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J.E. Chambers (NASA Ames Research Center and Armagh Observatory)
If Jupiter and Saturn formed via accretion of a solid core, and capture of an H-He envelope, the amount of solid material in the proto-planetary disk must have been substantially more than the `minimum mass' nebula. In this case the region now occupied by the main-belt asteroids must have contained (i) several Earth masses of solids, and (ii) planet-sized bodies. One scenario for removing these `asteroidal planets' is that they scattered one another into unstable resonances with the giant planets.
N-body integrations show that the modern Jupiter and Saturn can clear 90% of the original mass in the main belt within 100 million years. However, in the course of ejecting asteroidal planets, the orbital eccentricities of the giant planets decrease to almost zero. This implies Jupiter and Saturn initially had more eccentric orbits (roughly e=0.1), which increases the size and strength of the unstable resonances. In this case, they clear 90% of the mass in the asteroid belt in only a few million years. At this point, a few asteroidal planets remain, and these can survive for several hundred million years before being removed. During this time, they are likely to inject a steady stream of smaller asteroids into unstable resonances with the giant planets. Many of these asteroids will enter the region occupied by the terrestrial planets.
This scenario may simultaneously solve 2 observational problems: (i) how to remove most of the solid material in the asteroid belt quickly enough, and with few collisions, so that the fragile crust of Vesta is preserved, and (ii) how to continue to supply large impactors to the terrestrial planets until the end of the late heavy bombardment.