  In the Nice model of solar system formation, Uranus and Neptune undergo an
orbital upheaval, sweeping through a planetesimal disk. The region of the disk
from which material is accreted by the ice giants during this phase of their
evolution has not previously been identified. We perform direct N-body orbital
simulations of the four giant planets to determine the amount and origin of
solid accretion during this orbital upheaval. We find that the ice giants
undergo an extreme bombardment event, with collision rates as much as ~3 per
hour assuming km-sized planetesimals, increasing the total planet mass by up to
~0.35%. In all cases, the initially outermost ice giant experiences the largest
total enhancement. We determine that for some plausible planetesimal
properties, the resulting atmospheric enrichment could potentially produce
sufficient latent heat to alter the planetary cooling timescale according to
existing models. Our findings suggest that substantial accretion during this
phase of planetary evolution may have been sufficient to impact the atmospheric
composition and thermal evolution of the ice giants, motivating future work on
the fate of deposited solid material.
