HOUSTON, TEXAS Following years of rigorous study, the inside story of whether meteorite ALH 84001 the so-called "Mars rock" harbors evidence for past Martian biology remains steeped in debate.

It was a NASA-led research team that announced in August 1996 that the potato-shaped meteorite found in Antarctica might sport fossilized bacteria. They argued that "lines of evidence" pointed to the likelihood that a primitive form of microscopic life that flourished on the red planet three billion years ago had been found.

Now, fast-forward from 1996 to five-and-a-half years later.

It turns out that rock-solid evidence is hard to come by.

"We have concluded theres no way to transport the magnetite into the Martian carbonates after it fell on Earth," said David McKay, a senior scientist at NASAs Johnson Space Center. He led the team that first claimed ALH 84001 had a special story to tell.

McKay said that no plausible scenario could be found that allows any of these features to be formed in the Antarctic. "We conclude that we cannot justify the hypothesis that any of the biogenic magnetite is terrestrial contamination," he said.

Kathie Thomas-Keprta, a research scientist for Lockheed Martin, is also part of the science team who first brought ALH 84001 to world attention.

Thomas-Keprta began her talk with a power point quote from Mark Twain: "The scientist will never show any kindness for a theory which he did not start himself" Her overall message conveyed a "this is my story and Im sticking to it" attitude.

"No single process, either inorganic or biogenic, can explain the full distribution of magnetites observed in ALH 84001 carbonates. We propose that the origins of magnetites in ALH 84001 can best be explained as the product of multiple processes," Thomas-Keprta reported.

"This is a dangerous thing to say in this audiencewe continue to stand by our hypothesis," Thomas-Keprta said, reemphasizing her belief that ALH 84001 is rife with biogenic proof of Martian organisms.

But for skeptics, ALH 84001 as a showboat of Mars life is a dead issue.

"I think its over," said Harry McSween, noted space geologist in the department of geological sciences at the University of Tennessee in Knoxville.

"Each of the lines of evidence have been systematically dissected. The only thing that was left was the magnetites. There are just too many inorganic ways to make the magnetites nowways that look just like the magnetites in the meteorite," McSween said.

"Id love for this life hypothesis to be right. It may well be right for Mars. I just dont think its right for this meteorite," McSween told SPACE.com.

McSween said that the years of debate have been beneficial. Identifying what kinds of signatures of life are real and can be depended upon is crucial, particularly when spacecraft missions return the first Mars samples, or in trying to make on-the-spot judgements via instruments on the planet.

The Mars rock saga has shown how tricky it is to figure out biosignatures, McSween said. "A lot of people, including the McKay team, have done some exquisite work. This is going to be really useful to the community. This is all valuable stuff," he said.

ALH 84001 is a very complex rock, loaded with multiple sets of data, said Everett Gibson, senior scientist and co-leader of the Mars research team at the NASA Johnson Space Center.

Gibson was also part of the original team that linked the Mars rock with possible Martian microfossils.

"We think theres a small percentage of the processes recorded in the rock twenty-five percent that may represent the end product of biological processes. So you have to sort out what the vast majority of the signatures are, which are the inorganic, normal production of a rock and other processes on Mars. Biology is a small ingredient in the life of a planet. And in the case of Mars, its only a small component, yet it leaves trace signatures," Gibson said.

Gibson said those who see meteorite ALH 84001 as holding compelling evidence for Mars life remain on firm footing.

"This is an argument thats difficult for the person on the street to sort out and understand the subtleties. This is the way science works. Any radical idea in science is not accepted quickly," Gibson said.

Egos of those studying the Mars rock are getting bruised and scientific punches and counter punches are being thrown. And still the debate continues. In terms of being better prepared for handling Mars return samples in the future, its a "win-win situation for science," Gibson said.

But at the end of the conference day, following another rally of research papers the detective story that is ALH 84001 remains incomplete.

"The evidence for its biological nature is still there. Its strong. It hasnt been refuted. This hypothesis has been out for five-and-a-half years. Its still standing," Gibson said.