The existence of living organisms on Venus is impossible due to the almost complete absence of biologically available water and the presence of sulfuric acid. This conclusion was reached by scientists at Queens University in Belfast, who measured the activity of water in the clouds of the second planet from the Sun and other large objects of the Solar system. The astronomers published their findings in the journal Nature Astronomy.
Water activity (Aw) is defined as the ratio of the vapor pressure of water over a given material to the vapor pressure over clean water at the same temperature. This indicator is a measure of the availability of water for living organisms, in particular, microorganisms. Usually, water activity is used to assess the stability of food products. For example, the increased "humidity" of food contributes to the reproduction of pathogenic microbes and a short shelf life. Water activity can also be used as a criterion for the habitability of the environment.
Scientists used the data of measurements of interplanetary probes and determined the activity of water in the atmosphere of Venus, where the main part of the clouds are drops of sulfuric acid. It turned out that it is lower or equal to 0.004; at the same time, the minimum limit at which known extremophile organisms can still exist is 0.585. These data refuted the existence of hypothetical microorganisms, which was allegedly indicated by an abnormal level of phosphine detected in 2019. Then a group of scientists missed sulfur dioxide in the atmosphere of Venus, and decided that a powerful radio signal characteristic of it at a frequency of 266.94 gigahertz gives phosphine in such a large amount that it could be formed, most likely, due to the activity of living organisms. This anomaly was later recognized by other researchers as a mistake.
As for other planets, the formation of ice on Mars requires water activity of less than 0.537, which is slightly below the habitable range. And the clouds of Jupiter, on the contrary, from the point of view of Aw turned out to be a biologically favorable environment (>0.585), although the existence of living organisms here may be limited by other factors, for example, the chemical composition of the atmosphere of a gas giant or the level of ultraviolet radiation.
On Earth, the conditions in the troposphere are generally acceptable for living organisms to live, but over the middle part of the stratosphere, the atmosphere becomes too dry for active life. The authors of the study hope that their method of determining the activity of water will also be applicable to planets outside the Solar system.
Alexander Enikeev
