The rare gas analysis of the lunar surface has lead to important conclusions concerning the moon. The large amounts of rare gases found in the lunar soil and breccia indicate that the solar atmosphere is trapped in the lunar soil as no other source of such large amounts of gas is known. The cosmogenic products indicate that the exposure ages of the 17 lunar rocks measured vary from 20 to 400 million years with some grouping of the ages. The most striking feature is the old potassium-argon age which for the 14 rocks analyzed varies from 2.5 to 3.8 billion years. It is concluded that Mare Tranquillitatis crystallized about 4 billion years ago from a molten state produced by a large meteorite impact or volcanic flow.