The most famous Martian meteorite, ALHA84001, received attention because of the possibility that it houses ancient bacteria. But other rocks from the Red Planet have survived the long interplanetary journey to end up on Earth, too. Scientists were stumped, however, as to why the majority of these meteorites were relatively young compared to Mars's age. New findings published today in the journal Science can help explain the characteristics of the Martian meteorites.

Analysis of the Martian rocks indicated that they were ejected by six or seven separate impact events. Previous tests had predicted that a collision with an asteroid large enough to leave a crater 12-kilometers wide would be needed to send the meteorites to Earth. Because such huge impacts are extremely rare, it was unlikely that enough of them could have occurred to explain our planet's Martian meteorite collection. Now the higher-resolution models developed by James N. Head of the University of Arizona and his colleagues show that collisions that cause craters only three kilometers across can launch 10 million fragments, each about 10 centimeters across, into space. This distribution would be sufficient to expect some of them to show up on Earth, the scientists report. In addition, they found that sections of the planet that are covered by debris (which are likely to be made up of older terrain) would require larger, and hence rarer, impacts. As a result, meteorites that reach Earth are biased toward younger ages.