Making more terrestrial planets

The results of 16 new 3D N-body simulations of the final stage of the forma tion of the terrestrial planets are presented. These N-body integrations be gin with 150-160 lunar-to-Mars size planetary embryos, with semi-major axes 0.3 < a < 2.0 AU, and include perturbations from Jupiter and Saturn. Two i nitial mass distributions are examined: approximately uniform masses, and a bimodal distribution with a few large and many small bodies. In most of th e integrations, systems of three or four terrestrial planets form within ab out 200 million years. These planets have orbital separations similar to th e terrestrial planets, and the largest body contains 1/3-2/3 of the survivi ng mass. The final planets typically have larger eccentricities, e, and inc linations, i than the time-averaged values for Earth and Venus. However, th e values of e and i are lower than in earlier N-body integrations which sta rted with fewer embryos. The spin axes of the planets have approximately ra ndom orientations, unlike the terrestrial planets, and the high degree of m ass concentration in the region occupied by Earth and Venus is not reproduc ed in any of the simulations. The principal effect of using an initially bi modal mass distribution is to increase the final number of planets. Each si mulation ends with an object that is an approximate analogue of Earth in te rms of mass and heliocentric distance. These Earth analogues reach 50% (90% ) of their final mass with a median time of 20 (50) million years, and they typically accrete some material from all portions of the disk. (C) 2001 Ac ademic Press.