A 13,000 year old meteorite from Mars, found in 1984 in the

Allan Hills Region of Antarctica, is back in the news. The rock caused quite a

stir when NASA announced during an August 1996 press conference that it

contained evidence of past life on Mars.

The first paper in Science described micrometer-sized

carbonate deposits, shaped like pancakes, along tiny cracks and crevices in the

meteorite, known as ALH84001. Researchers theorized that the carbonates were

deposited from carbon dioxide-saturated fluids that were no more than 100

degrees Celsius — the temperature around which microorganisms on Earth

flourish.

They also found nanometer-sized iron sulfide and iron oxide

grains (which they theorized were produced by

bacteria) and organic compounds known at polyaromatic hydrocarbons, or PHAs,

which they suggested were the organic remains of Martian organisms.

The clincher, however, was the discovery of “worm-shaped

objects” within the meteorite’s fractures, which scientists proposed were the

fossilized remains of the organisms themselves.

The results were controversial from the start and were quickly followed by a flurry of

papers that refuted every leg of the argument including the life-friendly temperature of the water; the biological origin of the iron grains; and the prospect that the organic PAHs actually came from

Antarctica, not Mars.

“A quiet consensus has emerged that the ALH84001 meteorite

contains no evidence of past Martian life,” wrote Georgia Tech’s John Bradley.

This month, NASA scientists responded with a fresh volley of

research that used a new analysis technique, called ion beam milling, to study the iron grains. They conclude that there is “considerable

evidence” that the nanocrystal iron grains were not the result of geology, which

leaves biology as the only other alternative.

In advance of scientists deciding what this latest research

means, some members of the (British) press have decided that life (once again) has been found on Mars,

or at least inside this one Martian rock.

They may be right, of course. Or, quite possibly, given a couple

of weeks or even days, scientists may trump the results once again. That is the

nature of the scientific process.

We’ll have more on this story next week.

(ALH84001: A space crypt for Martian bugs? Credit: NASA)