PLANETS REGULARLY EXCHANGE ROCKS, COMPUTER STUDY FINDS.

Byline: Glennda Chui Knight-Ridder Tribune News Wire

Like kids in a gravel pile, the inner planets have been flinging rocks out into space for billions of years - and hitting each other with some regularity, according to new computer simulations done at Cornell University.

Among tens of thousands of meteorites that have been found sprinkled across the Earth, researchers think about two dozen were originally part of the moon or Mars. The rest are bits of asteroids.

But prospectors should also be keeping their eyes out for meteorites from Mercury or Venus - and for Earth rocks that catapulted into space during cataclysmic impacts, toured the solar system for a few million years and fell back down again, said Joseph A. Burns, a planetary scientist at Cornell.

The problem, he added, is that no one would recognize one of these exotic chunks now, even if they stumbled across it.

After all, people have brought home rocks from the moon and sent robots to probe and analyze the surface of Mars. Scientists know how to identify those rocks. But they've never sampled rocks from Mercury or Venus to determine the chemistry of the rocks there.

"You can see the bind you're in," Burns said. "If you find a weird rock it's not easy to see where it came from unless you've already been there."

Harry Y. McSween Jr., a meteorite expert at the University of Tennessee, agreed. "We know so little about Mercury," he said, "that it's even difficult to guess what Mercury rocks might look like."

The idea that planets could swap rocks with each other is fairly new, McSween said. But the Cornell study, published in Friday's issue of the journal Science, is the first to show, in detail, the paths those rocks might take and where they would likely come to rest.

Twenty years ago, scientists scoffed at the idea that a chunk of one planet could wind up on another. Sure, an asteroid could hit a planet hard enough to propel rocks from that planet high into the air. But one of those rocks could not escape the planet's gravitational grasp unless it accelerated, almost instantly, to very high speed - and then it would probably get hot enough to vaporize.

Then researchers began to identify meteorites that came from the moon and Mars. And H. Jay Melosh, a planetary scientist at the University of Arizona, figured out how a rock could fly fast enough to escape from a planet without serious damage.

For the new study, researchers used two powerful desktop computers to simulate the tracks of hundreds of individual rocks flying off the surface of Mars, Mercury or the moon and ping-ponging through the solar system. Each time one of the model rocks approaches a planet, it feels a tug from the planet's gravitational field and veers off in a new direction. After 100 million simulated years - compressed into two years of computer time - most of the rocks have hit something.

Of 900 rocks leaving Mars in the simulation, 7.5 percent eventually fell to Earth. An equal number landed on Venus, 38 percent plunged into the sun, and 15 percent rocketed out toward Jupiter.

Only one in 200 Mercury rocks wound up on Earth. This suggests they would be rare indeed, Burns said; still, you might expect scientists to have found a few.

It's probably even less likely that a rock could survive the escape from Earth or Venus and then end up on Earth, the team wrote. Both planets are much bigger than Mars, so a rock would have to go much faster to escape. Only the biggest rocks would survive the resulting trauma, and they would have to be launched by an extremely large and rare impact.