Mars meteorite poses puzzling questions.

Mars meteorite poses puzzling questions

Researchers confirmed last week that a recently identified meteorite of Martian origin ranks as the oldest piece of the Red Planet known to have struck Earth. Radioactive dating indicates that the meteorite, a 1.9-kilogram rock designated ALH84001, formed about 4.5 billion years ago, during the solar system's infancy and shortly after the Martian crust formed. "This meteorite is giving us a look at Mars early in its history, when it was a warmer, wetter planet," says Everett K. Gibson of NASA's Johnson Space Center (JSC) in Houston. He and his collaborators, along with several other research teams, reported their findings at the annual Lunar and Planetary Science Conference in Houston.

Originally misclassified as a fragment gouged from an asteroid (SN: 3/26/94, p.206), ALH84001 has several intriguing properties, says Allan H. Treiman of the Lunar and Planetary Institute in Houston. For instance, among the 11 meteorites identified as chunks of Mars, ALH84001 has the highest concentration of carbonates.

The high carbonate content appears to support the long-held notion that water once flowed on Mars. Researchers speculate that carbonates crystallized within the meteorite when water rich in dissolved carbon dioxide percolated through rock just beneath the Martian surface.

In another finding, mass spectroscopy and electron microscopy reveal that ALH84001 contains polycyclic aromatic hydrocarbons (PAHs), report Kathie L. Thomas of Lockheed Martin Engineering and Sciences Co. in Houston and her colleagues. These organic molecules may originate from material delivered to Mars by comets that struck the planet.

Another interpretation is that the PAHs appeared as precursors of primitive life. On Earth, some PAHs are products of biological activity. Measurements of the decay products of two radioactive elements confirm that ALH84001 is about 4.5 billion years old, Lawrence E. Nyquist of JSC and his colleagues announced at the conference. A team of German researchers reported a similar age last year.

Some scientists were reluctant to accept the earlier estimate, Nyquist notes, in part because the other known Martian meteorites are much younger. With only about 4 percent of the present Martian surface believed to have survived unchanged from such an early era, some researchers doubted that Earth would get "free delivery" of so old a sample.

The great age of ALH84001 suggests that the Martian crust formed in a hurry, no more than 100 million years after the birth of the sun, Treiman says. But age is only half the mystery, he adds. In the standard model for crust formation, low-density material floats to the surface of a young, molten planet like scum on a pond. However, ALH84001 contains relatively high-density material, including the mineral orthopyroxene. How could such a rock become part of the Martian crust?

"We're all puzzled," Treiman says.