  The solar system's dynamical state can be explained by an orbital instability
among the giant planets. A recent model has proposed that the giant planet
instability happened during terrestrial planet formation. This scenario has
been shown to match the inner solar system by stunting Mars' growth and
preventing planet formation in the asteroid belt. Here we present a large
sample of new simulations of the "Early Instability" scenario. We use an N-body
integration scheme that accounts for collisional fragmentation, and also
perform a large set of control simulations that do not include an early giant
planet instability. Since the total particle number decreases slower when
collisional fragmentation is accounted for, the growing planets' orbits are
damped more strongly via dynamical friction and encounters with small bodies
that dissipate angular momentum (eg: hit-and-run impacts). Compared with
simulations without collisional fragmentation, our fully evolved systems
provide better matches to the solar system's terrestrial planets in terms of
their compact mass distribution and dynamically cold orbits. Collisional
processes also tend to lengthen the dynamical accretion timescales of Earth
analogs, and shorten those of Mars analogs. This yields systems with relative
growth timescales more consistent with those inferred from isotopic dating.
Accounting for fragmentation is thus supremely important for any successful
evolutionary model of the inner solar system.
