Earth's early life endured long asteroid bombardment
IT WAS a blitzkrieg with no let-up. Earth may have been pounded by massive asteroids for a billion years longer than we thought, with the impacts only stopping about 3 billion years ago. If that is true, early life had to endure a bombardment that periodically melted Earth's surface.
The planet formed 4.5 billion years ago, and chunks of rock many kilometres across continued falling onto it for hundreds of millions of years. It seemed there was a final burst of impacts around 3.9 billion years ago – and by 3.8 billion years ago it was all over. The first fossils of life are very slightly younger.
That story is wrong, says Donald Lowe of Stanford University in California. The barrage continued far longer. "Its termination was not an abrupt drop-off but a gradual waning until 3 billion years ago," he says.
Lowe and his colleagues have spent 40 years studying a patch of ancient rocks in eastern South Africa called the Barberton Belt. Over 25 years ago they found four layers of spherical particles, which seemed to have condensed from clouds of vaporised rock. Lowe says they are the traces of four major meteorite impacts, and date from between 3.5 and 3.2 billion years ago.
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http://www.newscientist.com/article/dn26055-earths-early-life-endured-long-asteroid-bombardment.html