Martian meteorite ALH 84001--the one that researchers claimed in 1996 held evidence of past life on Mars--is back. This time researchers report that magnetite grains in the meteorite match those created by an Earth-bound bacterium, adding a new log to the fire over whether ALH 84001 once contained life.

The original argument for fossils in ALH 84001 featured several lines of evidence, such as the structure of carbonate globules and associated organic hydrocarbons. Skeptics chiseled away until the strongest piece left standing was that magnetite in the rock might have been produced by bacteria. But, as planetary scientist Kathie Thomas-Keprta of NASA's Johnson Space Center in Houston admits, "We really didn't go into a lot of detail in 1996. It was highly speculative." Now, Thomas-Keprta and colleagues have returned with the details.

The group extracted magnetite crystals from the meteor's carbonate globules and photographed them from many angles using transmission electron microscopy. The group established six criteria, such as chemical purity and crystal perfection, to judge whether bacteria had created the crystals. One type, shaped like an elongated prism, constituted 27% of all the magnetite, and it satisfied five of the six criteria (they were unable to test the sixth). Furthermore, these crystals appeared identical to ones from a bacterial strain called MV-1, the researchers report in the 1 December issue of Geochimica et Cosmochimica Acta. No inorganic process is known to create such magnetite crystals, and any such particles found on Earth, the authors say, would be readily classified as originating from bacteria that use magnetite to orient themselves.

The work is being greeted warmly but with skepticism. "I welcome this paper," says Richard Frankel of California Polytechnic State University in San Luis Obispo. "But I don't think this settles the issue of whether there was life on Mars." Critics believe the crystals could have been produced inorganically, saying we understand little about inorganically produced magnetite grains of such miniscule size. And many observers won't be satisfied until high-quality fossils are brought back from the Red Planet.

--JAY WITHGOTT

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