A gas hailed as a possible sign of life on Mars disappears from the planet's atmosphere hundreds of times faster than expected.

This suggests that Mars could in fact be an "extraordinarily harsh environment" for the survival of other organic molecules — the carbon-based chemicals on which life on Earth is based — said a new study published in Thursday's edition of Nature.

"This would leave little hope that life as we know it can exist at present or that evidence of past life can be preserved in the shallow surface layer," said the paper by University of Paris researchers Franck Lefèvre and François Forget.

They had created computer models to look more closely at variations in methane — a simple organic molecule that is the major component of natural gas.

In January, NASA researchers reported that 19,000 tonnes of methane were released in high concentrations over three specific areas in Mars's western hemisphere over a short period in summer 2003. Most of the methane had disappeared by 2006.

Molecules destroyed in an hour

The NASA researcher noted that most methane on Earth is produced by living things, and its presence on Mars boosts the probability that life existed on Mars in the past or still does.

Lefèvre and Forget took into account Mars's climate and standard methane chemistry in an effort to explain the NASA researchers' results.

They found that such large variations in the local concentration of methane could only exist if methane was destroyed on the surface of Mars within about one hour, compared to its expected lifetime of several centuries.

That is about 600 times faster than predicted by standard photochemistry — that is, the chemical reactions catalyzed by the sun that might normally be expected. They suggested very reactive chemicals in the soil, such as hydrogen peroxide, might be responsible.

The French researchers also found that variations in methane concentrations could be created on Mars by carbon dioxide vaporizing from dry ice to gaseous CO2 and condensing again, but the patterns of such a mechanism did not match those observed by the NASA researchers.