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Bacterial Muck on Mars?

HOUSTON--The authors of the life-on-Mars paper that rocked the world last summer (Science, 16 August 1996, p. 924) say they have found further evidence of past life in the famed meteorite: residue from bacterial secretions. The findings, reported here yesterday at the annual Lunar and Planetary Science Conference, were greeted with skepticism from other scientists at the meeting, suggesting that the debate over martian life isn't likely to end soon.

Geochemist Carlton Allen of Lockheed Martin Engineering and Sciences in Houston announced that his group has identified in meteorite ALH84001 the remains of possible "biofilms"--thin layers of carbon that might have been secreted by some tiny long-gone Martian bacteria. Biofilms are aggregates of bacteria attached to a surface and surrounded by a protective coat of mucus or slime, and they are commonplace on Earth; the best known, perhaps, is dental plaque.

Allen reported that the team--headed by David McKay of NASA's Johnson Space Center in Houston--had further analyzed carbonate globules similar to those that yielded the first putative traces of past life. The researchers lightly etched the globules' surfaces with acid to remove the overlying carbonate. Underneath, they found layers of filmy, flaky material, sometimes with a honeycombed structure, that they surmise are the remains of biofilms. These layers did not occur only around the tiny wormlike features that the team has suggested could be the remnants of ancient bacteria. Rather, said Allen, "we find biofilms all over the place" in the globules.

Experts are taking the preliminary analysis with a grain of salt. McKay and colleagues "have no evidence there are organisms in those features," says David Des Marais, a geochemist at NASA's Ames Research Center in Mountain View, California. The NASA team and others are continuing to analyze the meteorite to gather more clues and to identify possible organic matter.