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.
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.
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