These models of the interiors of Pluto and Charon are based on the reasonable guess that each object has a rock interior with an icey outer layer. The models are constrained by estimates of each body's density. McKinnon & Mueller, Nature, v. 335, p240, 1988. The interior composition of Pluto and Charon cannot be determined directly, but theorical calculations and the value for the mean density (about one third that of earth) imply that nearly equal amounts of rock and water ice are present, with rock probably providing slightly more than half of the total mass. With a density about tywice thaat of water, Pluto is apparently made of much rockier material than are the other planets of the ouuter solar system. This may be the result of the kind of cold-temperature/low-pressure chemical combinations that took place during the formation of the planet. Many astronomers think Pluto may be a former satellite of Neptune, knocked into a separate orbit during the early days of the solar system. Charon would then be an accumulation of the lighter materials resulting from the collision.