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Earth's beating lasted longer: bombardment persisted over 2 billion years, analysis finds.

It's no secret that the early Earth took a beating from above, but now it seems the planet sustained a longer bombardment than initially thought.

Scientists thought this pummeling--a spike of violent impacts during a period known as the Late Heavy Bombardment-lasted several hundred million years at most. But new simulations suggest that Earth's pummeling persisted for more than 2 billion years, two teams report in the May 3 Nature.

"It seems highly likely that these impacts affected the Earth's biosphere in profound ways," says study coauthor William Bottke of the Southwest Research Institute in Boulder, Colo.

Reconstructions of the early solar system peg the bombardment as beginning about 4.1 billion years ago, triggered by a cataclysmic rearrangement of the outer planets. At that time, changes in Jupiter and Saturn's orbits unleashed a gravitational tidal wave that sent their ice giant brethren farther out while flinging smaller asteroids and comets inward.

Earlier theories suggest the turbulence ended by about 3.7 billion years ago. Yet there's evidence for enormous impacts as recently as 1.8 billion years ago. "Where are these impactors coming from? We must be missing a source," Bottke says.

In his new version of events, the asteroid belt's inner boundary was closer to where Mars now resides, rather than out at 2.1 times the Earth-sun distance, where it is now. Simulations show that as the outer solar system spasmed, it perturbed this expanded reservoir, hurling asteroids toward the inner solar system. These asteroids--hotter and more cooked kinds than most of those around today--can explain the fresher impacts.

The longer bombardment matches the story told by tiny "impact spherules" embedded in Earth's sediments. These glassy spheres formed after giant impacts ejected plumes of vaporized rock, which coated the globe and condensed into part-extraterrestrial BBs. The age of the spherules suggests a gradual decline in Earth's pummeling. "This is a good marriage of geology in the field and models and calculations done in the lab," says Donald Lowe, a geologist at Stanford University.

The spherules form layers that linger after craters have been erased by tectonic activity. "If you find one of these spherule layers, you can estimate the thickness of it, and you can estimate the size of the asteroid that created that layer," says Brandon Johnson of Purdue University, a coauthor of the second study.


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Title Annotation:Atom & Cosmos
Author:Drake, Nadia
Publication:Science News
Geographic Code:1USA
Date:Jun 2, 2012
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