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 . 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 , 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 . 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.
 Greaves et al. (2020), Phosphine Gas in the Cloud Decks of Venus, Nature Astronomy
 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
 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.
 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
 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.
 Glindemann, D., Stottmeister, U., & Bergmann, A. (1996). Free phosphine from the anaerobic biosphere. Environmental Science and Pollution Research, 3(1), 17-19.
 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.
: 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.
 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.