摘要：Based on orbital calculations of Keplerian planetesimals incident on a planet with various initial orbital elements, we develop a numerical model which describes the accretional and dynamical evolution of planet-satellite systems in a swarm of planetesimals on heliocentric orbits with given spatial and velocity distributions. In the plane of orbital radius of the satellite vs. satellite/planet mass ratio, a satellite with some initial value moves quickly toward the balanced orbital radius, where accretion drag compensates with tidal repulsion, and then grows toward the equilibrium mass ratio. Using the model, we propose two types of co-accretion scenarios for the origin of the Moon, both of which satisfy the most fundamental dynamical constraints: the large angular momentum of the Earth-Moon system and the large Moon/Earth mass ratio. In the first scenario the Moon starts from a small embryo and grows in a swarm of planetesimals with low velocity dispersion and nonuniform spatial distribution, so that large spin angular momentum is supplied to the planet. Such a situation would be realized when the Earth grows up rapidly before dissipation of the solar nebula. Second one considers co-accretion after a giant impact during Earth accretion, which produces enough angular momentum as large as that of the present Earth-Moon system as well as a lunar-sized satellite. In this case, solar nebula would have already dissipated and random velocities of incident planetesimals are rather high, so that the Earth grows slowly. We find that the total angular momentum decreases by 5–25% during this co-accretion stage.
