By Eric Hand
Barlangi Rock, an ancient hill in the outback of Western Australia, is dimpled by the quarries of Aboriginal people who chiseled its fine-grained rocks into sharp tools. Now, geologists have added a much deeper layer of history to those rocks by showing they were forged 2.229 billion years ago, when an asteroid crashed into our planet. The finding makes Yarrabubba crater, the 70-kilometer-wide scar left by the collision, Earth’s oldest.
The geologists who reported the date last week, here at the Goldschmidt geochemistry conference, also point out a conspicuous coincidence: The impact came at the tail end of a planetwide deep freeze known as Snowball Earth. They say the impact may have helped thaw Earth by vaporizing thick ice sheets and lofting steam into the stratosphere, creating a powerful greenhouse effect.
“It’s intriguing to think what a moderate to large impact event could do in this time period,” says Timmons Erickson, a geochronologist at NASA’s Johnson Space Center in Houston, Texas, who led the study. “The temporal coincidence is striking,” agrees Eva Stüeken, a geobiologist at the University of St. Andrews in the United Kingdom. But she and other researchers are skeptical that Yarrabubba—which is just one-third the size of the crater left by the dinosaur-killing impact 66 million years ago—could have had such a profound effect on the climate. Still, Stüeken says, paleoclimate studies should consider the possible role of such violent collisions. “It forces us to think more about these impacts and these potential feedbacks.”
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