Five years ago, NASA scientists announced a remarkable discovery. A potato-size meteorite from Mars known as ALH84001, they said, contained evidence that the Red Planet once harbored primitive life forms. Not surprisingly, the assertion ignited a firestorm of controversy. Since then, three of the four original lines of evidence for ancient Martian life have been dismissed. Now new research may put the final nail in the coffin. According to a report published in the current issue of the Proceedings of the National Academy of Sciences, tiny magnetite crystals in the meteorite that were claimed to mirror those produced by certain terrestrial bacteria in fact exhibit no such unique resemblance.

Using an improved transmission electron microscope technology not available when the original analysis of the famed Martian meteorite was first conducted, Peter R. Buseck of Arizona State University and colleagues re-examined the three-dimensional shapes of the known bacterial nanocrystals. The morphologies they observed differed considerably from those described in the original report. Furthermore, the crystals of the various strains differed from one another, and none uniquely matched those reported from the meteorite. These findings call into question whether the forms of the meteoritic crystals are accurately known and suggest that any similarities between the meteoritic crystals and bacterial ones could simply be chance resemblances. In fact, the researchers write, "current knowledge about the magnetic crystals in ALH84001, when examined critically, is inadequate to support the proposed former existence of extraterrestrial life."

The next step, Buseck's team notes, is to subject the meteorite's magnetite crystals to the same rigorous measurement and reconstruction methods they describe. "Because it seems that the magnetite nanocrystals in ALH84001 are the only remaining, potentially definitive indicators of former life on Mars," the authors conclude, "such careful work is justified, and indeed, demanded."