Life on Venus

Long-standing unexplained Venus atmosphere observations and chemical anomalies point to unknown chemistry but also leave room for the possibility of life.

The unexplained observations include several gases out of thermodynamic equilibrium (e.g. tens of ppm O₂, the possible presence of PH₃ and NH₃, SO₂ and H₂O vertical abundance profiles), an unknown composition of large, lower cloud particles, and the “mysterious” absorber in the clouds.
Petkowski, J. J., Seager, S., Grinspoon, D. H., Bains, W., Ranjan, S., Rimmer, P. B., Buchanan, W. P., Agrawal, R., Mogul, R.,&Carr, C. E.(2023). Astrobiological Potential of Venus Atmosphere Chemical Anomalies and Other Unexplained Cloud Properties. Astrobiology, 00(00),00-00. Embargoed until publication.

Concentrated sulfuric acid (instead of water) could be the basis of Venusian biochemistry.

Venus clouds are composed of concentrated sulfuric acid—an aggressive chemical that destroys most of Earth life’s biochemicals and are thought to be sterile to life of any kind. Here we show that key molecules needed for life (nucleic acid bases) are stable in concentrated sulfuric acid, advancing the notion that the Venus atmosphere environment may be able to support complex chemicals needed for life.
Seager, S., Petkowski, J. J., Seager, M. D., Grimes Jr., J. H., Zinsli, Z., Vollmer-Snarr, H. R., Abd El-Rahman, M. K., Wishart, D. S., Lee, B. L., Gautam, V., Herrington, L., Bains, W., & Darrow, C.(2023). Stability of Nucleic Acid Bases in Concentrated Sulfuric Acid: Implications for the Habitability of Venus’Clouds. Proceedings of the National Academy of Sciences, 120(25), e2220007120.

Many features of Venus’clouds rule out the possibility that Earth life could live there, none of them however rule out the possibility of all life based on what we know of the physical principle of life on Earth.

The energy requirements for retaining water and capturing hydrogen atoms to build biomass are not excessive, the radiation environment is benign, defenses against sulfuric acid are conceivable and have terrestrial precedent, and the speculative possibility that life uses concentrated sulfuric acid as a solvent instead of water remains. The clouds can support biomass that could readily be detectable by future astrobiology-focused space missions from its impact on the atmosphere.
Bains, W., Petkowski, J. J., & Seager, S. (2023). Venus’ atmospheric chemistry and cloud characteristics are compatible with Venusian life. Astrobiology, 23(10), 00-00.

Potential presence of ammonia and phosphine in the atmosphere of Venus provide evidence of yet-unknown processes that are out of equilibrium.

The investigation of such unexpected observations on Venus should be open to a wide range of explanations, including unknown biological activity. Exploration of such unexplained observations with a skeptical eye towards tacit assumptions will increase the chances of making profound discoveries about the atmosphere of Venus and the diverse and often strange nature of planetary environments.
Cleland, C. E., & Rimmer, P. B. (2022). Ammonia and Phosphine in the Clouds of Venus as Potentially Biological Anomalies. Aerospace, 9(12), 752.

Building on the work of Rimmer et al. 2021, the model predicts that the clouds are not entirely made of sulfuric acid, but partially composed of ammonium salt slurries, which may be the result of biological production of ammonia in cloud droplets.

As a result, the clouds are no more acidic than some extreme terrestrial environments that harbor life. Life could be making its own environment on Venus. The model’s predictions for the abundance of gases in Venus’ atmosphere matches observation better than any previous model, and reconcile decades-long atmosphere anomalies.
Bains, W., Petkowski, J. J., Rimmer, P. B., & Seager, S. (2021). Production of Ammonia Makes Venusian Clouds Habitable and Explains Observed Cloud-Level Chemical Anomalies. Proceedings of the National Academy of Sciences, 118(52), e2110889118.

The mysterious UV absorber in the top clouds of Venus has defied explanation for close to a century.

A new study proposes that the UV absorber involves organic molecules inside the Venus cloud particles. Simple organic molecules can be formed from seed carbon compounds undergoing a chain of chemical reactions that lead to a rich variety of organic molecules. UV radiation is relevant for some reactions. The simple organic molecules originate from meteoritic delivery, photochemistry, or even possibly life itself.
Spacek, J. (2021). Organic Carbon Cycle in the Atmosphere of Venus. arXiv preprint arXiv:2108.02286.

The clouds of Venus might not be as inhospitable as previously thought.

