40Ar/39Ar dating of Apollo 12 impact spherules

We have used the 40Ar/39Ar isochron technique to determine ages of 81 lunar spherules from Apollo 12 soil sample 12023. Most spherules are created in meteoroid impacts, and their ages correspond to the timing of the impacts that formed them. Of the 81 impacts we have dated, most occurred in the last 500 million years. The abundance of spherules from the most recent ∼10% of the history of the Moon is consistent with an increase in the meteoroid bombardment of the inner Solar System, but does not require this explanation. The soil sample from which we extracted our spherules was from the ejecta of a recent impact; our spherule age measurements support models of lunar soil mechanics and impact cratering in which ejecta are stratified and inverted relative to the stratigraphy that was present before the impact.