
Reported Phosphine on Venus
Concentrated Sulfuric Acid and Biomolecules
The reported detection of phosphine (PH₃) on Venus has been controversial ever since the original announcement in 2020 (Greaves et al. 2020). Some question whether or not the signal is real.
Others find the signal to be real but question the attribution to PH₃. Still others assume the signal and attribution are legitimate, but then propose a nonbiological chemical source for phosphine (note that the original announcement did not claim PH₃ was definitively caused by life but explored the idea). Our team, with the data analysis led by Prof. Jane Greaves and the chemistry interpretation led by Janusz Petkowski and William Bains, has responded to nearly all published papers with a response published in the literature. Science is working in this back-and-forth process. To capture back and forth with live links to the published papers, we have created a living “PH₃ Chronology Table” at this link. https://docs.google.com/spreadsheets/d/18UX5Tr31C1WD1s-vrshFDqldo3ibvNri-fkliQSC2fM/edit#gid=377604647
Phosphorus (III) Trioxide (P₄O₆) Cannot be the Source of PH₃ on Venus.
The source of the reported phosphine (PH₃) detection on Venus is unknown. There could be an as yet unknown geochemical or photochemical process. Or, there could possibly be life in the Venus cloud layers producing PH₃ (1).
One proposed PH₃ atmospheric formation pathway is the reaction of phosphorus (III) oxide (P₄O₆) with water to transiently form phosphorous acid (H₃PO₃) which is unstable in the gas phase and undergoes disproportionation to phosphine and phosphoric acid (H₃PO₄).
The proposed reaction proceeds as follows:
P4O6 + 6 H₂O → [4 H₃PO₃] → PH₃ + 3 H₃PO₄
We have previously shown (1) that this reaction in the Venus atmosphere would not spontaneously generate sufficient PH₃ to explain the observed amounts (2). In contrast, Prof. Mathew Pasek and his team from the Department of Geosciences at the University of South Florida found that the reaction would generate PH₃, if P₄O₆ was present at low altitudes in the atmosphere of Venus (3). They did not however calculate whether P₄O₆ was indeed present.
To resolve this major discrepancy between the two contradictory studies, we joined forces with Prof. Pasek’s team. Together we have shown that the formation of phosphine from P₄O₆ in the Venusian atmosphere is thermodynamically unfavorable and therefore cannot be the source of PH₃ in the Venus atmosphere.
We conclude that the widely used values from the NIST/JANAF thermochemical database are almost certainly too low (specifically the Gibbs free energy of formation of P₄O₆) and therefore erroneously predict that P₄O₆ is more stable than is plausible. We showed that there is no combination of thermodynamic values that allows P₄O₆ to be present and allows PH₃ to be formed by disproportionation of P₄O₆ (4). This finding rules out P₄O₆ disproportionation as a source of PH₃ in Venus clouds, although it does not rule out other unknown abiotic sources for PH₃.
We emphasize that there is a need for more robust data on both the thermodynamics of phosphorus chemistry for astronomical and geological modelling in general, and for understanding the atmosphere of Venus in particular.
The paper detailing the results is published in the American Chemical Society Earth and Space Chemistry Journal (4).
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W. Bains, et al., Phosphine on Venus Cannot be Explained by Conventional Processes. Astrobiology 21, 1277–1304 (2021).
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J. S. Greaves, et al., Low levels of sulphur dioxide contamination of Venusian phosphine spectra. Mon. Not. R. Astron. Soc. 514, 2994–3001 (2022).
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A. Omran, et al., Phosphine Generation Pathways on Rocky Planets. Astrobiology 21, 1264–1276 (2021).
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W. Bains, et al., Large Uncertainties in the Thermodynamics of Phosphorus (III) Oxide (P₄O₆) Have Significant Implications for Phosphorus Species in Planetary Atmospheres. ACS Earth Sp. Chem. (2023) https:/doi.org/10.1021/acsearthspacechem.3c00016.

