Ancient asteroid strikes more frequent than thought

Posted by Kate Taylor

The Earth experienced many more giant asteroid strikes in the past than previously believed, new NASA research shows.

The most famous strike - the one believed to have killed off the dinosaurs 65 million years ago - is believed to have been almost six miles across.

And, by studying ancient rocks in Australia and using computer models, the team estimates that around 70 asteroids the same size or larger hit the Earth between 1.8 and 3.8 billion years ago, with about four similarly-sized objects hitting the moon.

The evidence for these impacts on Earth comes from thin rock layers containing round, sand-sized droplets called spherules - once the molten droplets that were ejected within the huge plumes created by mega-impacts on Earth.

"The beds speak to an intense period of bombardment of Earth," says William Bottke, principal investigator of the impact study team at the Southwest Research Institute (SwRI). "Their source long has been a mystery."

The team's findings support the theory that Jupiter, Saturn, Uranus and Neptune formed in different orbits nearly 4.5 billion years ago, migrating to their current orbits about 4 billion years ago through the interplay of gravitational forces.

This event triggered a solar system-wide bombardment of comets and asteroids called the Late Heavy Bombardment. After modeling the main asteroid belt at the time, the team discovered thatthe innermost portion of the belt became destabilized and could have delivered numerous big impacts to Earth and the moon.

The frequency of the impacts indicated in the computer models matches the number of reliably-dated spherule beds. At least 12 mega-impacts produced spherule beds during the Archean period 2.5 to 3.7 billion years ago.

"The Archean beds contain enough extraterrestrial material to rule out alternative sources for the spherules, such as volcanoes," says Bruce Simonson, a geologist from Oberlin College in Ohio.