"Our team stands stronger today than when we wrote the manuscript three years ago," said Everett Gibson, a geochemist at NASA's Johnson Space Center who co-authored, with David McKay, the 1996 paper that shocked the world with the implication life may be quite common in the universe.
Gibson said Thursday that further examination of the Martian Antarctic meteorite and studies on two other Martian meteorites show minerals that could only have been produced by living organisms. He told an audience at the International Mars Society conference that despite three years of criticism and attack from the scientific community, none of the data from the 1996 study had been weakened.
Gibson's and McKay's findings have been questioned by much of the scientific community. Some have said the organic material in the meteorite came from Earth. These scientists have shown that the meteorite was "contaminated" with terrestrial material during the thousands of years it sat in the Antarctic ice sheet. Others contend that the minerals Gibson and his co-workers call the remains of fossil bacteria were actually created by non-biological processes.
One of the scientists who expressed both criticisms is Timothy Jull, a physicist at the University of Arizona who specializes in carbon dating. Jull analyzed a sample of the Antarctic meteorite in 1997, and more recently looked at another Martian meteorite named Nakhla. Gibson said yesterday that some of the new evidence for life was based on analysis of Nakhla, which landed in Egypt in 1911.
Comparing the atomic signatures of carbon in the rocks to the signatures of rocks from Earth, Jull found much of the organic material in both meteorites to be of earthly origin, he said. The meteorites did contain some carbon molecules that were clearly extraterrestrial, he said. But those molecules don't prove life existed on Mars, he said.
"I don't think they require the intervention of bacteria to create them," Jull said. "It is not a biotic form of carbon. It is a molecule that can be created by chemical reactions."
Gibson agreed that not all carbon molecules require life to produce. He said much of the criticism of his team's work has come from attacks on individual lines of evidence, when his team has actually pointed to several peculiar formations in the meteorites, that together are best explained as the product of minute Martian organisms.
He emphasized the conclusion of the 1996 paper. "None of these observations is in itself conclusive for the existence of past life," he said, reading a line from the published article. "Although there are alternative explanations for each of these phenomena taken individually, when considered collectively, . . . we conclude that they are evidence for primitive life on Mars."
Skeptics, though, find problems with each of the phenomena Gibson and McKay catalogued. Allan Treiman, senior staff scientist at the Lunar and Planetary Institute is a skeptic. "The bugs in that research are pretty bad," Treiman said.
Most of the so-called biotic minerals are not from biological processes at all, he said. "Some are clearly little mineral grains, others are artifacts from the surface preparation of the samples, they're artifacts that come from the laboratory, not from Mars."
Still, even Treiman agrees there is some good evidence for life on Mars. The best, both Treiman and Gibson agree, does not concern an organic compound, but a pure, crystalline form of iron oxide called magnetite.
Some bacteria on Earth produce microscopic crystals of magnetite to help them determine direction. The tiny iron oxide crystals are affected by Earth's magnetic field, and microorganisms use them to tell up from down, scientists believe.
Gibson and McKay have taken electron micrograph images of sections of the Martian meteorites that show tiny strings and crowds of magnetite crystals. The crystals, which are embedded in globules of Martian minerals, are about 1000 times smaller than the diameter of a human hair. They often line up in chains that look precisely like those produced by bacteria on Earth.
These chains impress Treiman.
"These crystals are dead ringers for some of the magnetites found on earth produced by bacteria," Treiman said. "They are the same size, same shapes, same chemical composition. What I don't understand, though, is what magnet-seeking bacteria are doing deep in rock that is already highly magnetized."
Magnetite-producing bacteria generally live in fine-grained muddy sediment, not inside rock, Treiman said. "The fact that I can't understand how these things happened though, doesn't mean they didnt" Treiman acknowledged. There is still a good deal of debate about whether these crystals were produced on Mars, or whether the structures require living organisms to produce them.
Gibson and his colleagues have submitted for publication some of the new findings. Gibson calls the new work even more exciting and conclusive than the 1996 announcement, which he said still stands firm.
"Not a single one of our data points has been refuted," Gibson said. "Scientists have had different interpretations of that data, that's the way the scientific process works."
Gibson said he expects more skepticism this time, but said several opponents are slowly becoming convinced that Mars may have once been, and may still be, home to primitive bacteria.
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