What Boulders on Mars Are Telling NASA About Their Life Stories

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.

"This tells us that the powers operating on Mars share in common something we've been seeing on Earth," says Dr. Marshall. Later missions may be able to tell scientists whether any of this volcanic activity has led to the creation of huge continental plates similar to those on Earth.

Scientists chose a flood plain for Pathfinder's landing site, hoping to find a larger variety of rocks that the flood would have left behind. Scientists believe much of Mars's water has either evaporated or is frozen now at the planet's north and south poles, but at one time, it was warm enough to flow in rivers, streams, and floods.

If Sojourner bumps into any sedimentary rocks, like the sandstone found on Earth, this would indicate that water had washed around Mars long enough to erode rocks and form layers upon layers of sediment. If put under enough pressure, these layers form long, flat rocks.

The prospect of finding sedimentary rock on Mars tends to make geologists quite animated. But Jiba Ganguly, a geoscience professor at the University of Arizona in Tucson, is holding out for metamorphic rock, perhaps a sedimentary rock that has been placed under heat or pressure and turned into something entirely different.

"If we see metamorphic rock, I would be very excited," says Dr. Ganguly. "I think I've watched more TV in the past few days than I've watched in the past six months. Whenever I go home, I switch on the TV."

But amid the hoopla, there are some geologists who won't be satisfied until they can get their hands on a piece of Mars. "There's no substitute for bringing rocks back," says John Wood, a geologist at Harvard University's Smithsonian Observatory in Cambridge, Mass. "The first thing a geologist does with a rock on Earth, he takes his pick and opens it up to get a fresh surface."

In addition, he worries that a Martian sandstorm may eventually put Sojourner's solar panels out of commission. "They need windshield wipers on that thing."