“After the Sun, the Moon of all the heavenly bodies is that which interests us the most; its phases afford us a measure of time so remarkable that it has been primitively in use among all people” [1]. The Moon and Mercury represent special cases even by the standards of the solar system. Mercury is unique due to its high density, with an iron/silicate ratio about twice that of the other inner planets (Section 11.1). In contrast, the Moon is of interest because of its low density and low metal/silicate ratio [2]. Explanations for the peculiar nature of both bodies have a long history and much effort has been expended in attempts to fit one or both into overall schemes of planetary formation, but without conspicuous success.

The Moon has played a central role in the recent development of theories of the origin and evolution of the solar system. This is not without irony, as it has proven one of the most difficult objects to explain. It is in plain sight, accessible even to naked-eye observation, as Harold Urey (1893–1981), who persuaded NASA to go to the Moon, was accustomed to remind us. The Moon was often thought to be a kind of Rosetta Stone, so that the general belief in pre-Apollo times was that we could discover much about the origin of the solar system by going to the Moon [3].