Alternative interpretations to in situ measurements yield potentially habitable conditions with water activities (0.585) and buffered acidities pH ~0 that lie within the limits of terrestrial microbial growth.
Mogul, R., Limaye, S. S., Lee, Y. J., & Pasillas, M. (2021). Potential for phototrophy in Venus’ clouds. Astrobiology, 21(10), 1237-1249.

A new model of the chemistry of the atmosphere of Venus suggests that the clouds are not entirely made of sulfuric acid, but of salts.

The model predicts that the acidity of some cloud particles is much higher than previously thought and could reach the pH of around 1.
Rimmer, P. B., Jordan, S., Constantinou, T., Woitke, P., Shorttle, O., Hobbs, R., & Paschodimas, A. (2021). Hydroxide salts in the clouds of Venus: their effect on the sulfur cycle and cloud droplet pH. The Planetary Science Journal, 2(4), 133.

People wonder if water is be the only possible solvent for life. If life on Venus exist then it can use concentrated sulfuric acid as a solvent rather than water.

Our theoretical study suggest that concentrated sulfuric acid can in principle support a rich and diverse organic chemistry. However, if sulfuric acid-based life exists then it has to be completely different than Earth-like, water-based life.
Bains, W., Petkowski, J. J., Zhan, Z., & Seager, S. (2021). Evaluating Alternatives to Water as Solvents for Life: The Example of Sulfuric Acid. Life, 11(5), 400.

We have created a database of sulfuric acid reactivity of organic chemicals as a tool to study the possibility of life in concentrated sulfuric acid solvent.

Bains, W., Petkowski, J. J., & Seager, S. (2021). A Data Resource for Sulfuric Acid Reactivity of Organic Chemicals. Data, 6(3), 24.

Our hypothesized life cycle for the hypothetical microbial life in the Venusian atmosphere.

The paper includes a detailed description of  the extreme challenges to life of any kind in the Venusian atmosphere.
Seager, S., Petkowski, J. J., Gao, P., Bains, W., Bryan, N. C., Ranjan, S., & Greaves, J. (2021).
The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere. Astrobiology, 21(10), 1206-1223.

Phosphine

We show that the formation of phosphine from P₄O₆ in the Venusian atmosphere is thermodynamically unfavorable.

Bains, W.,Pasek,M. A., Ranjan, S., Petkowski, J. J.,Omran,A., & Seager, S.(2023). LargeUncertainties in the Thermodynamics of Phosphorus (III) Oxide (P₄O₆) Have Significant Implications for Phosphorus Species in Planetary Atmospheres ACS Earth and SpaceChemistry, 7(6), 1219-1226.

A review of the phosphine debate so far that discusses the discovery of phosphine, conflicting and confirming observations and analyses, and discusses future observational campaigns.

Clements, D. L. (2023). Venus, Phosphine and the Possibility of Life Contemporary Physics, 63(3), 180-199

Recovery of Venusian phosphine in SOFIA spectra by reducing contaminating signals; the PH₃ abundance is ~3 part-per billion (ppb) above the clouds. Just like it is on Earth there might be a difference in PH₃ abundance between Venusian day and night observations.

Greaves, J. S., Petkowski, J. J., Richards, A. M., Sousa-Silva, C., Seager, S., & Clements, D. L. (2023). Recovering Phosphine in Venus’ Atmosphere from SOFIA Observations arXiv:2211.09852.

Sources of PH₃ proposed by Truong and Lunine 2021 would require extraordinary explosive volcanism that does not happen on Venus.

Bains, W., Shorttle, O., Ranjan, S., Rimmer, P. B., Petkowski, J. J., Greaves, J. S., & Seager, S. (2022). Constraints on the production of phosphine by Venusian volcanoes Universe, 8(1), 54.

Bains, W., Shorttle, O., Ranjan, S., Rimmer, P. B., Petkowski, J. J., Greaves, J. S., & Seager, S. (2022). Only extraordinary volcanism can explain the presence of parts per billion phosphine on Venus Proceedings of the National Academy of Sciences, 119(7), e2121702119.

A short summary paper on the Venusian PH₃ discovery and the scientific debate that arose since the original candidate detection in September 2020.

Bains, W., Petkowski, J. J., Seager, S., Ranjan, S., Sousa-Silva, C., Rimmer, P. B., … & Richards, A. M. (2022). Venusian phosphine: a ‘Wow!’ signal in chemistry? Phosphorus Sulfur and Silicon and the Related Elements, 197(5-6), 438-443.

