Lunar impact history from Ar-40/Ar-39 dating of glass spherules

Lunar spherules are small glass beads that are formed mainly as a result of small impacts on the lunar surface; the ages of these impacts can be deter mined by the Ar-40/Ar-39 isochron technique. Here, 155 spherules separated from 1 gram of Apollo 14 soil were analyzed using this technique. The data show that over the Last similar to 3.5 billion years, the cratering rate de creased by a factor of 2 to 3 to a low about 500 to 600 million years ago, then increased by a factor of 3.7 +/- 1.2 in the last 400 million years. Th is latter period coincided with rapid biotic evolutionary radiation on Eart h.