'Conclusive evidence' for Martian life

ALH84001 was not recognised as Martian at first

By BBC News Online's Ivan Noble

Scientists have published what they claim is "conclusive evidence" that bacteria once lived on Mars.

The study is a follow-up to the famous 1996 research that purported to show ancient bacterial forms in a meteorite from Mars. Critics at the time said the evidence was too thin to prove anything.

Now, a Nasa-backed team from the US, Spain and Germany has published further work which it says strengthens the earlier claims.

It's conclusive evidence that Martian bacteria were in this meteorite

Dr Imre Friedmann
Nasa scientist

But some British experts, at least, are still sceptical, arguing the latest study falls short of absolute proof.

The argument centres on tiny crystals found in a potato-sized meteorite picked up 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, the team says.

Signature of life

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.

Robby Score found the meteorite in Antarctica

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 it 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 found the missing sixth element.

Crystal chain clue

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 Center says his team 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 says.

Mission to Mars

Professor Colin Pillinger, lead scientist on 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.

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

Prof Colin Pillinger, Open University

"The bigger the claim, the more perfect the evidence has to be," he told BBC News Online. "This doesn't actually prove that the evidence in the meteorite is for life on Mars.

"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. 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."

Rock origins

One major objection to the claims has always been that the magnetite crystals could have come from bacteria which infiltrated the rock after it landed on Earth.

But Dr Friedmann says 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 analysed 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 realised it was a Martian meteorite until 1993. There 15 meteorites now known to be of Martian origin.