Doubts surface over life on Mars

Life on Mars? The question mark is getting bigger. A study by American scientists has cast fresh doubts on the announcement last August by the US space agency Nasa that it had found fossils of living cells in a meteorite from Mars.

A team at the University of Hawaii, led by Edward Scott, said their analysis indicated that carbonates in the rock - taken as key evidence of life - were formed as part of a high-impact shock, and not long-term processes conducive to life.

The research, published today in the science journal Nature, adds weight to those opposing the Nasa conclusion. While the original paper - published in Science magazine in the US - sparked enormous public excitement, the research papers that have since cast doubt on it have followed have received comparatively little attention.

Last August, a multi-disciplinary team led by Nasa announced that tiny holes in a 16-million-year-old piece of meteorite known as ALH 84001 might be the remains of ancient Martian bacteria.

Since then, the meteorite has been examined by teams of researchers around the world. Those who say it could harbour fossils cite evidence that the carbonates inside the rock formed at moderate temperatures over long periods of time.

The idea is that mineral-rich water percolated through tiny cracks in the rock, creating an environment in which bacteria could grow.

But others say it looks like the carbonate molecules formed in a hot flash - like that caused by a meteorite impact - which would make it less likely that living bacteria were once in there. Scott's group backed the "hot flash" camp.

"We find that carbonate, plagioclase and silica were melted and partly redistributed by the same shock event responsible for the intense local crushing of pyroxene in the meteorite," they write.

Nasa is planning missions to Mars to scoop up and analyse rock and soil to see if any similar traces can be found.

t The weather forecast for Nasa's Pathfinder probe, due to arrive on Mars on 4 July, is: Changeable. Pink skies, no clouds, temperatures rising to minus 40C. Planetwide dust storms. Later, clear blue skies, colder, minus 87C, brilliant ice clouds, no dust.