Primordial Excitation and Depletion of the Main Belt

The asteroid belt is characterized by a large dynamical excitation, an important mass depletion compared to its original density, and a radial mixing of bodies with different physical and chemical properties. Two mechanisms have been proposed to explain this structure: the sweeping of secular resonances through the belt, and the scattering action by embryos later ejected from the solar system by planetary perturbations. The resonance sweeping is due to the dissipation of the gas-drag-inducing primordial nebula. Resonance sweeping and gas drag are responsible for mass depletion and orbital migration. Embryos are naturally created in the inner solar system during planet formation, producing a strong dynamical excitation. Although both approaches are fairly good at structuring the asteroid belt and depleting it, it is likely that both mechanisms occurred at one time or another. More work is needed to assess the combined effect of the nebula and the excitation by embryos.

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