  Current lunar origin scenarios suggest that Earth's Moon may have resulted
from the merger of two (or more) smaller moonlets. Dynamical studies of
multiple moons find that these satellite systems are not stable, resulting in
moonlet collision or loss of one or more of the moonlets. We perform Smoothed
Particle Hydrodynamic (SPH) impact simulations of two orbiting moonlets inside
the planetary gravitational potential and find that the classical outcome of
two bodies impacting in free space is altered as erosive mass loss is more
significant with decreasing distance to the planet. Depending on the conditions
of accretion, each moonlet could have a distinct isotopic signature, therefore,
we assess the initial mixing during their merger, in order to estimate whether
future measurements of surface variations could distinguish between lunar
origin scenarios (single vs. multiple moonlets). We find that for
comparable-size impacting bodies in the accretionary regime, surface mixing is
efficient, but in the hit-and-run regime, only a small amount of material is
transferred between the bodies. However, sequences of hit-and-run impacts are
expected, which will enhance the surface mixing. Overall, our results show that
large scale heterogeneities can arise only from the merger of drastically
different component masses. Surfaces of moons resulting from the merger of
comparable-sized components have little material heterogeneities, and such
impacts are preferred, as the relatively massive impactor generates more melt,
extending the lunar magma ocean phase.
