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).

1. W. Bains, et al., Phosphine on Venus Cannot be Explained by Conventional Processes. Astrobiology 21, 1277–1304 (2021).
2. J. S. Greaves, et al., Low levels of sulphur dioxide contamination of Venusian phosphine spectra. Mon. Not. R. Astron. Soc. 514, 2994–3001 (2022).
3. A. Omran, et al., Phosphine Generation Pathways on Rocky Planets. Astrobiology 21, 1264–1276 (2021).
4. 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.