Rather than settling the question of whether the meteorite once harboured microscopic life, the scientists have become embroiled in a bitter war of words, accusing each other of bias and intellectual blindness. ''It's more than just being polarised into two opposing camps,'' says Professor Bruce Jakosky of the University of Colorado. ''People are digging in their heels. The arguments are getting vitriolic. It's getting personal.''
The debate centres on the claims made by scientists from the American National Aeronautics and Space Administration (Nasa), in collaboration with researchers from Stanford University in California. At a press conference organised by Nasa, the scientists made the boldest claims yet in support of life on Mars. ''The evidence strongly suggests primitive life may have existed on Mars more than 3.6 billion years ago,'' they said. The fact that their statements had the imprimatur of Nasa - and effectively the US government - ensured that the press coverage went ballistic.
Buried in the maelstrom of words and interviews dating back to that August announcement, was a challenge from David McKay, a planetary scientist from Nasa's Johnson Space Center in Houston, Texas and leader of the research team. ''We are putting this evidence out to the scientific community for other investigators to verify, enhance, attack - disprove if they can - as part of the scientific process,'' Dr McKay had said. ''Then, within a year or two, we hope to resolve the question one way or the other. What we have found to be the most reasonable interpretation is of such radical nature that it will only be accepted or rejected after other groups either confirm our findings or overturn them.''
That two-year deadline is now up, and scientists seem to be further apart than ever on a meteorite called ALH84001. Dr McKay said this week that the critics have been unable to prove anything that undermines the original contention that the potato-sized lump of Martian rock bears the signature of extra-terrestrial life. ''We feel a lot of the criticism is unfair or just flat wrong. There's a lot of resentment against the publicity we have had. Some of our critics are jealous because they want their own publicity. Some of our critics are simply just resentful.''
In the opposing camp, however, things look very different. ''The weight of evidence has been against McKay's interpretation of ALH8001. Many different lines of research are saying they are wrong,'' said Ed Scott, a planetary scientist from the Hawaii Institute of Geophysics and Planetology. Scott seems far from alone in his views. ''If you were to take a vote on the possibility of lifeforms in the meteorite,'' said Professor Jeffrey Bada, a leading expert on extra-terrestrial chemistry at the Scripps Institution of Oceanography, in La Jolla, California, ''it would be overwhelmingly on the negative side. Everything I have seen points to problems with the original work.''
It is almost impossible to document every claim and counterclaim made about the Martian meteorite over the past two years. Each piece of the original argument put forward by McKay's team has been exhaustively scrutinised. From the very start McKay had emphasised that each piece of evidence on its own does not amount to very much, but taken together, the picture of Martian life emerges convincingly from the jigsaw puzzle.
But even this logic has been criticised. ''After all, if A, B, C and D are not conclusive evidence for life, why would A plus B plus C plus D equal life?'' says Professor Bada.
McKay and his team highlighted several key findings in support of the existence of fossilised microbes on the ALH80001 meteorite. They said they had found tiny globules of carbonate chemicals, which could have been deposited by living organisms. They also discovered organic compounds known as polycyclic aromatic hydrocarbons (PAHs) in the vicinity of the carbonate deposits, which again indicated a biological origin. McKay also reported finding tiny crystals of a substance called magnetite, which looked surprisingly similar in shape and size to magnetite crystals created by terrestrial bacteria.
But it was the fourth line of evidence that generated the greatest stir. The Nasa team showed highly magnified images of rod-shaped structures, some with recognisable segments. Could these be the Martian lifeforms themselves? One problem was that they were extremely small and some eminent microbiologists said they were just not big enough to hold the basic biological machinery necessary for life. Other scientists said they were effectively an optical illusion and had nothing to do with biology.
The other bits of the jigsaw were also attacked. Ed Scott says the carbonates could only have been formed under very high temperatures - too high to have a biological origin. Jeffrey Bada said the PAHs had resulted from terrestrial contamination rather than coming from Mars, and John Kerridge, a chemist at the University of California, San Diego, dismissed the magnetite evidence because these crystals are so ubiquitous that they were almost certainly not deposited by living organisms.
Each line of criticism has itself generated its own criticism with the result that all but a few have been able to follow the arcane nature of the dispute. Dozens if not hundreds of research papers have been published detailing every turn in the debate with each claim followed by a riposte. Yet the Nasa and Stanford scientists have held their ground. The rock ALH84001, they insist, bears the best evidence to date that there is, or once was, extra-terrestrial life.
Debates between the two opposing camps have taken place at several scientific conferences this summer with little or no consensus being reached. ''Unfortunately this has polarised the scientific community and this is not good for the public perception of the life on Mars debate. The original authors have gone out of their way to criticise their critics instead of producing other new evidence to back up their claims,'' Professor Bada says. ''I feel the sides are moving further apart rather than trying to resolve the issue. They simply have to start considering that their original claims were very inconclusive.''
Richard Zare, professor of natural science at Stanford University and a member of the McKay team, said the original arguments for life on Mars were only a hypothesis. ''This hypothesis remains unproved and untested,'' he said. If there has been any public misunderstanding then the news media, rather than the scientists, are to blame. ''At first, the news media were probably too believing in our hypothesis. Many ignored the caveats that we voiced repeatedly. In the last few months, the pendulum has swung and the coverage has become overly sceptical. If the public is confused, the news media are more responsible for that confusion than the scientists who are trying to understand this question,'' Professor Zare said.
Nevertheless, it is probably fair to say that there are now more sceptics about the ALH80001 question among the scientific community than there were two years ago. The McKay group has promised fresh research in the coming months to bolster their increasingly isolated position. Until they do, the conclusion of many scientists will be that, although they accept the possibility of life on Mars, the ALH84001 meteorite has failed to answer the key question - are we alone in the universe?
THE METEORITE was the first to be found in the Alan Hills area of Antarctica in 1984 - which accounts for its code name. It weighs 4.2 pounds and is estimated to be 4.5 billion years old. About 16 million years ago a huge comet struck Mars and ejected the rock into space, where if floated for millions of years. It eventually fell to Earth about 13,000 years ago where it rested on an ice sheet until its discovery in 1984. It is now the most studied lump of rock in history.