Authors: Robin M. Canup & Craig B. Agnor

Status: In Origin of the Earth and Moon (eds. R. M. Canup and K. Righter) 2000. University of Arizona Press, Tucson

Abstract: Current models for the formation of the terrestrial planets suggest that the final stage of planetary accretion is characterized by collisions between tens to hundreds of lunar to Marssized planetary embryos. In this view, large impacts are an inevitable outcome as a system of embryos destabilizes to yield the final few planets. One such impact is believed to be responsible for the origin of the Moon. Improvements in numerical methods have recently allowed for the first direct orbit integrations of the final stage of accretion, which is believed to persist for ~108 yr. The planetary systems produced by these simulations bear a general resemblence to the terrestrial planets, but on average differ from our system in the final number of planets (fewer), their orbital spacings (wider) and their eccentricities and inclinations (larger). The discrepancy between these predictions and the nearly circular orbits of both Earth and Venus is significant, and is likely a result of the approximations inherent to the late-stage accretion simulations performed to date. Results from these works further highlight the important role of stochastic impact events in determining final planetary characteristics. In particular, impacts capable of supplying the angular momentum of the Earth-Moon system are predicted to be common.

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