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Sticks and stones: the Martian Meteorite debate rages on

By Michelle / April 12, 2002


Mars has always been a provocateur. The planet has a long history of making us uneasy, from the portents of violence our ancestors associated with its red glow, to our science-fiction nightmares of malicious, technologically superior alien invaders.

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And Mars is still stirring things up in the scientific community. For several years now, there has been an on-going debate as to whether a meteorite from Mars contains the fossilized remnants of microbial life. Some scientists think we no longer have to wonder about whether there is other life in the universe; we have the remains of tiny Martian cousins in our laboratories at this very moment. Others remain skeptical, claiming that every structure and chemical in the meteorite could have been formed by natural processes that have nothing to do with life, like chemical weathering and heating. Despite the controversy, the Martian Meteorite debate has already taught us a lot about what kind of questions to ask the next time we get our hands on a sample of Martian soil, as well as shown us how little we understand about the threshold of life itself.

Backing up a little, how in the world did a piece of Mars find its way to Earth? Would you recognize a Martian rock if it were sitting in your backyard? The idea isn't as outlandish as it seems. We know of 24 meteorites that were originally part of Mars.

The first was recovered after it boomed down into a field outside of Chassigny, France, in 1815 (although the people of the time had no way of knowing that it came from Mars). The famous Martian Meteorite (designated ALH 84001) that has spurred all the debate was found more recently, lying on the Antarctic ice in 1984. Antarctica turns out to be a rich ground for meteorite hunters, as all of the indigenous rocks are buried beneath thousands of feet of glacial ice. If you find a rock sitting on top of the ice, there's a good chance it landed there from somewhere else. One of the meteorite-hunting teams even has a mascot of a penguin standing with a baseball glove aimed at the sky. Deserts like the Sahara and the Mojave are also a good bet, as the meteorites stand out from the sandy, eroded rocks around them.

So now that you've found a rock from space, how do you know it came from Mars? We've never brought a rock back from Mars, and our robotic landers have only been able to do crude chemical analysis of the rocks and soil there. Interestingly, that issue is not part of the debate. Scientists are almost certain that these meteorites are bits of the planet Mars due to careful chemical analysis of bits of the Martian atmosphere trapped in the rocks. We know the chemical composition of Mar's atmosphere very well (from spectroscopic measurements), and the rocks match it exactly. Really, there's no where else they could have come from.

Which leaves the next big question: How did they get here?

That might be the most amazing part of the whole story. The only way a rock could get to here from there is to be blasted off the surface of Mars at 11,000 miles per hour (that's the speed needed to escape Mars' gravity). There's no physical process on any planet that we know of that can achieve such speeds. Even rocks hurled out of giant volcanic explosions don't go anywhere near that fast. So, in fact, the only thing that can create a Martian meteorite is another meteorite. Probably a very big one, as big as a mile across.

That's right, scientists think these meteorites were chunks of Mars that got blasted into space after a violent (think dinosaurs) impact from a meteor or a comet. Judging from our maps of the Martian surface, this hasn't happened for a while. We have no way of knowing exactly when this impact took place, but we do have some idea how long ALH 84001 stayed drifting around between Earth and Mars. High energy particles called cosmic rays irradiate anything in space, leaving radioactive traces. ALH 84001 seems to have been exposed to these particles for about sixteen million years, although if it was the inner part of a larger meteor that broke up, it could have been in space much longer. And we didn't find it the moment it fell to Earth either, not by a long shot. Judging by other radioactive decay processes, the rock had been cooling its heels in Antarctica for about 13,000 years.

Right away, planetary scientists knew they'd found something interesting, as the rock showed signs of having been flooded with liquid water at least a billion years ago, perhaps as much as three billion. This piqued everyone's imagination, as the meteorite seemed to come from a lost age on Mars, when life might have taken hold.