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Some of the material blasted into space coalesced to form Earth's nearest neighbor, the thinking goes.
But it's been hard to pin down exactly when this impact, or these impacts, occurred, Barboni said.
If, in the year AD 1600, you had asked an educated European how old the planet Earth was and to recount its history he would have said that it was about 6000 years old and that its ancient history was given by the biblical account in Genesis.
If you asked the same question of an educated European in AD 1900 you would have received a quite different answer.
"That's one of the biggest problems — the whole-rock record on the moon is not there." So Barboni and her team decided to take a different tack.