“(April 25, 2012, phys.org) New research reveals that the Archean era — a formative time for early life from 3.8 billion years ago to 2.5 billion years ago — experienced far more major asteroid impacts than had been previously thought, with a few impacts perhaps even rivaling those that produced the largest craters on the Moon, according to a paper published online today in Nature.

The fingerprints of these gigantic blasts are millimeter- to centimeter-thick rock layers on Earth that contain impact debris: sand-sized droplets, or spherules, of molten rock that rained down from the huge molten plumes thrown up by mega-impacts. This barrage of asteroids appears to have originated in an extended portion of the inner asteroid belt that is now mostly extinct. Computer models suggest the zone was likely destabilized about 4 billion years ago by the late migration of the giant planets from the orbits they formed on to where we find them today.

The team conducting this study includes members or associates of the NASA Lunar Science Institute's Center of Lunar Origin and Evolution (CLOE), based at the Southwest Research Institute (SwRI) in Boulder, Colo.

Archean rocks are scarcer than rocks of any other age, and impact spherule beds have been found only in terrains where conditions were ideal for capture and preservation, such as in shales deposited on the seafloor below the reach of waves. At least 12 spherule beds deposited between 3.47 and 1.7 billion years ago (Ga) have been found, with most in the Archean; 7 between 3.23-3.47 Ga, 4 between 2.49-2.63 Ga and 1 between 1.7-2.1 Ga…”

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