Researchers have found the first terrestrial evidence that the Earth was heavily bombarded by meteorites around four billion years ago.
Until now, scientists suspected the Earth and its neighboring planets must have been pelted in the same meteorite event—known as the "Late Heavy Bombardment"—that left the moon scarred by giant impact craters still visible today. Some of the moon's impact craters are more than 600 miles (1,000 kilometers) across.
Over time Earth's surface has changed dramatically as its dynamic plates reshaped the continents, all but erasing the evidence of the ancient bombardment.
Now researchers have discovered for the first time evidence in rocks—found in Canada and Greenland—that they say confirms Earth was bombarded as heavily as the moon.
"There is no other conceivable explanation for new-found traces of a tungsten isotope in rocks 3.7 billion years old from Greenland and Canada," said Ronny Schoenberg of the University of Queensland, Brisbane, Australia, and colleagues in this week's Nature.
The researchers said a form of tungsten found in the rocks—which came from the Earth's early crust—is usually found in meteorites.
"There is no plausible mechanism by which tungsten isotope anomalies could have been preserved in the Earth's dynamic crust-mantle environment," said Ken Collerson, also of the University of Queensland. "Therefore, we conclude these rocks must contain a compound derived from meteorites," he said in an article on the university Web site.
"We have in effect found a chemical fingerprint in the earth's oldest terrestrial rocks of a heavy meteorite bombardment 3.8 to 4 billion years ago," he said.
The origin of the meteorites is not fully resolved, but is thought to be material remaining after all the other planets had formed.
The findings may also be relevant to the debate about the earliest signs of life on Earth.
The debate revolves around the origin of tiny graphite particles that have been found in the same ancient rocks. The presence of extraterrestrial tungsten raises the possibility that this graphite could also be of meteoritic origin.
"This finding has implications for the origin of life on Earth as these giant impacts would have annihilated any possible existing life-forms but also delivered complex molecules from carbonaceous chondrites—a type of meteorite—to the Earth's surface," another member of the research team, Balz Kamber, said.
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