The announcement of the potential presence of PH₃ on Venus resulted in the community showing a small but significant increase in the notion that life in the clouds of Venus is a possibility. Nevertheless, the community still considers Venus to be the least likely abode of life ranking far behind Europa, Enceladus, Mars and even behind Titan.

Bains, W. & Petkowski, J. (2021). Astrobiologists are rational but not Bayesian. International Journal of Astrobiology,20(4), 312-318.

Several authors suggested that the signal from JCMT telescope can be attributed to mesospheric SO₂ instead of PH₃. It is however unlikely that the contested absorption line comes from the SO₂; simultaneous observation of other SO₂absorptions show that the potential SO₂ line-contamination is less than <10% of the observed signal.

Greaves, J. S., Rimmer, P. B., Richards, A., Petkowski, J. J., Bains, W., Ranjan, S., … & Fraser, H. J. (2021). Low levels of sulphur dioxide contamination of Venusian phosphine spectraMonthly Notices of the Royal Astronomical Society, 514(2), 2994-3001.

A response to papers by Snellen et al. 2020, Villanueva et al. 2021 and Thompson 2021 that question the data processing and data interpretation of the original Venusian PH₃ discovery paper. The paper shows the recovery of the PH₃ in the atmosphere of Venus in data taken with ALMA, using three different calibration methods and confirms that the JCMT detection of PH₃ remains robust.

Greaves, J. S., Richards, A. M., Bains, W., Rimmer, P. B., Clements, D. L., Seager, S., … & Fraser, H. J. (2021). Reply to: No evidence of phosphine in the atmosphere of Venus from independent analyses. Nature Astronomy, 5(7), 636-639.

A response to papers by Snellen et al. 2020, Villanueva et al. 2021, and Thompson 2021 that question the data processing and data interpretation of the original Venusian PH₃ discovery paper. The response demonstrates that PH₃-identification was not a post-hoc rationalization of a feature found after complex data processing.

Greaves, J. S., Richards, A. M., Bains, W., Rimmer, P. B., Sagawa, H., Clements, D. L., … & Hoge, J. (2021). Addendum: Phosphine gas in the cloud deck of Venus. Nature Astronomy, 5(7), 726-728.

Re-analyzed data from the Pioneer Venus Large Probe Neutral Gas Mass Spectrometer (LNMS) reveal the presence of several trace chemical species in Venus’ clouds including phosphine, hydrogen sulfide, nitrous acid (nitrite), nitric acid (nitrate), hydrogen cyanide, and possibly ammonia.

Mogul, R., Limaye, S. S., Way, M. J., & Cordova, J. A. (2021). Venus’ mass spectra show signs of disequilibria in the middle clouds. Geophysical Research Letters, 48(7), e2020GL091327.

Discovery paper of PH₃ in the Venusian atmosphere and its implications for the presence of life.

Greaves, J. S., Richards, A., Bains, W., Rimmer, P. B., Sagawa, H., Clements, D. L., … & Hoge, J. (2021). Phosphine gas in the cloud decks of Venus. Nature Astronomy, 5(7), 655-664.

A supplementary paper to the PH₃ on Venus discovery paper. Here we expand on the arguments that no known atmospheric, surface, or subsurface chemistry can explain the presence of atmospheric ppb by volume levels of PH₃. The paper also discusses the volcanic sources of PH₃ proposed by Truong and Lunine 2021 and other PH₃ formation pathways postulated by Omran et al 2021.

Bains, W., Petkowski, J. J., Seager, S., Ranjan, S., Sousa-Silva, C., Rimmer, P. B., … & Richards, A. M. (2021). Phosphine on Venus cannot be explained by conventional processes. Astrobiology, 21(10), 1277-1304.

We present a new model for the biological production of phosphine (PH₃) in specific anaerobic environments, where the combined action of phosphate reducing and phosphite disproportionating bacteria can produce phosphine.

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.

Many scientists are not aware or do not believe phosphine is produced by life. In this paper we list and then dispel the four main arguments against (too toxic, no direct evidence of production from a specific species, too energetically costly to produce, unstable in O₂-rich environment). We postulate that anaerobic life persisting in anoxic (O₂-free) environments may exploit trivalent phosphorus chemistry much more extensively, in contrast to O₂-dependent life for which phosphine is highly toxic.

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.

We make the case for PH₃ as a biosignature gas on exoplanets. The main conclusion is massive amounts of PH₃ need to be produced by life on an exoplanet in order to generate enough PH₃ to accumulate in the atmosphere for hypothetical remote sensing detection.

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.