THE acidic clouds of Venus could in fact be hiding life. Unlikely as it sounds, the presence of microbes could neatly explain several mysterious observations of the planet鈥檚 atmosphere.
Venus is usually written off as a potential haven for life because of its hellishly hot and acidic surface. But conditions in the atmosphere at an altitude of around 50 kilometres are relatively hospitable: the temperature is about 70 掳C, with a pressure of about 1 atmosphere. Although the clouds are very acidic, this region also has the highest concentration of water droplets in the Venusian atmosphere. 鈥淔rom an astrobiology point of view, Venus is not hopeless,鈥� says Dirk Schulze-Makuch from the University of Texas at El Paso.
To look for possible signs of life, Schulze-Makuch and his colleague Louis Irwin looked at existing data on Venus from the Russian Venera space missions and the US Pioneer Venus and Magellan probes. They noticed some peculiar things about the chemical composition of Venus鈥檚 atmosphere. Solar radiation and lightning should produce large quantities of carbon monoxide in the planet鈥檚 atmosphere, but instead it is scarce, as if something is removing it. They also found hydrogen sulphide and sulphur dioxide. These two gases react with each other, and so are never normally found together unless something is producing them.
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Even more mysterious is the presence of carbonyl sulphide. This gas is so difficult to produce inorganically that it is sometimes considered an unambiguous indicator of biological activity. 鈥淭here may be non-biological ways to produce the hydrogen sulphide or carbonyl sulphide that we don鈥檛 know about, but both reactions need catalysts to proceed efficiently,鈥� says Schulze-Makuch. 鈥淥n Earth, the most efficient catalysts are microbes.鈥�
Schulze-Makuch thinks that bugs living in the Venusian clouds could be combining sulphur dioxide with carbon monoxide and possibly hydrogen to produce either hydrogen sulphide or carbonyl sulphide in a metabolism similar to that of some early Earth bugs. He suggests the bugs could be using ultraviolet light from the Sun as an energy source. If they are absorbing UV, that would explain the presence of mysterious dark patches on ultraviolet images of the planet. He presented his theory at the Second European Workshop on Astrobiology in Graz, Austria, last week.
Not everyone is convinced. 鈥淚 am reluctant to believe this result,鈥� says Andr茅 Brack from the Centre for Molecular Biophysics in Orl茅ans, France. 鈥淔or life, you need a volume of water, not just tiny droplets.鈥� But Schulze-Makuch points out that there is chemical evidence that Venus was once cooler and had oceans. 鈥淟ife could have started there and retreated to stable niches once the runaway greenhouse effect began,鈥� he says.
But we may have to wait several years for any firm answers. The European Space Agency鈥檚 Venus Express mission, which will investigate the planet鈥檚 atmosphere, is due for launch in 2005. Meanwhile the Swedish Space Agency is looking for international partners to develop their idea for a mission to return a sample of the atmosphere from Venus around the turn of the decade.