Dynamic Model for the Formation of the Earth-Moon System

Some geochemical characteristics of the Moon are such that they contradict the hypothesis of the Moon's formation from the Earth's mantle. We propose a new alternative to the currently accepted giant impact hypothesis. It was shown that 40% evaporation of a material of chondritic chemistry yields a composition sim- ilar to that of the Moon, including low iron content. It is known that evaporation is accompanied by isotope fractionation, whereas no isotope effects were detected in lunar soils within the analytical accuracy. However, isotope fractionation can be absent, if matter evaporates under equilibrium conditions. Such conditions occur in a cloud of hot particles. In order to justify this concept, we developed a computer model for the formation of the Earth and Moon from a single cloud of primitive (chondritic) composition. The model is based on the mod- ified method of particle dynamics. We introduced the following additional interactions between particles: long- range gravitational attraction, short-range viscoelastic interaction related to collisions, and gas dynamic repul- sion due to the evaporation of matter from particle surface. It was shown that the gas dynamic repulsion reduces the interaction energy and allows fragmentation of a cloud whose momentum corresponds to that of the Earth- Moon system. Computer modeling indicated that the accumulation of dispersed dust material provides a faster growth rate of the larger of the two bodies. This is why the Moon retained relatively low iron abundance, whereas the Earth accumulated most of the remaining dust cloud and acquired its high iron content. If the pro- : posed model is valid, it is necessary to revise current concepts on the formation of planet-satellite systems.