In a study published in the Aug. 23 issue of the journal Science, the research team reports that an ancient meteorite slammed into Earth 3.47 billion years ago.
Scientists have yet to locate any trace of the extraterrestrial object itself or the gigantic crater it produced, but other geological evidence collected on two continents suggests that the meteorite was approximately 12 miles (20 kilometers) wide roughly twice as big as the one that contributed to the demise of the dinosaurs some 65 million years ago.
"We are reporting on a single meteorite impact that has left deposits in both South Africa and Australia," said Donald R. Lowe, a Stanford professor of geological and environmental sciences who co-authored the Science study. "We have no idea where the actual impact might have been."
To pinpoint when the huge meteorite collided with Earth, Lowe and his colleagues performed highly sensitive geochemical analyses of rock samples collected from two ancient formations well known to geologists: South Africa's Barberton greenstone belt and Australia's Pilbara block. The two sites include rocks that formed during the Archean eon more than 3 billion years ago when Earth was "only" a billion years old and single-celled bacteria were the only living things on the planet.
"In our study, we're looking at the oldest well-preserved sedimentary and volcanic rocks on Earth," Lowe noted. "They are still quite pristine and give us the oldest window that we have on the formative period in Earth's history. There are older rocks elsewhere, but they've been cooked, heated, twisted and folded, so they don't tell us very much about what the surface of the early Earth was really like."