Scientists involved in the now-suspect discovery of signs of fossilized life on Mars say they have found evidence of past life in a second martian meteorite. But the latest announcement, made this week at the annual Lunar and Planetary Science Conference in Houston, so far has left colleagues underwhelmed.
Using a powerful scanning electron microscope, geologist David McKay and a team at the Johnson Space Center found rounded or spherical "units" measuring 0.2 to 1 micrometer in a meteorite called Nakhla, discovered in Egypt in 1911. The units, they say, resemble small fossilized bacteria. McKay declined to be interviewed before his talk, given after Science went to press, but his online abstract says some groupings are reminiscent of dividing bacteria. It argues that their textured surface, the presence of lacy material resembling the mineralized biofilm produced by bacteria, and an occasional fibril-like filament also suggest "possible bacteria in Nakhla."
Researchers have heard all this before. McKay's group made similar "if it looks biological it's probably biological" arguments in 1996 for microfossils in ALH84001, notes microscopist John Bradley of MVA Inc. in Norcross, Georgia. But the group has since retracted those claims, conceding that some apparent microfossils were too small ever to have been alive and others were parts of the underlying rock.
"It's very hard to prove" such phenomena are biological, says meteoriticist Horton Newsom of the University of New Mexico, Albuquerque. To see them properly requires a sophisticated scanning electron microscope, but at that fine scale it is impossible to determine composition. So, he says, "they have a long row to hoe to convince people."