Venus Phosphine Update June 2023
The Venusian atmospheric phosphine debate continues in 2023. Recently Cordiner et al. (2022) find no phosphine in Venus’ atmosphere, using the airborne SOFIA observatory (1). Professor Jane Greaves and the phosphine team, however, have reanalyzed the SOFIA data and recovered a 5.7 σ candidate detection, at ~3 ppb of PH3above the clouds of Venus (2). Moreover, we hypothesize that the detections (3, 4) and upper limit observations (i.e. the non-detections) (5, 6) can be reconciled if the former are from ‘mornings’ in Venus’ atmosphere and the latter from ‘evenings’. Sunlight reduces the amount of phosphine in Earth’s atmosphere by more than ten times during the day as compared to the night (7), so similarly on Venus, we might expect lower abundances of PH₃ when the part of the atmosphere observed has passed through sunlight (see Figure below; adapted from (2)). If the available observations of Venus can be reconciled in this way, further modelling of possible sources of PH₃(e.g. through photochemical processes, disequilibrium chemistry, or extant life) seems worthwhile. Modeling work and experiments aimed at finding novel, previously unknown, chemical routes of phosphine formation in the Venusian atmosphere have recently been proposed (8) and if completed should contribute valuable data to the debate on the source of PH3.
The debate regarding phosphine in Venus’ atmosphere is likely to continue for some time. Attempts to reconcile conflicting observations are ongoing. A new JCMT survey led by Dr. David Clements and Prof. Jane Greaves is currently underway. Ultimately the Venusian phosphine debate can likely only be resolved by new direct sampling of Venus atmosphere, potentially with the powerful Venus Tunable Laser Spectrometer (VTLS) instrument on board the DAVINCI descent probe (9).

The sketch of the trend of phosphine abundances by altitude. Symbols indicate candidate detections plus best upper limits for phosphine abundances. Rising arrows indicate observations made where the super-rotating atmosphere was rising into sunlight and falling arrows indicate observations made where the atmosphere was descending towards the nightside (see key). Large and small symbols indicate that a large fraction of the planet area was observed, or that a small region was sampled, respectively. For comparison, the blue arrow indicates the ten-fold increase of terrestrial phosphine from day to night – note the arrow’s plotted position is arbitrary; Earth hosts much lower PH3 abundance than Venus.
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M. A. Cordiner, et al., Phosphine in the Venusian Atmosphere: A Strict Upper Limit from SOFIA GREAT Observations. Geophys. Res. Lett., e2022GL101055 (2022).
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J. S. Greaves, et al., Recovery of Phosphine in Venus’ Atmosphere from SOFIA Observations. arXiv Prepr. arXiv2211.09852 (2022).
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J. S. Greaves, et al., Phosphine gas in the cloud decks of Venus. Nat. Astron. 5, 655–664 (2021).
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R. Mogul, S. S. Limaye, M. J. Way, J. A. Cordova, Venus’ Mass Spectra Show Signs of Disequilibria in the Middle Clouds. Geophys. Res. Lett., e2020GL091327 (2021).
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T. Encrenaz, et al., A stringent upper limit of the PH3 abundance at the cloud top of Venus. Astron. Astrophys. 643, L5 (2020).
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L. Trompet, et al., Phosphine in Venus’ atmosphere: Detection attempts and upper limits above the cloud top assessed from the SOIR/VEx spectra. Astron. Astrophys. 645, L4 (2020).
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D. Glindemann, A. Bergmann, U. Stottmeister, G. Gassmann, Phosphine in the lower terrestrial troposphere. Naturwissenschaften 83, 131–133 (1996).
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M. Ferus, et al., Abiotic chemical routes towards the phosphine synthesis in the atmosphere of Venus in European Planetary Science Congress, (2022), pp. EPSC2022-198.
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J. B. Garvin, et al., Revealing the Mysteries of Venus: The DAVINCI Mission. Planet. Sci. J. 3, 117 (2022).