Venus, nicknamed “our sister planet” by the view of the lens of the ship Mariner 10. The image in visible and ultraviolet light gives a peaceful impression. However, the planet’s surface shrouded in acid clouds is crushing high atmospheric pressure and suffering from water shortages. Credit: NASA.
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Life on Venus experiences periodic ups and downs. We do not know, of course, whether this is the case in reality, in the atmosphere of Venus, but it certainly applies to the view of its probability among the professional public and astrobiological enthusiasts.
The whole debate was recently stirred up by the discovery of phosphane in Venus clouds (Greaves et al., 2020), which was later questioned (Akins et al., 2021; Villanueva et al., 2021) and resuscitated again (Greaves et al., 2021). its biological interpretation (Truong and Lunine, 2021). Reanalysis of Pioneer Venus data added other potentially biogenic gases: methane and other hydrocarbons, hydrogen sulfide, and possibly ammonia (Mogul et al., 2021).
Picrophilus torridus. Credit: Wolfgang Liebl. Georg-August-University Goettingen.
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Now an international team of scientists led by John E. Hallsworth (the first author of the study) has looked at how terrestrial organisms would like Venus in the clouds. They recently published their results in a professional study in the prestigious journal Nature Astronomy. We can sum them up in one word: I didn’t like it.
They took the water to Paškal. Water is the basis of life, but it is not enough on its own. If unavailable, it is of little use to living organisms. Substances dissolved in water bind some of the water molecules and do not like to get rid of them. From the point of view of a living organism, there is a drought in such a liquid. Not only the castaways on the high seas, but also sea fish and sea snakes are struggling with an unquenchable thirst because they have trouble pulling enough water out of the salt sea. Salted meat, candied fruit or even sweet syrup will not spoil, because salt or sugar take up all the water for themselves and simply dry out all harmful organisms.
Technical activity refers to water activity, which is a quantity corresponding to the relative humidity of the air at which a given solution neither dries out nor draws water vapor, but is in equilibrium with the air. The water activity of pure water is equal to one, the concentration of the solution decreases. If the water activity of the solution is higher than the air humidity, the solution dries, if it is lower, it behaves hygroscopically and draws water vapor from the surroundings. The water activity of seawater is 0.98, and most microbes begin to thirst below 0.92. Molds are a bit more resistant – that’s why sugar-preserved syrups and jams are more moldy than they would normally “spoil”. The most extreme known “suchomil”, a mold Aspergillus penicillioides, stops growing at water activity below 0.585.
Venus in size compared to Earth. The image of the Earth is from the workshop of the Apollo 17 crew. Credit: NASA.
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The clouds of Venus are neither salt nor sugar, but sulfuric acid. No problem, you would say, bacteria in volcanic springs usually “give” it. But not so fast. The most resistant acidophiles tolerate sulfur solutions with a concentration of about 11 weight percent, while on Venus it is more than about eighty (75-96%) – this is more of a small amount of water dissolved in the acid than vice versa. By the way, the authors of the current study came to the surprising finding that the earthly record holder in the field of acidity – extreme acidophile Picrophilus torridus from the realm of the arches – in fact, in its flight it is not limited so much by the acidity of its environment as by water activity. Because sulfuric acid is sulfuric acid, it does not share water.
Terrestrial organisms, perhaps according to the motto “many dogs – hare’s death”, poorly tolerate the concurrence of several extremes. Low water activity thus deprives them of the strength they need to fight acidity, or vice versa.
And how is that Venus? The water activity in the cloud cover is lower than 0.004 – two orders of magnitude smaller than that needed by terrestrial organisms, and more than 60 times lower than, for example, in the Sahara.
So we certainly don’t have to worry about finding something too similar to terrestrial organisms on today’s Venus. An exotic life that would play by completely different rules is, of course, a different question (I touched it a bit in the previous article “Metanogeni and Enceladu“).
Water activity on Venus has previously been considered a limiting factor by the well-known astrobiologist Dirk Schulze-Makuch, who pointed out that typical conditions on a given body may not be essential for the survival of organisms, but even small microenvironments or fleeting opportunities are sufficient. Many organisms can be dry or frozen most of the time, and they only need to come to life here and there. It is easier to imagine such fleeting opportunities on Mars than in the clouds of Venus, which appear to be a stable and homogeneous environment, but this can only be given by the degree of our ignorance.
The authors of the study, once they were done with Venus, looked at other planets. They found that the environment on the surface of Mars shows a water activity of about 0.537, which is outside the limits of known life, but only narrowly. More surprising were the results for Jupiter: the king of the planets has in its clouds, in places where there is a pressure of about 5 bar and at the same time quite terrestrial temperature, water activity compatible with terrestrial life: over 0.585. In addition, there is a lot of organic matter. This somewhat revives ancient visions of organisms hovering among the colorful clouds of this giant planet, as outlined by Carl Sagan, AC Clarke, and others.
Video: The first real images from Venus
Resources:
https://astronomycommunity.nature.com/posts/on-venus-water-everywhere-nor-any-drop-to-drink-3b8a3a6f-5282-4477-8bf5-d34aaea2917f
(pictures can also be drawn from here)
Akins, A. B., Lincowski, A. P., Meadows, V. S. & Steffes, P. G. Preprint at https://arxiv.org/abs/2101.09831 (2021).
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.
Villanueva, G. L., Cordiner, M., Irwin, P. G. J., De Pater, I., Butler, B., Gurwell, M., … & Kopparapu, R. (2021). No evidence of phosphine in the atmosphere of Venus from independent analyses. Nature Astronomy, 5(7), 631-635.
Truong, N., & Lunine, J. I. (2021). Volcanically extruded phosphides as an abiotic source of Venusian phosphine. Proceedings of the National Academy of Sciences, 118(29).
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.
Greaves, J. S., Richards, A., Bains, W., Rimmer, P. B., Clements, D. L., Seager, S., … & Fraser, H. J. Re-analysis of phosphine in Venus’ clouds. arXiv preprint arXiv:2011.08176.
Hallsworth, J. E., Koop, T., Dallas, T. D., Zorzano, M. P., Burkhardt, J., Golyshina, O. V., … & McKay, C. P. (2021). Water activity in Venus’s uninhabitable clouds and other planetary atmospheres. Nature Astronomy, 1-11. doi.org/10.1038/s41550-021-01391-3
Schulze-Makuch, D. (2021). The case (or not) for life in the Venusian clouds. Life, 11(3), 255.
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