9 June 1998

Is The Martian Meteorite A Dud?

When research teams at NASA's Johnson Space Centre and Stanford University announced in August 1996 that meteorite ALH84001 contained brown-coloured carbonates resembling the earliest microfossils on Earth, the possibility of past life on Mars did more to sell clip-on "antennae" than all the B-movies ever made. The rock may not have contained the footprints of little green men, but the "signature" of life was written all over it. Which is why chemists from the University of California, San Diego, risk being called the year's biggest wet blankets when they say that all that fuss was but a case of mistaken identity.

Reporting their findings in the current issue of Science, the researchers refute the idea that grains of carbonate minerals believed to signal past life on Mars are biologic in origin. Instead, they say, the potato-sized rock is laced with carbonates that resulted from reactions with atmospheric carbon dioxide.

"This data suggests that the carbonates were made by the interaction with the atmosphere rather than with the water on the surface, as would be required for a biologic process", says Mark Thiemens, professor of chemistry and biochemistry at UCSD and principal investigator of the study.

In the first ever multi-oxygen isotopic examination of carbonate globules in the meteorite, Thiemens and his team examined "isotopic signatures" in much the same way a detectives scrutinizes fingerprints. If the signatures pointed toward a water origin, the case for life would be strengthened, since the genesis for all life in the Universe is thought to be water.

"So if these things were biogenic, they should have equilibrated with water," says Thiemens. "They didn't. They equilibrated with the atmosphere." Party poopers.