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Latest research casts new doubt on evidence for fossil life in Martian meteorite
New analyses of the famous Martian meteorite, ALH84001, have cast additional doubt on the likelihood that it contains the fossilized remains of ancient Martian microbes.
Two studies published last week find that much of the organic material in the meteorite appears to be terrestrial, rather than extraterrestrial, in origin. Richard Zare, the Marguerite Blake Wilbur Professor of Chemistry at Stanford who headed the team that discovered organic material of possible Martian origin in the potato-sized rock, says that the new findings do not directly refute the original research. One of the analyses, however, does suggest that the meteorite contains considerably more terrestrial contamination than he had thought, Zare acknowledges.
ALH84001 was thrust into the limelight in August 1996 when a team of scientists published a controversial analysis in the journal Science. They argued that they had discovered organic material, unusual mineralogical features and electron microscope images showing tiny oval and worm-shaped features that, when taken together, provided compelling circumstantial evidence that the meteorite was inhabited by Martian microorganisms more than three billion years ago.
Scientists at NASA's Johnson Space Center in Houston provided the electron microscope images of the putative nanofossils. Researchers from the University of Georgia and McGill University contributed the mineralogical evidence. Zare's research group produced data showing that the meteorite contained a family of organic compounds called polycyclic aromatic hydrocarbons (PAHs) that could have been produced by the decomposition of alien microorganisms.
In the 17 months since the research was announced, other scientists have published dozens of independent analyses that have both supported and attacked the Martian microbe hypothesis. In the last month, however, the weight of new research appears to be stacking up against the pro-life position.
In December, John Bradley of MVA Inc. and Ralph Harvey of Case Western Reserve University published a paper in the journal Nature that attacked the NASA group's interpretation that the oval and worm-like shapes that it reported could be the fossils of microorganisms. Duplicating the NASA researchers' methods, Bradley and Harvey reported that all the shapes that they could find in the meteorite are non-biological in nature and consist of the fractured surfaces of common crystals. In the same issue of the journal, the NASA team strongly contested this interpretation.
The two analyses of the organic material in the meteorite appeared in the Jan. 16 issue of the journal Science.
A team from Scripps Institution of Oceanography, headed by Jeffrey Bada, analyzed the meteorite for amino acids, the building blocks of life. They found amino acids present in very low concentrations (between 7 parts per million and 100 parts per billion). "What we found was that, yes, there are amino acids in the meteorite at very low levels, but they are clearly terrestrial and they look similar to amino acids we see in the surrounding Antarctic ice," Bada said in a Scripps news release. (The meteorite spent an estimated 13,000 years in the Antarctic ice before it was discovered.)
Bada based his conclusion that the amino acids were due to terrestrial contamination on the results of an analytic technique called liquid chromatography. The method determines the "handedness" of the amino acids. Terrestrial organisms produce only left-handed amino acids, where non-biological processes produce a mixture of left- and right-handed molecules. Bada found that the amino acids in the meteorite were left-handed and so concludes that they must be terrestrial. There is at least a 50 percent chance, however, that Martian life (if it exists) would also favor left-handed molecules. So the experiment is by no means conclusive, Zare said.
Even if the amino acids in the meteorite come from terrestrial contamination, Zare says this does not prove that the PAHs which his group found are also terrestrial in origin. "Amino acids are soluble in water. So water provides a mechanism for carrying them into the interior of the meteorite. But PAHs are highly insoluble and I don't know of any mechanism that would transport them into the rock's interior where we found them," he said.
Zare finds the second analysis, performed by a University of Arizona research team headed by A. J. Timothy Jull, much more interesting and compelling. "It is state-of-the-art and an extremely valuable study of the degree of contamination in the meteorite," he said.
Jull's group burned samples of the meteorite at two different temperatures to separate the organic carbon from the carbon contained in inorganic minerals, which burn off at higher temperatures. They then analyzed the isotopic ratios of the carbon from the two sources.
In previous work, Jull had determined that the carbonates in ALH84001 are substantially enriched in the isotope carbon-13 compared to those on Earth. He and his colleagues interpret this as an indication that the carbon dioxide in the early Martian atmosphere was also enriched in carbon-13. If that is the case, then the tissue of Martian organisms would also have elevated levels of carbon-13. When the team analyzed the ratio of carbon isotopes in the organic carbon, however, it found that fully four-fifths of the material had the same isotopic signature as terrestrial carbon. The other 20 percent appears to have a preterrestrial origin, they found.
"It looks like regular terrestrial organic material, with the exception of one small component in ALH84001," Jull said in a University of Arizona news release.
The analysis "indicates a much greater degree of terrestrial contamination in the meteorite than I suspected was present two years ago," Zare said. "In that sense, Jull's study does cast new doubt on our hypothesis that the meteorite contains evidence of past Martian life."
On the other hand, the Stanford chemist does not believe that the study completely rules out an extraterrestrial origin for the PAHs. "Jull's work is for the whole rock. As in real estate, location is everything. His study does not give any indication of the locations from which these different carbon isotope fractions are coming. So I cannot tell where the PAHs, which are concentrated around carbonate spheroids in the meteorite's interior, fall in the terrestrial or preterrestrial fraction."
The saga of the provocative rock is far from over. Last summer NASA and the National Science Foundation awarded grants for 23 new investigations of AHL84001 as part of a coordinated program designed to determine whether it contains traces of alien life. These studies will be producing results in the next two to three years.
Although it may be decades before the significance of the meteorite is determined conclusively, Zare sees several beneficial effects that are independent of the debate's ultimate outcome. These include a revitalization of research on meteorites, increased efforts to extend the boundaries of the scientific ability to measure trace quantities of chemical compounds in materials, and its illustration of the critical importance of multidisciplinary research.
Most important, he says, it has given a major new impetus to research that addresses the closely related questions of "How did life begin on Earth?" and "Is there life beyond Earth?" A concrete example of this is NASA's decision to found a new $7 million to $10 million-per-year Astrobiology Institute specifically for this purpose. Zare is serving as chair of the search committee for the institute's first director.
By David F. Salisbury