Scientific reaction was fast, and at times, furious. The evidence for life, said critics, was only circumstantial. As long as alternate explanations existed, a claim to have found the first indication of life anywhere but on Earth should require more definitive proof. After three years and dozens of scientific papers, researchers have been unable to resolve the issue. There is still no overwhelming evidence that the meteorite carries remnants of ancient Martian life. There’s also no conclusive proof that it doesn’t.
The most immediately compelling line of evidence–the presence of bacteria-shaped nodules of rock–has also been the most problematic. Most appear too small to be fossils of viable cells, and may be the result of chemical and physical processes. With just the one rock, there’s no real chance to test theories. The result? Impasse. Two other potential signs of ancient life have been similarly inconclusive, but a fourth is more troublesome for skeptics. ALH84001 contains crystals of magnetite, an iron compound used by some bacteria on Earth to sense and move along magnetic fields. Of the three forms of the mineral found in ALH84001, two can be formed by nonbiological means. But a third is known on Earth only as a bacterial product. “It’s hard to argue that they’re not similar” to bacterial magnetites on Earth, says Allan Treiman, who studies meteorites at the Lunar and Planetary Institute in Houston, “because they are.” But questions remain about, among other things, the location of the magnetite within the meteorite. “I hold no ill will to the hypothesis,” says Treiman, “but I don’t think it has been proved. I don’t know if it can be.”
The ALH84001 debate may never be resolved, but the investigations have helped to refine procedures for the coming search for Martian life. “The work McKay’s group has started is in effect a dry run for analyzing returned samples,” says Treiman. “Whether or not their original hypothesis is accepted, it has turned out to be very positive.” ALH84001, now one of the most thoroughly studied objects on our planet, may yet help prove–or disprove–the existence of life on others.
