautioned that even both gases together would not show life without direct measurements. In July 2025 the Royal Astronomical Society quoted her saying the team's latest data had found more evidence of ammonia, and said papers were expected later that year; this page found no refereed ammonia paper by 4 November 2026. At IKI in Moscow, planetary physicist Dmitry Gorinov said in May 2025 that ammonia on Venus is extremely scarce or absent. A December 2025 Peking University laboratory study proposed a non-biological source: sunlit sulfur turning nitrate into ammonia in acid.

Even if one or both gases are real, a biology needs somewhere to live. In June 2021 John Hallsworth and colleagues computed water activity, a measure of how available water is to organisms, from observed temperatures and water vapour, and found 0.004 or less for the sulfuric acid droplets, two orders of magnitude below 0.585, the limit for known extremophiles. In 2024 Charles Cockell, Hallsworth and colleagues described the Venus cloud deck as temperate but incompatible with known life, and used it to separate habitability for known organisms from habitability for any possible life. Supporters of cloud life have two replies: ammonia could neutralise some droplets, and the 2025 Pioneer Venus reanalysis suggests the aerosols hold far more water than pure acid would. Both rest on models or on reinterpreted 1978 data.

The MIT-led group has asked what survives in the acid itself. In 2023 Seager, Petkowski and colleagues reported in PNAS that the five nucleic acid bases of DNA and RNA are stable across the cloud range of temperature and acid strength. In July 2024 the same group tested 20 dipeptides, pairs of amino acids joined by a peptide bond, found that most broke down within weeks, that none was stable at both 81 and 98 percent acid, and concluded that Venus life would likely need a substitute for the peptide bond. In March 2025 they reported that short chains of peptide nucleic acid (PNA), a candidate genetic-like polymer, lost between 0.4 and 28.6 percent in 14 days in 98 percent acid at about 25 °C. In September 2026 a PNAS paper by Jia Yi Zhang, Aurelio Dregni, Petkowski, Seager and Mei Hong reported, from NMR spectroscopy and structural modelling, that three peptides fold into omega-loop structures in concentrated acid, where scarce water spares the peptide bonds from hydrolysis; Petkowski's Wrocław university quoted him saying they stayed fully stable in 98 percent acid for at least two weeks. Martin Rahm of Chalmers University, who was not involved, told Eos he was 'less sure' the structures are omega loops, and Eos noted that the modelling software was not designed for sulfuric acid. Whether a molecule holds its shape is a separate question from whether the droplets can support life, and the water activity objection still applies.

The discoverers call for measurements from inside the clouds, and by early November 2026 no instrument built for the purpose had flown. One candidate is Rocket Lab's small entry probe carrying an autofluorescence nephelometer, the first of what Seager calls the Morning Star Missions; she told Space.com, in an article of 3 September 2026, that Rocket Lab had moved it to the Neutron rocket and the team was 'awaiting Neutron readiness' (see Venus Life Finder); on 14 October 2026 Rocket Lab put Neutron's first launch in 2027. In July 2025 Greaves's team presented VERVE, a mission concept for a CubeSat-sized probe budgeted at 50 million euros that would ride with ESA's EnVision and map phosphine and ammonia. In Moscow, Russian Academy of Sciences president Gennady Krasnikov told President Putin on 9 December 2025 that Venera-D would look for markers of life, and vice-president Sergey Chernyshev said in April 2026 that traces of ancient life, such as complex organic compounds, might be found by the mission planned for 2036. One chance has gone by: in 2021 the Max Planck Institute for Solar System Research expected the submillimetre instrument on ESA's JUICE to detect phosphine at concentrations a thousand times lower, but JUICE passed Venus on 31 August 2025 with its high-gain antenna as a sunshield, and ESA said its remote sensing instruments could not be active.

Three separate questions tend to be flattened into one headline. Is there a real absorption feature? Independent reanalyses found none that is statistically significant, the discoverers keep recovering one, and the answer depends on how the spectrum of a very bright planet is cleaned. Is it phosphine? The discoverers put sulfur dioxide's share at a tenth or less; Lincowski's team found ordinary sulfur dioxide enough to explain the line, and infrared and spacecraft limits above the clouds sit well below the millimetre claims, which the discoverers attribute to altitude, time of day and sunlight. Is it biological? Clements wrote in 2024 that the discovery papers were explicit that they did not constitute evidence for life, only of phosphine. In a July 2026 preprint chapter, Daliah Bibas and Clément Vidal set the phosphine claim beside the 1976 Viking life-detection experiments on Mars as two high-uncertainty claims, and argued that such claims need transparent routes to verification.