BBC News Online, Monday, February 26, 2001
Scientists have published what they claim is conclusive evidence that bacteria once lived on Mars.
In 1996 the American space agency Nasa announced the discovery of remnants of ancient bacteria in a meteorite from Mars, but critics at the time said the evidence was too thin. Now a Nasa-backed team from the US, Spain and Germany has published new work strengthening the case.
But some British experts are skeptical, saying the study falls short of absolute proof.
The argument centres on tiny crystals found in a potato-sized meteorite discovered in Antarctica in 1984. The international team says the crystals are the same as those deposited by earthly microorganisms known as magnetotactic bacteria. Since the meteorite is from Mars, it can only be assumed that the same bacteria must have lived on Mars as well, they say.
The crystals in question are made of magnetite, a compound of iron and oxygen. The team's aim was to show that they must have been deposited by living organisms, rather than a simpler chemical process. Magnetite crystals deposited by bacteria on Earth show six key features that distinguish them from crystals of non-biological origin. This biosignature is what researchers have been looking for. Last year another group of researchers announced that they had found some, but not all, of the six key features.
Now, in a paper published in the journal Proceedings of the National Academy of Sciences (PNAS), Imre Friedmann, Jacek Wierzchos, Carmen Ascaso and Michael Winklhofer say they have now found the missing sixth element. The final clue was the arrangement of the crystals in the meteorite. Crystals left behind by bacteria on Earth form characteristic chains.
Dr Imre Friedmann of Nasa's Ames Research Centre says they used a new method of electron microscopy to show that the Martian crystals are in chains, too.
"These crystals match all the criteria for biologically-formed chains, and as far as I'm concerned it's conclusive evidence that Martian bacteria were in this meteorite. I cannot guess how my colleagues will react, but in my opinion no reasonable person can doubt it any more. The evidence is so strong. When you put all the elements together there can be no other explanation," Dr Friedmann told the Press Association news agency.
Professor Colin Pillinger, lead scientist in the British-led Beagle 2 effort to land on Mars in 2003, describes the NASA-backed study as very interesting, but says it falls short of absolute proof.
"The bigger the claim, the more perfect the evidence has to be. This doesn't actually prove that the evidence in the meteorite is for life on Mars," he told BBC News Online. "We cannot say absolutely hand on heart that this is something which happened on Mars until we find organic matter in a genuine Martian sample," he said. "We have to go to Mars and if there is doubt we will have to bring samples back. If there is still doubt we will have to send a person there to carry out the experiments in situ. "Our Mars mission experiments are designed to test for organic matter and isotopic fractionation - the best measure I know to detect organic life," he added.
One major objection to NASA's claim has been that the magnetite crystals could have come from bacteria which infiltrated the rock after it landed on Earth. Dr Friedmann says that the crystals are undoubtedly from Mars, because they are inside other globules of rock whose origin is uncontested. The Viking missions to Mars in the 1970s analyzed Martian rock and the Martian atmosphere. These measurements allow scientists to work out whether a meteorite bears the chemical signature of something formed on Mars. The meteorite in this study - known as ALH84001 - was found in Antarctica in 1984 but no-one realized it was a Martian meteorite until 1993.
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Filel Date: 3.10.01