By Charles Seife in Washington DC THE NASA team that stunned the world in 1996 by announcing that it had found evidence of life on Mars now claims to have discovered fossilised bacteria in a second Martian meteorite. But many scientists are as sceptical of the new evidence as they are of the original claim. The team’s latest work is on the Nakhla meteorite, which exploded in the sky over Egypt on 28 June 1911. Forty Martian fragments fell to Earth, one allegedly killing a dog—though this has never been verified. Collectors gathered the shards, and a well-preserved specimen is now in the Natural History Museum in London. As with ALH84001, the meteorite at the centre of the earlier claims, the chemical composition of the Nakhla meteorite indicates that it was blasted off the surface of Mars. Next week, at the Lunar and Planetary Conference in Houston, researchers led by David McKay of NASA’s Johnson Space Center, also in Houston, will announce that the Nakhla meteorite contains tiny structures that could be fossilised microorganisms. “Some are spheroidal, some are sausage-looking,” says team member Kathie Thomas-Keprta. The structures are much larger than those found in the ALH84001 meteorite. At 0.2 to 1 micrometres long, they fall within the size range of Earth bacteria. Some have thin tendril-like structures that resemble bacterial flagellae. The largest of the structures found in ALH84001 was 0.2 micrometres long which led some microbiologists to argue that they couldn’t be fossilised bacteria. Thomas-Keprta, who is also based at the Johnson Space Center, notes that the structures are found in widely separated clusters, just like bacterial colonies. “If they were chemical precipitates, you’d expect to find them all over,” she says. But other experts are sceptical. “Their biggest problem is that the meteorite sat around on Earth for nearly 100 years,” says Ralph Harvey, a meteorite specialist at Case Western Reserve University in Cleveland, Ohio, and one of the main critics of the team’s previous work. “It will be very hard to prove that it’s not contaminated.” Thomas-Keprta concedes that this is likely to be a problem. “To distinguish between Martian and terrestrial contamination is going to be difficult,” she admits. “There is work left to do.” John Bradley of MVA, a company in Norcross, Georgia, specialising in microscopic analysis, says it will take much stronger evidence than a superficial similarity between structures in Martian meteorites and living bacteria to persuade the sceptics. “Since neither I nor Ralph have looked at Nakhla before, all options are open,” he says. “But it appears they’re using the `If it looks like a bug it must be a bug’ hypothesis.” After the ALH84001 announcement, there was a flurry of activity as other researchers sought to test McKay’s claims. Harvey, Bradley and others have since come up with an inorganic process to explain the rock’s chemistry. They have also argued that the microscopic structures formed at temperatures too high to support life. But researchers are unlikely to go to the same lengths again. “You can’t spend the rest of your life disproving others’ work,” says Bradley. “How much of our time is it worth?