Signs of life in a Martian meteorite were due to contamination from the earth, scientists have found.

The journal Science, which caused a sensation when it published the original findings in 1996, has now published two separate papers which prove the rock was contaminated by the Antarctic ice in which it was found.

The structures that convinced Nasa scientists

The original work by Nasa scientists seemed to indicate the presence of fossilised primitive micro-organisms and mineral features characteristic of biological activity within the potato-sized meteorite ALH84001.

Organic molecules were also found adding weight to Nasa's claims.

There was skepticism at the time and in subsequent months over whether the "fossils" could have been created by other means than organic lifeforms, but the crucial evidence lies with those organic molecules.

Scientists still think Mars could once have supported life (Nasa)

Tests conducted at the University of Arizona at Tucson show that the carbon atoms in the organic material could only have come from Earth whereas the mineral features are most definitely extraterrestrial.

In other words the organic molecules are not evidence that the "fossils" were ever living organisms.

Professor Jeff Bada: No evidence of uniquely Martian molecules

And according to Professor Jeff Bada, who led a team at the Scripps Institution of Oceanography, their investigations were similarly conclusive.

"The important issue we've addressed is the question is there any evidence of unique, Martian organic compounds and the answer is no," he said.

Professor Bada's team searched for evidence of amino acids, essential for biological activity. None of the amino acids present in the meteorite could be traced back to a Martian origin.

Scientist say life may exist on Mars, or more likely may once have existed but this meteorite can no longer be used as evidence of it.

A space probe will be sent to bring back samples from the surface of Mars - perhaps as early as 2005

"Neither paper changes our original hypothesis," said Everett K. Gibson Jr., a NASA researcher. "They don't shake our belief one bit."

Gibson and David S. McKay, both of the Johnson Space Center in Houston, announced in August 1996 that they had found the evidence of life in a meteorite from Mars.

Gibson said that Bada's team analysed a sample that included minerals from around the life-like structures in the rock that he and McKay found. A true test, he said, would be to analyse only the lifelike structures.

For instance, he said, an analysis of a chocolate chip cookie would give different results, depending on what part of the cookie is tested.

In that analogy, said Gibson, the NASA team examined only the chocolate chips, while the Scripps team looked at the whole cookie.
 

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