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What Boulders on Mars Are Telling NASA About Their Life Stories

IF ROCKS COULD TALK

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Here on Earth, all those Martian rocks with their Hanna-Barbera names may look rather ordinary. Hardly worth a 309-million-mile flight.

But to geologists, rocks hold clues for how the Red Planet was formed and what has happened to it in the last million years or so.

To be sure, intergalactic rock hounds haven't been this excited since the Apollo missions of the 1970s, when astronauts brought back boxes of gray basaltic moon rocks. Then this spring's discovery in Antarctica of a Martian asteroid, which contained bacteria-like fossils, further raised their expectations - and those of the public.

Just a week into the Mars Pathfinder mission, scientists know more now than they did from the previous unmanned Viking mission 20 years ago. Through pictures and data beamed to Earth from their robotic rover, Sojourner, they can tell that some of the larger rocks are made of volcanic coarse-grained matter. They can tell that the fine flour-like dust on the ground has traces of iron, and that it's starting to rust. And they can tell that all these rocks and boulders were dropped in their current spots by a biblical-scale flood.

"I'm not sure this mission is going to say much about the actual origin of the planet," says Fred Marshall, a geology professor at Principia College in Elsah, Ill. "You have windstorms on Mars, and the features we are seeing occurred fairly long after the origin of the planet. But we can tell you what may have happened since the planet was formed."

The two rocks visited thus far show that Mars is similar to Earth in its composition. Like Earth, Mars has a molten core that sends magma to the surface like bubbles in a boiling kettle of soup. Pressure from the magma is strong enough to push through the hardened crust of Mars's surface, forming volcanoes.

But unlike the basaltic magmas found on Hawaii, the ocean floor, and Earth's moon, the Martian magma has cooled very slowly, forming a coarse-grained rock. This material is so close to a form of granite found on Earth that mission-control scientists have identified it as andesite, a rock found in the Andes of South America.

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