Lunar Impact History from ⁴⁰ Ar/ ³⁹ Ar 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 determined by the 40 Ar/ 39 Ar 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 ∼3.5 billion years, the cratering rate decreased 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. This latter period coincided with rapid biotic evolutionary radiation on Earth.