The case of the misclassified meteorite.

For 8 years, researchers had classified a meteorite found in the Allen Hills of Antarctica as a fairly ordinary chunk of rock--a fragment gouged from a familiar resident of the asteroid belt. Tucked away in a nitrogen-filled cabinet at NASA's Johnson Space Center in Houston, the meteorite has borne its commonplace label ever since its discovery in 1984.

That's how things stood until last October, when David W. Mittlefehldt, a geologist with Lockheed Engineering and Sciences Co. in Houston, examined a thin slice of the carefully preserved rock. A detailed analysis of its chemical composition has now stripped the 1.9-kilogram meteorite of its plebeian origin and assigned it celebrity status. The Allen Hills meteorite doesn't come from an asteroid, reports Mittlefehldt, it comes from Mars.

Only the 10th meteorite known to originate from the Red Planet, the Allen Hills rock is the first meteorite identified as having formed beneath the Martian surface. Because of its underground origins, it may provide a wealth of information on the geological processes that helped shape the planet, Mittlefehldt says.

When Mittlefehldt first sampled a section of the meteorite, he wasn't thinking about Mars. He simply wanted to take a close look at a meteorite that researchers had classified as a diogenite. This type of rock is believed to have come from a well-studied asteroid, 4 Vesta.

A devotee of diogenites for several years, Mittlefehldt analyzed the mineral content of the Allen Hills fragment, known as ALH84001. To his surprise, the mineral composition indicated that the meteorite wasn't a diogenite and couldn't have come from 4 Vesta. But the chemical fingerprints did match those of the nine known meteorites believed to be pieces of Mars.

Mittlefehldt says he first suspected that scientists had misclassified ALH84001 when he found an iron oxide with an abundance of triply ionized iron, a characteristic of Martian meteorites. In contrast, the same oxide in diogenites contains iron in the doubly ionized form. The chemical form of sulfide minerals in the meteorite clinched the Martian connection, he says.

One of the most remarkable features of ALH84001 is its concentration of carbon compounds, which may be higher than for any of the other Martian meteorites, notes Mittlefehldt. The meteorite appears to have incorporated carbon dioxide at higher temperatures than the other rocks, indicating that it acquired the gas beneath the Martian surface, perhaps from magma fluids.

The higher pressure below the surface probably prevented more carbon dioxide from bubbling out of the rocky body; a significant fraction remained trapped in the meteorite as carbonates, he suggests. In contrast, other Martian meteorites probably incorporated carbon by soaking up carbon dioxide from the planet's atmosphere.

"This new finding is probably the first convincing case for [a tangible storehouse] of primordial carbon inside Mars," says Mittlefehldt.