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's 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 °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. "From an astrobiology point of view, Venus is not hopeless," 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's atmosphere. Solar radiation and lightning should produce large quantities of carbon monoxide in the planet's 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.

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. "There may be non-biological ways to produce the hydrogen sulphide or carbonyl sulphide that we don't know about, but both reactions need catalysts to proceed efficiently," says Schulze-Makuch. "On Earth, the most efficient catalysts are microbes."

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. "I am reluctant to believe this result," says André Brack from the Centre for Molecular Biophysics in Orléans, France. "For life, you need a volume of water, not just tiny droplets." But Schulze-Makuch points out that there is chemical evidence that Venus was once cooler and had oceans. "Life could have started there and retreated to stable niches once the runaway greenhouse effect began," he says.

But we may have to wait several years for any firm answers. The European Space Agency's Venus Express mission, which will investigate the planet's 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.

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Author: Stuart Clark

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Contact: Claire Bowles, claire.bowles@rbi.co.uk, 44-207-331-2751, New Scientist

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