  The main asteroid belt lies between the orbits of Mars and Jupiter, but the
region is not uniformly filled with asteroids. There are gaps, known as the
Kirkwood gaps, in the asteroid distribution in distinct locations that are
associated with orbital resonances with the giant planets; asteroids placed in
these locations follow chaotic orbits and escape from the asteroid belt. Here
we show that the observed distribution of main belt asteroids does not fill
uniformly even those regions that are dynamically stable over the age of the
solar system. We find a pattern of excess depletion of asteroids, particularly
just outward of the Kirkwood Gaps associated with the 5:2, the 7:3, and the 2:1
jovian resonances. These features are not accounted for by planetary
perturbations in the current structure of the solar system, but are consistent
with dynamical ejection of asteroids by the sweeping of gravitational
resonances during the migration of Jupiter and Saturn ~4 gigayears ago.
