NEW YORK, March 6 -- A chunk of dark rock found on the ice in Antarctica -- a lump plain enough to kick aside without remembering -- may be a little piece of Mars about 4 billion years old. It fell to Earth as a meteorite, one of seven to 12 meteorites that scientists now classify as martian, said Richard Ash from the University of Manchester, England. He and colleagues announced Wednesday that the Antarctic meteorite called Allan Hills 84001, is by far the oldest Mars rock knowingly handled by a human being. The meteorite is 'kind of a gray green,' he said. 'It doesn't look particularly special.' Location is everything though, and working on a splinter of ancient Mars struck Ash as 'just mind blowing.' Meteorites are, so far, the only source of martian rock available to scientists, since neither of the two Viking landers that visited the planet sent back samples. Meteorites are 'the cheap man's space program,' said Ash, in an interview from the American Museum of Natural History in New York, where he is working this year. Previously studied martian meteorites are no older than 1.3 billion years, but the ancient rock gives scientists a glimpse of a controversial chapter in the solar system's history. In an era that astronomers call the late heavy bombardment, 3.8 billion to 4 billion years ago, a storm of matter from space pounded the moon, digging many of the multi-ringed craters on the familiar, pock-marked face. What caused the bombardment and what other planets took a beating has caused considerable debate.
The ancient meteorite received a geological shock, such as the impact of a bombardment, during the controversial period, said Ash and colleagues in a paper in the new issue of the journal Nature. The shock did not toss the rock into space, however. Ash thinks that yet another impact 15 million to 16 million years ago finally knocked the lump off Mars. Then 'it floated around in space' and eventually crashed to Earth. It has been lying in Antarctica for tens of thousands of years. 'If a meteorite fell in Washington, you'd have trouble recognizing it unless you saw it fall,' Ash said. As rain and snow wear away the burned crust of meteorites, they become even harder to distinguish from other rocks. However, the cold desert of Antarctica preserves meteorites. 'Because they fall in the deep freeze, they can be around for tens of thousands of years,' Ash said. In certain parts of Antarctica, meteorite spotting is easy because rocks are so unusual in the vast fields of ice. Ash credits the Japanese with starting the hunt by picking up some 'strange black rocks' in 1969. Once scientists realized what the rocks were, NASA began an annual Antarctic meteorite hunt. Researchers have collected several thousand meteorites. 'Most are believed to come from asteroids,' said Ash. Some might be splinters of comets. 'About a dozen have been chipped off the moon,' he said. Up to a dozen of the meteorites failed to fit into any other class, and astronomers discovered that gas bubbles trapped in the rocks match the Martian atmosphere reported by exploring spacecraft. Bits of Mars can fall anywhere on Earth, said Ash. One of the meteorites recently classified as martian thumped down in Egypt in 1911, killing a dog. (Written from Washington by UPI Science Writer Susan Milius)
