The paper, published in a prestigious journal concerning the chemistry of rocks and meteorites, examined the ratio of different isotopes of residues of iron sulphide, more commonly known as "fool's gold", in the meteorite ALH84001, which came from Mars about 4 billion years ago. Last month Nasa scientists announced that they believed they had found signs of past life buried deep within ALH84001.
In the paper, a team of scientists from the University of New Mexico said that the ratio of isotopes was too small to indicate biological activity. Different isotopes of the same chemical have fractionally different weights and geological processes such as fluids washing over a rock for a long period filter them slowly.
Normally, living organisms create a large ratio of isotopes - up to a factor of 80 - because they act as more efficient filters. The New Mexico team only found a ratio of three.
But Monica Grady, an expert in meteorites at the Natural History Museum, London, said that the result does not invalidate the Nasa work. "It could be caused by life on Mars which is not as we know it."
The paper was submitted to the journal Geochimica et Cosmochimica Acta, which is published in Oxford, in February, which means that the New Mexico scientists were not aware of the Nasa research when they did their examination. The comment on the apparent lack of biological activity was added at a late stage.
Nasa's conclusion that life might have existed on Mars 4 billion years ago was based on residues of carbonates which were discovered inside the meteorite, found in 1984. The announcement by Nasa was regarded by some observers as an attempt to garner publicity at a time when budgets for space expeditions were being cut.Reuse content