  Of the solar system's four terrestrial planets, the origin of Mercury is
perhaps the most mysterious. Modern numerical simulations designed to model the
dynamics of terrestrial planet formation systematically fail to replicate
Mercury; which possesses just 5% the mass of Earth and the highest orbital
eccentricity and inclination among the planets. However, Mercury's large
iron-rich core and low volatile inventory stand out among the inner planets,
and seem to imply a violent collisional origin. Because most algorithms used
for simulating terrestrial accretion do not consider the effects of collisional
fragmentation, it has been difficult to test these collisional hypotheses
within the larger context of planet formation. Here, we analyze a large suite
of terrestrial accretion models that account for the fragmentation of colliding
bodies. We find that planets with core mass fractions boosted as a result of
repeated hit-and-run collisions are produced in 90% of our simulations. While
many of these planets are similar to Mercury in mass, they rarely lie on
Mercury-like orbits. Furthermore, we perform an additional batch of simulations
designed to specifically test the single giant impact origin scenario. We find
less than a 1% probability of simultaneously replicating the Mercury-Venus
dynamical spacing and the terrestrial system's degree of orbital excitation
after such an event. While dynamical models have made great strides in
understanding Mars' low mass, their inability to form accurate Mercury analogs
remains a glaring problem.
