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| Earth on Apr 26, 2012

Study: 70 killer asteroids struck Earth 1.8 to 3.8 billion years ago

Rocks in Australia contain sand-sized circles and particles, thought to be formerly molten droplets ejected into space when asteroids struck Earth.

A study of ancient rocks in Australia – combined with computer modeling of the early solar system – has led researchers to estimate that approximately 70 killer asteroids of the same size that caused the extinction of the dinosaurs or larger struck Earth 1.8 to 3.8 billion years ago. During the same period, approximately four similarly-sized objects hit the moon. Scientists with NASA’s Lunar Science Institute and international scientists published these results yesterday (April 25, 2012) in the journal Nature.

Part of the evidence for these early impacts on Earth by killer asteroids comes from thin rock layers in Australia. The rocks contain debris of nearly spherical, sand-sized droplets called spherules. The scientists say these rocks consist of formerly molten droplets ejected into space within the huge plumes created by mega-impacts on Earth.

Researchers estimate that approximately 70 asteroids of the same size that killed the dinosaurs - or larger - impacted Earth 1.8 to 3.8 billion years ago. Image Credit: NASA Lunar Science Institute

The demise of the dinosaurs occurred more recently than the asteroids suggested by this study, only about 65 million years ago in contrast to 1.8 to 3.8 billion years ago. Scientists believe that the killer asteroid that impacted Earth 65 million years ago would have been almost 6 miles (10 kilometers) in diameter.

This science team used computer modeling to depict a time in the early solar system, billions of years ago. They call on the giant planets Jupiter, Saturn, Uranus and Neptune to help send killer asteroids hurtling to Earth. They say their study supports the idea that the giant planets formed in different orbits from where we see them today nearly 4.5 billion years ago. The theory is that the giant worlds migrated to their current orbits about 4 billion years ago from the interplay of gravitational forces in the young solar system. According to the theory, this event triggered a solar system-wide bombardment of comets and asteroids called the Late Heavy Bombardment by scientists. In yesterday’s Nature paper, the team describe a model they created with computers, showing what the asteroid belt might have looked like at that early time. They then tracked what would have happened when the orbits of the giant planets changed.

They discovered the innermost portion of the asteroid belt became destabilized and could have delivered numerous big impacts to Earth and the moon over long time periods.

The millimeter-scale circles and irregular gray particles in this sample are sand-sized, about a millimeter (0.039 inch) in scale. Scientists are saying these circles and particles are formerly molten droplets ejected into space when an asteroid hit the Earth 2.63 billion years ago. Image Credit: Bruce Simonson, Oberlin College and Conservatory.

Further evidence for these early impacts on Earth by killer asteroids comes from thin rock layers in Australia. The rocks contain debris of nearly spherical, sand-sized droplets called spherules. These circles and particles are formerly molten droplets ejected into space when an asteroid hit the Earth 2.63 billion years ago, according to this study.

Read more from NASA’s Lunar Science Institute

Bottom line: Scientists studying ancient rocks in Australia and using a computer model to depict the early solar system now say that approximately 70 killer asteroids 6 miles (10 kilometers) – or larger – impacted Earth 1.8 to 3.8 billion years ago, and approximately four similarly-sized objects hit the moon around that time. Scientists with NASA’s Lunar Science Institute and international scientists published these results April 25, 2012 in the journal Nature..

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