The squiggly little forms in martian meteorite ALH48001 have been flashed around the world as evidence of extraterrestrial life. But in this week's Nature, a group of three meteoriticists argues that there's nothing lifelike about these martian "bugs." Rather, they're simply a trick of the eye abetted by the peculiarities of the powerful microscopes used to image them
In a short paper, John Bradley of MVA Inc., in Norcross, Georgia, Ralph Harvey of Case Western Reserve University in Cleveland, and Harry McSween of the University of Tennessee, Knoxville, present their own nanometer-scale images of the meteorite, taken with the same sophisticated field-emission scanning electron microscope technology as the originals. The team argues that most of the wormlike forms are nothing more than narrow ledges of mineral, where the natural layering of the minerals pyroxene and carbonate has been accentuated the way individual sheets stand out on the edge of a loose sheaf of papers. "What we are reporting," he says, "is a whole population of elongated forms that bear an uncanny resemblance to the proposed nanofossils. I can't see the difference."
The originators of the nanofossils from Mars idea, led by David McKay of NASA's Johnson Space Center in Houston, defend themselves in a response accompanying the Bradley paper. "We know that the [mineral structures] are there, but that's not what we're calling the martian 'bugs,'" says team member Kathie Thomas-Keprta of Lockheed Martin in Houston. They counter that most of Bradley's mineral structures are too small and too well-ordered to be mistaken for the wormlike forms.
The exchange leaves other researchers wondering what kind of evidence might end the debate. The problem, says interplanetary dust specialist Donald Brownlee of the University of Washington, Seattle, is that the shape of a structure is very weak evidence concerning the existence of life. If microscopic views of structures can't settle the issue, perhaps the McKay group's planned dissection of a microfossil will help. But the claim of life on Mars may have to stand or fall on other evidence. "It may not be possible to prove they are microfossils from Mars," says Brownlee.