  The solar and extra solar gas giants appear to have diverse internal
structure and metallicities. We examine a potential cause for these dispersions
in the context of the conventional sequential accretion formation scenario. In
principle, gas accretion onto cores with masses below several times that of the
Earth is suppressed by the energy released from the bombardment of residual
planetesimals. We show that these planetesimals generally cannot pass through
the mean motion resonances of the cores, and the suppression of planetesimal
bombardment rate enables the cores to accrete gas with little interruption.
During growth from the cores to protoplanets, as the resonances overlap with
each other, the trapped planetesimals become dynamically unstable and their
eccentricity excitation is strongly enhanced. Subsequent gas drag induces the
planetesimals to migrate and collide with the protoplanet. This process leads
to the resumption and a surge of planetesimal bombardment during the advanced
stage of the protoplanet growth. This mechanism may account for the diversity
of the core-envelope structure between Jupiter, Saturn and the metallicity
dispersion inferred from the transiting extra solar planets.
