The Moon and the Earth have been companions for several billion years, and one Moon-formation theory proposes that they have been so since the beginning of the solar system. Another suggests that the Moon was formed elsewhere but was captured by the Earth's gravitational pull. Yet another theory has it that they were once a single body but that the Moon was then split away by a gigantic asteroid collision. Detailed studies of differences in composition between the Moon and the Earth suggest a fourth alternative: that in the early solar system it was the Moon itself - then larger - that collided with the Earth and that the portion that ricocheted away became the Moon we know.

Radiometric age-dating of rocks brought back by the 1969-72 Apollo missions from different parts of the Moon disclosed evidence of its geologic history. The oldest particles of material found in every locality are 4.5 to 4.6 billion years old. Because this coincides approximately with the radiometric ages of the oldest known chondritic meteorites, the entire solar system with all its constituents may well be 4.6 billion years old. Because no material that old survives in larger pieces, these must have been shattered and transported all over the Moon in the course of an initial heavy bombardment of the lunar surface during the first 200 or 300 million years of its existence, before the supply of interplanetary material available for bombardment became largely exhausted.

The dating results also indicated that a large part of the crater-forming impacts that disfigured the mountainous parts of the Moon occurred in the first half-billion years of lunar existence. The largest of them, which gave rise to scars known as circular maria, occurred 400 to 800 million years after the Moon was formed. The flooding of the basins excavated by these impacts with basaltic magmas occurred some 400 to 700 million years later, or 3.3 to 3.8 billion years before the present time. No more basalts appeared on the lunar surface in the first 800 million years of its existence, nor were any added more than 600 million years later.

In the past 3 billion years, more than two-thirds of the age of the satellite, nothing much has happened on the Moon. It has continued to expose its stony face to cosmic weather and keeps accumulating scars of new impacts, but at a diminishing rate. In the aeons to come, the surface of the Moon is likely to become more and more withdrawn in its petrified grimace, reflecting the state of the solar system in the days long gone by, a fossil reminder of the past that no internal convulsion can awaken from its slumber.