War of words on alien life

March 14, 1997

The possibility of life on Mars has aroused heated debate within the scientific community, THES reporters examine the issues.

BRITISH scientists are expected this week to endorse claims that a Martian meteorite found in Antarctica may contain evidence of former extraterrestrial life.

But there is disagreement over whether samples from the meteorite were formed at hot or cold temperatures - a factor which some scientists believe makes a crucial difference to the chances of the existence of life.

Since Nasa scientists first announced last August that they had discovered remains of early life in a potato-sized lump of rock thought to have broken from the surface of Mars some 15 million years ago, chemists, microbiologist and geologists have fiercely contested the claims.

But now scientists at Edinburgh University, working in collaboration with a team from the University of Wisconsin, Caltech and Nasa in the United States, say they have strong evidence that the carbonate globules in the rock were formed at low temperatures - making life more likely.

Their claims run counter to those of earth scientists at Manchester University, who say the carbonates were deposited from fluids at high temperatures.

Grenville Turner, Manchester team leader, argues that the calculated deposition temperatures of the carbonates are "ambiguous" and depend on the oxygen source.

"If it comes from silicate minerals, low deposition temperatures are implied. If it is predominantly from carbon dioxide, the deposition could have been at high temperatures," he said.

The Manchester project's evidence suggests traces of "light" oxygen, indicating high temperatures.

But findings of the team from Edinburgh and their US collaborators, who carried out the analysis on a sample from the Martian meteorite ALH84001, suggest the globules formed at temperatures below 300 degrees centigrade and possibly, for part of the time, below 100 degrees - temperatures thought to be low enough to potentially allow life.

They will report their conclusions to the 28th Lunar and Planetary Science Conference in Houston next week.

Colin Graham, the geochemist leading the Edinburgh group, said: "You find bacteria growing on earth in geysers at a little over 100 degrees centigrade." The team has used microanalysis, being pioneered in Edinburgh, to look at the proportion of different isotopes of oxygen and carbon in the sample.

They used an ion microprobe to make about 500 analyses of carbonate from Earth, so that the instrument's calibration was accurate. This then measured the abundance of isotopes of carbon and oxygen in the Martian sample.

Not enough is known about the history of water on the surface of Mars to draw a safe analogy with Earth. But the researchers have also found evidence for very small-scale chemical and oxygen disequilibrium both within the carbonates and between the carbonates and the surrounding rock.

"If you had high temperatures, it would be very hard to preserve this very small-scale disequilibrium in chemistry. The carbonate globules would have reacted chemically with the silicate to give new minerals and everything would have mixed together and you would get something close to chemical equilibrium. There is no evidence for such a reaction."

Chris Romanek, of the University of Georgia, was one of the Nasa scientists to originally postulate life on the Antarctic Martian meteorite.

He says formation temperature, and the possibilities of terrestrial contamination, are the questions which need to be addressed in the short term. "Personally I think it's going to be a low temperature deposit," he said.

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