Phosphine Gas Detected on Venus

In the September 14, 2020 Nature Astronomy issue Dr. Jane Greaves reports the discovery of 20 ppb of phosphine (PH₃) gas in Venus’ atmosphere, based on millimeter-wave observations [1]. This spectacular discovery is simply astounding. PH₃ should not exist or be produced at the measured levels in an oxidized planetary atmosphere like Venus’. We can argue there is no plausible way for PH₃ to be produced at the detected levels via any known atmosphere, surface, or subsurface chemistry [1,2]. We are left with the incredible possibility that PH₃ might be produced by life. Life on Venus could not survive at the inhospitably hot surface and so would have to live in the temperate permanent cloud deck. [3,4].

Very few people realize that PH₃ is produced by life on Earth. PH₃ is detected in anoxic habitats such as marshlands, swamps, and even animal digestive tracts [5,6], as well as in the laboratory from complex mixtures of bacterial species [7], although the enzymes and pathways responsible for making PH₃ are still unknown.  PH₃ is also created by us humans, e.g., as a pesticide. A point in favor of PH₃ as a biosignature gas is that it is so thermodynamically unfavorable on Earth, there is no non-life process that can produce it beyond non-negligible amounts [8,9].

Radio astronomer Dr. Jane Greaves found the somewhat obscure PH₃ biology papers [6,7] while looking for ideas to search for life in the Venusian atmosphere. Independently from Jane, since about 2015, my team had been exploring PH₃ as a potential biosignature gas for future telescopes to search for life in exoplanet atmospheres [9]. My close colleague Dr. William Bains had an even longer-standing interest in PH₃, and had just completed two papers supporting PH₃ as produced by life on Earth [5,8]. As word got around of my team’s expertise on the incredibly niche topic of PH₃ and life, we were connected with Jane through a mutual contact. Together with others on her team we worked on detailed interpretation of PH₃’s planetary production pathways in an anoxic atmospheres, despite many unknown reaction rates.

Have we found evidence for life on Venus? No. But we have found a very intriguing signal that will motivate work for years to come. We have Venus data from IRTF and team members are proposing to other infrared-capable ground-based telescopes (including the airborne SOFIA), although PH₃ only has very weak signals in the infrared.  Two Discovery-Class Venus Missions are currently in a phase A competition, but are not focused on life or signs of life detection.

A small Venus mission with a small payload could be launched within the next few years to confirm the presence of PH₃ gas and measure its vertical distribution in the atmosphere (along with other gases). A series of nimble and focused missions could seek for further signs of life and even life itself. A golden era of Venus exploration lies ahead.

References 

[1] Greaves et al. (2020), Phosphine Gas in the Cloud Decks of Venus, Nature Astronomy

[2] Bains, W., Petkowski, J. J., Seager, S. et al. (2020) submitted to Astrobiology, https://ef138b75-3870-4e60-a95e-a70887b14bfb.filesusr.com/ugd/874d8b_0c6a7490550b475bb03162e68b12f2d2.pdf

[3] Limaye, S. S., Mogul, R., Smith, D. J., Ansari, A. H., Słowik, G. P., Vaishampayan, P. (2018). Venus’ Spectral Signatures and the Potential for Life in the Clouds Astrobiology, 18(9), 1181-1198.

[4] Seager, S., Petkowski, J. J, Gao, P., Bains, W., Bryan, N., Ranjan, S., Greaves, J. (2020). The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere. Astrobiology, published ahead of print https://www.liebertpub.com/doi/full/10.1089/ast.2020.2244

[5] Bains, W., Petkowski, J. J., Sousa-Silva, C., & Seager, S. (2019). Trivalent phosphorus and phosphines as components of biochemistry in anoxic environments. Astrobiology, 19(7), 885-902.

[6] Glindemann, D., Stottmeister, U., & Bergmann, A. (1996). Free phosphine from the anaerobic biosphere. Environmental Science and Pollution Research, 3(1), 17-19.

[7] Jenkins, R. O., Morris, T. A., Craig, P. J., Ritchie, A. W., & Ostah, N. (2000). Phosphine generation by mixed-and monoseptic-cultures of anaerobic bacteria. Science of the total environment, 250(1-3), 73-81.

[8]: Bains, W., Petkowski, J. J., Sousa-Silva, C., & Seager, S. (2019). New environmental model for thermodynamic ecology of biological phosphine production. Science of The Total Environment, 658, 521-536.

[9] Sousa-Silva, C., Seager, S., Ranjan, S., Petkowski, J. J., Zhan, Z., Hu, R., & Bains, W. (2020). Phosphine as a biosignature gas in exoplanet atmospheres. Astrobiology, 20(2), 235-268.