Cosmic Bombardment History
The exact timing and magnitude of the impacts that smashed Earth during the Hadean are unknown. To get an idea of the effects of this bombardment, Marchi and his colleagues looked at the moon, whose heavily cratered surface helped model the battering that its close neighbor Earth must have experienced back then.
"We also looked at highly siderophile elements (elements that bind tightly to iron), such as gold, delivered to Earth as a result of these early collisions, and the amounts of these elements tells us the total mass accreted by Earth as the result of these collisions," Marchi said. Prior research suggests these impacts probably contributed less than 0.5 percent of the Earth's present-day mass.
The researchers discovered that "the surface of the Earth during the Hadean was heavily affected by very large collisions, by impactors larger than 100 kilometers (60 miles) or so — really, really big impactors," Marchi said. "When Earth has a collision with an object that big, that melts a large volume of the Earth's crust and mantle, covering a large fraction of the surface," Marchi added.
These findings suggest that Earth's surface was buried over and over again by large volumes of molten rock — enough to cover the surface of the Earth several times. This helps explain why so few rocks survive from the Hadean, the researchers said.
However, although these findings might suggest that the Hadean was a hellish eon, the researchers found that "there were time gaps between these large collisions," Marchi said. "Generally speaking, there may have been something on the order of 20 or 30 impactors larger than 200 km (120 miles) across during the 500 million years of the Hadean, so the time between such impactors was relatively long," Marchi said.
Any water vaporized near these impacts "would rain down again," Marchi said, and "there may have been quiet tranquil times between collisions — there could have been liquid water on the surface."
The researchers suggested that life emerging during the Hadean was probably resistant to the high temperatures of the time. Marchi and his colleagues detailed their findings in the July 31 issue of the journal Nature.
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