Graphic by Jim Houghton

ITHACA, N.Y. -- Planets and their satellites in the inner solar system -- including Earth -- have been sharing bits and pieces of themselves for billions of years, as even today rocks and particles shorn off from ongoing collisions continue their interplanetary voyage, new research shows.

This recent study helps to confirm that some meteorites found on Earth are from the moon and from Mars. Cornell University scientists have shown that the inner planets, bombarded in high-speed collisions with asteroids and comets, eject something of themselves into space. After launch, those pieces, from minuscule particles to grapefruit-sized rocks or larger, travel along orbits continually perturbed by the planets themselves.

"We found that planets and satellites are transferring material all the time," said Burns, Brett J. Gladman and Pascal Lee, Cornell doctoral students; Martin Duncan of Queen's University, Ontario; and Harold F. Levison of the Southwest Research Institute in Boulder, Colo., reported the research in the journal Science (March 8, 1996).

Using several high-speed desktop computers for several years, the researchers simulated throwing thousands of rocks off the moon, Mars, Venus and Mercury and followed their paths through space over millions of years, given the speed of their ejection from their home planets and gravitational tugs of the various planets.

Material is thrown out of craters whenever there is a collision, such as with an asteroid or meteor the size of a big building or larger, like the July 1994 crash of Comet Shoemaker-Levy 9 into Jupiter, or the event near Chicxulub, Mexico, that may have wiped out the dinosaurs on Earth. Such collisions are massive enough to send some fraction of the rocks and particles they launch hurtling into space, fast enough to escape the gravitational grasp of their parent planet.

This work shows accurately for the first time the history of planetary ejecta, and is a first step toward answering the puzzling question of why an equal number of meteorites found on Earth come from the moon and from Mars. With the moon so close and so much easier to escape, one might expect 100 or 1,000 times as much debris from there to be found on Earth. Yet a dozen meteorites from the moon and a dozen from Mars have been identified -- mostly since 1980 and mostly on the Antarctic ice sheet where they are relatively easy to spot.

Results of the simulations show that half the lunar material gets to Earth very fast -- within the first 50,000 years. But material from Mars tends to take much longer -- up to 15 million years. The earliest Martian meteorite on Earth took 700,000 years to arrive.

Like billions of celestial pinballs, these extraterrestrial rocks dance about the solar system, buoyed by gravitational kicks from the planets and possibly being catastrophically disrupted by collision in the asteroid belt. Over millions of years, their orbits become nothing like what they were when they started their solar system tour.

"After a few million years wandering, they've forgotten where they are from," said Gladman, whose doctoral dissertation comprises this study. The results appear to confirm the properties and ages of lunar and Martian meteorites found on Earth.

"Long-range gravitational effects strongly influence the orbits of many meteoroids, increasing their collision rates with other planets and the sun," the authors write. "These effects and collisional destruction in the asteroid belt result in shortened time scales and higher fluxes than previously believed, especially for Martian meteorites." They also say that it is possible that some meteorites on Earth may in fact have originated on Mercury or elsewhere.

The NASA-funded study shows that about 40 percent of the material launched from the moon eventually lands on Earth. Yet only 4 percent of material ejected from Mars makes it to this planet. Less than 1 percent of particles launched from Mercury can find their way to Earth.

"The details remain to be worked out, but we are hot on the trail," Burns said.

The study of these celestial dynamics and of meteorites in general could shed light on the formation of the solar system. While most meteorites on Earth come from asteroids, scientists now should be aware that a few others have originated on nearby planets or moons, said Burns, an expert in planetary mechanics. Further, material ejected into space from Earth during collisions millions of years ago could, and likely did, land on other planets. That material could have contained certain chemicals, such as the precursor amino acids necessary for life. "All of this is possible," Burns said.

The study also has practical implications for unmanned space travel to other planets, Burns added. "One must question whether it makes sense to spend billions of dollars to sterilize a spacecraft going to another planet, when that planet already may have been 'contaminated' by Earth through this natural ejecta process," he said.

The researchers stressed that there is more work to do to understand these results. Said Gladman, who expects his Cornell doctorate in August: "This is just the first step. The dynamical model correctly reproduces what we're able to deduce from the lunar and Martian meteorites. We expect the moon would be hit with a collision big enough to launch meteoroids every 10,000 years, and Mars every 1 million years or so. The Earth will get some of that material launched from each of these collisions. Now we must insure that the puzzle fits together correctly."

 Abstract of the article from Science web page