The world's most famous meteorite is much more heavily contaminated with earthly substances than previously known, according to new research that shakes -- but does not eliminate -- the hypothesis that the rock may contain signs of ancient Martian organisms.
A whopping 80 percent of the potato-sized rock's organic materials -- that is, the various components that may be related to biological activity -- come from terrestrial contamination, most likely seepage from Antarctic ice meltwater where it was found, separate teams led by Timothy Jull, of the University of Arizona in Tucson, and Jeff Bada of the Scripps Institution of Oceanography, near San Diego, reported yesterday.
Scientists had determined earlier that the meteorite fell to Earth some 13,000 years ago. Comparison with samples of actual Martian atmosphere from the 1970s Viking craft led to the conclusion it must have been blasted from the surface of Mars, some 16 million years ago, and cruised the solar system for eons before landing here.
Both of the new research papers, which are published in today's issue of the journal Science, leave open the question of possible Martian biological activity. Each concludes that the rock contains a fraction of material that is of unknown origin and could be from Mars.
But chemist Richard Zare, of Stanford University, a member of the team that first reported the detection of possible life signs in the rock, said the work done by the Jull team in particular "really does cast doubt on the hypothesis, though it does not constitute a refutation."
Some contamination is inevitable in any meteorite that falls to Earth, he said. "But until I saw [the Jull results] I didn't appreciate this meteorite had so much contamination in it."
The unexpectedly complex rock, with its microscopic mysteries, has strained existing technologies to their limits in its 17 months of fame, triggering a flood of new questions about how life arises -- and how to recognize it when it does.
Though the rock has generated numerous papers, claims and counterclaims, Bada and Jull are the first to publish results from tests of the organic material contained in the meteorite, known as Allan Hills 84001 (or ALH84001, for short), since the controversy began.
In August 1996, the NASA/Stanford team made global headlines with the detection of what they described as several mineral features characteristic of biological activity, as well as possible microscopic fossils of primitive microbes, inside the meteorite.
At the very least, it seemed, they had found the first organic molecules of apparent Martian origin (in a form known as polycyclic aromatic hydrocarbons, or PAHs), molecules that could have been produced in either biological or nonbiological processes. Neither of the new papers analyzed the meteorite's PAHs, which constitute only about 1 percent of the organic material.
Among the suggestions made by the NASA team was that primitive bacteria-like organisms may have contributed to the formation of the carbonate material in the rock, through decay or other biological processes.
Jull's team used carbon isotope tracers to determine the origin of those carbonate minerals and also of the organic carbon in the meteorite. (A low carbon-14 content indicates no terrestrial contamination.) He concluded that the organic material contains carbon 14, but "the carbonate doesn't, because the carbonate came from somewhere in space, presumably Mars, and the organic material is a recent addition that took place while the meteorite was sitting on the ice. So there is no connection between the two things."
Further analysis, however, also indicated that "ALH84001 contains a small preterrestrial carbon component of unknown origin," the authors wrote.
In a different analysis that yielded similar results, Bada said his team at Scripps focused on amino acids because, unlike PAHs, they play an essential role in biochemistry. The team used a technique known as high-performance liquid chromatography to determine the "handedness" of the amino acids. He found the bulk of them consisted of the "left-handed" forms similar to that seen in the ice where the meteorite was found. "What we and Tim Jull's team have shown is that there is no evidence in our hands that the meteorite contains any compounds that we could definitely trace to Mars, except some tiny mysterious component that we don't understand at this point," Bada said.
David McKay, leader of the NASA/Stanford team that made the initial claims, and team member Everett Gibson, both of NASA's Johnson Space Center, said the new findings are "interesting" but they are standing by the thrust of their initial interpretation.
"We see that our original interpretation may have been oversimplified, in that we didn't consider all the episodes" of the rock's complex history, Gibson said.
They said the Jull and Bada teams had based their work on a gross analysis of minute quantities of bulk rock rather than a study of specific components from a specific location in the rock. This means it is not clear where the contamination is located and whether it has affected the extremely tiny features at issue deep in the rock, the scientists said.
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