  The solar system's terrestrial planets are thought to have accreted over
millions of years out of a sea of smaller embryos and planetesimals. Because it
is impossible to know the surface density profile for solids and size frequency
distribution in the primordial solar nebula, distinguishing between the various
proposed evolutionary schemes has been historically difficult. Nearly all
previous simulations of terrestrial planet formation assume that Moon to Mars
massed embryos formed throughout the inner solar system during the primordial
gas-disk phase. However, validating this assumption through models of embryo
accretion is computationally challenging because of the large number of bodies
required. Here, we reevaluate this problem with GPU-accelerated, direct N-body
simulations of embryo growth starting from r~100 km planetesimals. We find that
embryos emerging from the primordial gas phase at a given radial distance
already have masses similar to the largest objects at the same semi-major axis
in the modern solar system. Thus, Earth and Venus attain ~50% of their modern
mass, Mars-massed embryos form in the Mars region, and Ceres-massed objects are
prevalent throughout asteroid belt. Consistent with other recent work, our new
initial conditions for terrestrial accretion models produce markedly improved
solar system analogs when evolved through the giant impact phase of planet
formation. However, we still conclude that an additional dynamical mechanism
such as giant planet migration is required to prevent Earth-massed Mars analogs
from growing.
