Mother of Selene

Abstract Scientists pursued the giant impact theory along two tracks: (1) testing it against the measured geochemistry of the Earth and the Moon, especially the isotopic composition of oxygen in the two bodies, and (2) performing computer simulations of a collision between two huge objects in space. The question they were trying to solve was how much of the Moon that resulted from the collision came from the proto-Earth and how much from the impactor, which they named Theia, the mother of Selene, goddess of the Moon in Greek mythology. The computer simulations showed that most of the Moon came from Theia. But the Moon and the Earth have virtually identical oxygen isotope ratios. If the Moon had come from some other part of the solar system, this would have required an enormous coincidence, something that scientists abhor. The other possibility was that the proto-Earth and Theia had different isotope ratios, but they had been homogenized in the fiery vapor cloud that resulted from the giant collision.