the the largest impact basin in the solar system and Orientale on the western limb (as seen from Earth; in the center of the image at left) which is a splendid example of a multi-ring crater. A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, 20 years after the last Moon landing, scientists still study these precious samples. Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a fortuitous match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the Solar System not available on the Earth. Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories: co-accretion which asserted that the Moon and the Earth formed at the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and capture which held that the Moon formed elsewhere and was subsequently captured by the Earth. None of these work very well. But the new and detailed information from the Moon rocks led to the impact theory: that the Earth collided with a very large object (as big as Mars or more) and that the Moon formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted. The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism indicating that there may have been a global magnetic field early in the Moon's history. With no atmosphere and no magnetic field, the Moon's surface is exposed directly to the solar wind. Over its 4 billion year lifetime many hydrogen ions from the solar wind have become embedded in the Moon's regolith. Thus samples of regolith returned ...