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3.3 Present Life
Could bacterial life exist in the atmosphere of Venus today? Although this is considered unlikely, the possibility of life in the clouds or the middle atmosphere of Venus has not been ruled out by any observations made to date. While the atmosphere is both dry and acidic,extremophilic life has adapted to far more harsh conditions on Earth.
There is some evidence that the trace-gas constituents of the Venus atmosphere are not in chemical equilibrium with each other. On Earth, the primary source of disequilibrium in the atmospheric chemistry is the activities of biological processing; could disequilibrium on Venus also be a sign of life? In 1997, David Grinspoon made the suggestion that microbes in the clouds and middle atmosphere could be the source of the disequilibrium. In 2002, Dirk Schulze-Makuch independently proposed that observations of the Venus atmosphere by space probes showed signatures of possible biological activity.
As noted by Grinspoon and Schulze-Makuch, the Venus atmosphere has several trace gasses which are not in chemical equilibrium. The Venera missions and the Pioneer Venus and Magellan probes found that carbon monoxide is scarce in the planet's atmosphere, although solar radiation and lightning should produce it abundantly from carbon dioxide. Hydrogen sulfide and sulfur dioxide, two gases which react with each other and thus should not be found together, are also both present, indicating some process (possibly biological?) is producing them. Finally, although carbonyl sulfide is difficult to produce inorganically, it is present in the Venusian atmosphere. On Earth, this gas would be considered an unambiguous indicator of biological activity. While none of these chemical combinations are in themselves an unambiguous sign of life, it is interesting enough to warrant a more careful look at the atmospheric chemistry.
Another interesting sign is the nature of the ultraviolet-absorbing aerosols that form the markings seen in UV images of the planet (figure 2). The nature of these aerosols, and whether they are biological in origin, is still unknown.
On Earth, viable microorganisms are found in clouds...