"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.
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.
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 eventsthey're not part of a heavy bombardment process."
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES