On the Character and Consequences of Large Impacts in the Late Stage of Terrestrial Planet Formation

Abstract

We perform three-dimensional N-body integrations of the final stages of terrestrial planet formation. We report the results of 10 simulations beginning with 22–50 initial planetary embryos spanning the range 0.5–1.5 AU, each with an initial mass of 0.04–0.13 M ⊕. Collisions are treated as inelastic mergers. We follow the evolution of each system for 2×10 8 years at which time a few terrestrial type planets remain. On average, our simulations produced two planets larger than 0.5 M ⊕ in the terrestrial region (1 simulation with one m≥0.5 M ⊕ planet, 8 simulations with two m≥0.5 M ⊕ planets, and 1 simulation with three m≥0.5 M ⊕ planets). These Earth-like planets have eccentricities and orbital spacing considerably larger than the terrestrial planets of comparable mass (e.g., Earth and Venus). We also examine the angular momentum contributions of each collision to the final spin angular momentum of a planet, with an emphasis on the type of impact which is believed to have triggered the formation of the Earth's Moon. There was an average of two impacts per simulation that contributed more angular momentum to a planet than is currently present in the Earth/Moon system. We determine the spin angular momentum states of the growing planets by summing the contributions from each collisional encounter. Our results show that the spin angular momentum states of the final planets are generally the result of contributions made by the last few large impacts. Our results suggest that the current angular momentum of the Earth/Moon system may be the result of more than one large impact rather than a single impact. Further, upon suffering their first collision, the planetary embryos in our simulations are spinning rapidly throughout the final accretion of the planets, suggesting the proto-Earth may have been rotating rapidly prior to the Moon-forming impact event.