  It remains an elusive goal to simultaneously model the astrophysics of Solar
System accretion while reproducing the mantle chemistry of more than one inner
terrestrial planet. Here, we used a multistage core-mantle differentiation
model based on Rubie et al. (2011,2015) to track the formation and composition
of Earth and Mars in various Grand Tack formation simulations. Prior studies
showed that in order to recreate Earth's mantle composition, it must grow first
from reduced (Fe-metal rich and O-poor) building blocks and then from
increasingly oxidized (FeO rich) material. This accretion chemistry occurs when
an oxidation gradient exists across the disk so that the innermost solids are
reduced and increasingly oxidized material is found at greater heliocentric
distances. For a suite of Grand Tack simulations, we investigated whether Earth
and Mars can be simultaneously produced by the same oxidation gradient. Our
model did not find an oxidation gradient that simultaneously reproduces the
mantle composition of Earth and Mars. Due to its small mass and rapid
formation, the formation history of Mars-like planets is very stochastic which
decreases the likelihood of compatibility with an Earth-producing oxidation
gradient in any given realization. To reconcile the accretion history and ideal
chemistry of the Mars-like planet with the oxidation gradient of an
Earth-producing disk, we determined where in the Earth-producing disk Mars must
have formed. We find that the FeO-rich composition of the Martian mantle
requires that Mars' building blocks must originate exterior to 1.0 astronomical
units (AU).
