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鈥檚 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鈥攖hough 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鈥檚 Johnson Space Center, also in Houston, will announce
that the Nakhla meteorite contains tiny structures that could be fossilised
microorganisms. 鈥淪ome are spheroidal, some are sausage-looking,鈥� says team
member Kathie Thomas-Keprta.
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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鈥檛 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.
鈥淚f they were chemical precipitates, you鈥檇 expect to find them all over,鈥� she
says.
But other experts are sceptical. 鈥淭heir biggest problem is that the meteorite
sat around on Earth for nearly 100 years,鈥� says Ralph Harvey, a meteorite
specialist at Case Western Reserve University in Cleveland, Ohio, and one of the
main critics of the team鈥檚 previous work. 鈥淚t will be very hard to prove that
it鈥檚 not contaminated.鈥�
Thomas-Keprta concedes that this is likely to be a problem. 鈥淭o distinguish
between Martian and terrestrial contamination is going to be difficult,鈥� she
admits. 鈥淭here is work left to do.鈥�
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. 鈥淪ince neither I nor Ralph have looked at
Nakhla before, all options are open,鈥� he says. 鈥淏ut it appears they鈥檙e using the
`If it looks like a bug it must be a bug鈥� hypothesis.鈥�
After the ALH84001 announcement, there was a flurry of activity as other
researchers sought to test McKay鈥檚 claims. Harvey, Bradley and others have since
come up with an inorganic process to explain the rock鈥檚 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. 鈥淵ou can鈥檛
spend the rest of your life disproving others鈥� work,鈥� says Bradley. 鈥淗ow much of
our time is it worth?鈥