LEAGUE CITY, Texas – NASA’s Mars Meteorite Research Team reopened a 14-year-old controversy on extraterrestrial life last week, reaffirming and offering support for its widely challenged assertion that a 4-billion-year-old meteorite that landed thousands of years ago on Antarctica shows evidence of microscopic life on Mars.

In addition to presenting research that they said disproved some of their critics, the scientists reported that additional Martian meteorites appear to house distinct and identifiable microbial fossils that point even more strongly to the existence of life.

“We feel more confident than ever that Mars probably once was, and maybe still is, home to life,” team leader David McKay said at a NASA-sponsored conference on astrobiology.

The researchers’ presentations were not met with any of the excited frenzy that greeted the original 1996 announcement about the meteorite — which led to a televised statement by President Bill Clinton in which he announced a “space summit,” the formation of a commission to examine its implications and the birth of a NASA-funded astrobiology program.

Fourteen years of relentless criticism have turned many scientists against the McKay results, and the Mars meteorite “discovery” has remained an unresolved and somewhat awkward issue. This has continued even though the team’s central finding — that Mars once had living creatures — has gained broad acceptance among the biologists, chemists, geologists, astronomers and other scientists who make up the astrobiology community.

Speaking at a four-day conference near NASA’s Johnson Space Center, McKay’s team didn’t claim it had definitive proof that the meteorites they are studying — which can be identified as Martian because the gases inside them match the Martian atmosphere — contain the remains of living organisms. Rather, the researchers described their re-energized confidence as emerging from a process of nitty-gritty science, based on inference, simulated testing and a kind of interplanetary forensics.

McKay cited years of work by team members Kathie Thomas-Keprta and Simon Clemett that he said rebuts a central critique of the meteorite’s significance. He also pointed to the presence of what appear to be fossilized microbes in other Martian meteorites, as well as the steady flow of discoveries by others pointing to a Mars that at one time could have supported life — wet, warmer and within an atmosphere and magnetic field.

The Thomas-Keprta work, published late last year in the journal Geochemica, centers on the origin of iron-based crystals called magnetites in the original Mars meteorite, called ALH84001. Magnetites on Earth are sometimes created by bacteria that respond to the planet’s magnetic field; the McKay team argued that some of the Martian magnetites were of this biologically created type.

Critics had said that the magnetites could have just as easily existed without bacteria or biology — that they sometimes form as a result of the shock and searing heat that could come, for instance, from an asteroid strike. But in the recent paper, Thomas-Keprta, an expert in the use of electron beam technology to look inside rocks, reported that the purity of the magnetites made that explanation impossible.

Reflecting both the contentiousness and drama of the debate, Thomas-Keprta finished her talk by referring to a recent article in a science journal that said the astrobiology community had “mostly abandoned” the biological explanations for the makeup of ALH84001. Her retort: “As Mark Twain put it, ‘Reports of our death have been greatly exaggerated.’ “

McKay said that not enough attention had been paid to work such as Thomas- Keprta’s.

“All the criticisms of our original paper got widely distributed, but when we did the work to prove the critics were wrong, it hardly made a ripple,” he said. “We’re now in a position to say we’ve knocked down all the criticisms — and our biological explanation is the one left standing.”

Mary Voytek, director of NASA’s astrobiology program, praised McKay and his team for their continued research into Mars meteorites, saying they have been crucial to the field.

She said, however, that the astrobiology community as a whole remained unconvinced of their findings, in part because “the bar is so high.”

Because the stakes involved with any announcement of possible or likely extraterrestrial life are so high – both for science and for the societal and religious implications of such a discovery — the issue brings out very strong feelings. At the conference, a leading cautionary voice in astrobiology proposed that a special protocol be established to oversee release of any journal articles making dramatic extraterrestrial claims.