the possibility of life on Mars has been given a jolt by two new analyses, one published in Geochimica et Cosmochimica Acta and the other accepted for publication by the same journal. This could well end claims made by nasa scientist David McKay and his team in August 1996, of the existence of Martian life. Evidence to Martian life centres around the meteorite alh 84001, which chipped off the surface of Mars some 15 million years ago and landed in the Allan Hills region of Antarctica about 13,000 years ago.
Inside the fissures in the meteorite are carbonate globules containing tiny tubular structures that look like fossilised bacteria, crystals of magnetite and iron sulphide that are similar to those produced by some terrestrial bacteria, and polycyclic aromatic hydrocarbons (pahs) -- oily organic molecules often formed during the decomposition of living organisms.
"The biological explanation is becoming less and less plausible," says John Kerridge, a planetary scientist at the University of California at San Diego, who is familiar with both the studies.
In one paper, the magnetite particles in alh 84001 are analysed by geoscientists Ralph Harvey of Case Western Reserve University in Cleveland, Ohio, and Harry McSween of the University of Tennessee in Knoxville, both working with John Bradley of mva in Norcross, Georgia, a company that specialises in microscopic analysis. After cutting a thin section from one of the carbon globules of the meteorite, they used a jet of argon ions to erode the slice until it was just 50 nanometres thick -- a process known as ion milling -- and examined it under an electron microscope.
Wherever the section was sliced through a magnetite particle lengthwise, the researchers could see a dark line running up its centre. "These things grew like a tightly wound spiral staircase, and (this line is) the axis around which the staircase winds," says McSween.
This crystal growth pattern, known as an axial screw dislocation, is rare in terrestrial magnetites and is totally unknown in those produced by living organisms, says McSween. These organisms are found only at fumaroles -- volcanic vents that release hot gases at temperatures between 500c to 800c. The gases condense to form long 'whiskers' that grow in the shape of a staircase. This finding ties in with McSween's earlier chemical analysis, which argued that the carbonate globules must have formed at more than 450c.
"I know of no biogenic mechanism for growing whiskers, and I don't know of any biogenic magnetite that contains screw dislocations," agrees Peter Buseck, a geochemist at Arizona State University in Tempe. But he concedes that the possibility cannot be completely ruled out.
In the second study, Luann Becker and her colleagues at the University of California report that the pahs in the meteorites could be contaminants from the Antarctic ice. All the pahs that McKay's team analysed in the Martian meteorite were discovered in the Antarctic samples analysed by Becker under a mass spectrometer. "This meteorite sat in the ice for at least 12,000 years. That gives you plenty of time to accumulate pahs," says Becker.
McKay's team insists that biological activity is the best explanation for the pahs, magnetite, iron sulphide and carbonates being found together. Hojatolla Vali, a member of McKay's team says, "You have to explain all those things together, and as far as I know the new studies don't address this. As long as they don't do so, I will go for the interpretation that it might be biological."