Re-Evaluating the Primordial Excitation and Clearing of the Main Belt

David P. O'Brien and Alessandro Morbidelli
Observatoire de la Cote d'Azur

William F. Bottke
Southwest Research Institute


The asteroid belt was originally much more massive than it currently is.
Dynamical processes in the early solar system removed most of the original 
mass and put the asteroid belt into its current dynamical state.  As reviewed 
by Petit et al. (2002), there are two main processes that are potentially
responsible for the excitation and depletion of the main belt: planetary 
embryos embedded in the main belt, and sweeping secular resonances with 
Jupiter and Saturn as the solar nebula was dissipating.  Most of the modeling 
of these processes assumes that Jupiter and Saturn were always on their current
orbits.

Recent outer planet migration models (Gomes et al. 2005, Tsiganis et al. 2005) 
point to a scenario in which the outer planets were initially a much more 
compact system (all within ~15 AU of the Sun) on nearly circular and co-planar 
orbits.  Thus, the resonance structure and gravitational perturbations in the 
primordial main belt due to Jupiter and Saturn would be substantially different 
than what is found in simulations that assume the current planet positions.  
For example, given the low initial planetary eccentricities, the effective 
widths and strengths of secular and mean-motion resonances would be smaller.       

We will present the results of simulations re-evaluating the primordial 
excitation and clearing of the main belt by sweeping secular resonances and 
by embedded planetary embryos, given the original orbits of Jupiter and Saturn 
as predicted by recent models (Gomes et al. 2005, Tsiganis et al. 2005).  
Preliminary results suggest that embedded planetary embryos are more effective 
than sweeping secular resonances at exciting and clearing the main belt, and 
the mass depletion driven by embedded planetary embryos is less rapid and 
pronounced than in simulations assuming the current orbits of Jupiter and 
Saturn.  

Gomes et al. 2005, Nature, in press.
Petit et al. 2002.  Asteroids III, pp.711-723.
Tsiganis et al. 2005.  Nature, in press.