Origin of the Earth and Moon

Since the beginning of civilization, the origins of the Earth and Moon have been the subjects of continuing interest, speculation, and enquiry. These are also among the most challenging of all scientific problems. They are, perhaps to a unique degree, interdisciplinary, having attracted the attention of philosophers, astronomers, mathematicians, geologists, chemists, and physicists. A large and diverse literature has developed, far beyond the capacity of individuals to assimilate adequately. Consequently, most of those who attempt to present review-syntheses in the area tend to reflect the perspectives of their own particular disciplines. The present authors approach is that of a geochemist, strongly influenced by the basic phil- osophy of Harold Urey. Whereas most astronomical phenomena are controlled by gravitational and magnetic fields, and by nuclear interactions, Urey (1952) emphasized that the formation of the solar system occurred in a pressure-temperature regime wherein the chemical properties of matter were at least as important as those of gravitational and magnetic fields. This was the principal theme of his 1952 book, "The Planets," which revolutionized our approach to this subject. In many subsequent papers, Urey strongly emphasized the importance of meteorites in providing critical evidence of chemical conditions in the primordial solar nebula, and of the chemical fractionation processes which occurred during formation of the terrestrial planets. This approach has been followed by most subsequent geochemists and cosmochemists.