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
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)