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Earth Asteroid Bombardment Mystery Solved?

James Owen
for National Geographic News
September 15, 2005
 
Scientists turned planetary detectives say they may have solved a solar system whodunit: What caused a cataclysmic asteroid assault on Earth and neighboring planets some 3.9 billion years ago?

Researchers suspect that the devastating bombardment, which lasted between 20 and 200 million years, originated in the main asteroid belt between Jupiter and Mars.

The U.S.-led team based their conclusion on evidence found in the unhealed wounds of ancient asteroid collisions left on the moon and the inner planets Mars, Mercury, and Venus.

Writing in tomorrow's issue of the journal Science, the scientists note that the intense period of crater-making impacts, known as the late heavy bombardment, was likely caused by repositioning of the solar system's giant outer planets.

Pockmarked Planets

During their study, the team investigated the most pockmarked celestial surfaces in the solar system, such as those visible on the moon and Mars.

The researchers compared the number and size of craters on older terrains with newer surface regions, such as the volcanic plains on Mars.

Younger planetary surfaces revealed impacts by relatively fewer, smaller projectiles. Craters in heavily scarred, older regions closely matched the size range of objects in the main asteroid belt.

Renu Malhotra, a planetary sciences professor at the University of Arizona, Tucson, co-authored the study. She says that the more recent impacts were likely caused by much smaller, less numerous near-Earth asteroids. Such impacts still occur today.

Malhotra notes that in the main asteroid belt small asteroids greatly outnumber large ones: There are about ten times as many asteroids measuring 300 meters (1,000 feet) in diameter than those measuring 3 kilometers (2 miles).

Smaller Asteroids

The scientist says that while the surface of the Earth would once have shown similar evidence of a 3.9-billion-year-old pummeling by asteroids, the scars have long since healed.

"Geological activity erases craters and 'renews' surfaces," she said. "The moon, Mercury, and Mars have large parts of their surfaces that have been unaltered by internal geological processes for nearly four [billion] years. In contrast, Earth has been very active geologically."

Venus also presented a difficult subject for the team's investigations, because asteroids tend to fragment in the planet's dense atmosphere.

"We were able to 'reconstruct' the original size of [Venus's] impactors by tracking the clusters of craters [made by] the fragmented asteroids," Malhotra said.

The team used its crater findings to suggest that movement of the big outer planets Jupiter and Saturn may have disturbed the main asteroid belt before the solar system became relatively stable, sending a barrage of asteroids careering towards Earth.

The research represents an "extremely interesting" new lead in the search to explain an intense period of bombardment of the inner planets, says Richard P. Nelson, an astronomy lecturer at Queen Mary, University of London, England.

Space Debris

Another possible explanation, says Nelson, is that the projectiles were made up of rock-and-ice space debris, which the outer planets Uranus and Neptune scattered into the solar system, "using their gravity rather like a slingshot."

"Uranus and Neptune then moved outwards, replacing the material they'd scattered inwards," he said.

But Nelson believes the study team's theory is equally probable.

"The stuff in the asteroid belt is in a sense a failed planet, which was forced to fail by the other planets because of their gravitational pull," he explained.

"If Jupiter had existed further out in the solar system and had migrated in towards the main asteroid belt, then that might have disturbed the [asteroid] belt, causing the period of late heavy bombardment."

"Either or both of these possible scenarios could be correct," Nelson said.

Future Bombardment?

So what are the chances of the main asteroid belt unleashing itself at Earth in the future?

"Very, very unlikely," Nelson said. "The solar system is pretty stable and settled now. There's evidence that during the late heavy bombardment it went through a period of instability when the main asteroid belt was cleaning itself up after formation. I think that process is more or less finished now."

He adds that planetary stability doesn't mean the Earth is safe from future impacts from big asteroids.

"Large objects hit the Earth every 100,000 years, or something like that. But these are just isolated, individual events—they're not part of a heavy bombardment process."

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