It may look like a boring old moss-covered rock, but this chestnut-size greenish chunk is actually a bit of meteorite from Mars and hence one of the most exciting things a planetary scientist can lay her sterilized gloves on. The Nakhla meteorite, named for where it fell in Egypt in 1911, made headlines last year when scientists claimed that, like an earlier rock in 1996, it contained possible evidence of microbial life on Mars. The clumps of minerals in the meteorites found on Earth—20,000 and counting—offer clues to the origin of our solar system. You can read up on the basic classes of meteorites at this Web site from a commercial lab. For a gigantic gallery of meteorite images annotated with scientific details, check out this site run by a collector. For more on the 14 known martian meteorites, try this NASA site.