  During the last stages of the terrestrial planet formation, planets grow
mainly through giant-impacts with large planetary embryos. The Earth's Moon was
suggested to form through one of these impacts. However, since the proto-Earth
has experienced many giant-impacts, several moons are naturally expected to
form through a sequence of multiple (including smaller scale) impacts. Each
impact potentially forms a sub-Lunar mass moonlet that interacts
gravitationally with the proto-Earth and possibly with previously-formed
moonlets. Such interactions result in either moonlet-moonlet mergers, moonlet
ejections or infall of moonlets on the Earth. The latter possibility, leading
to low-velocity moonlet-Earth collisions is explored here for the first time.
We make use of SPH simulations and consider a range of moonlet masses,
collision impact-angles and initial proto-Earth rotation rates. We find that
grazing/tidal-collisions are the most frequent and produce comparable fractions
of accreted-material and debris. The latter typically clump in smaller moonlets
that can potentially later interact with other moonlets. Other collision
geometries are more rare. Head-on collisions do not produce much debris and are
effectively perfect mergers. Intermediate impact angles result in debris
mass-fractions in the range of 2-25% where most of the material is unbound.
Retrograde collisions produce more debris than prograde collisions, whose
fractions depend on the proto-Earth initial rotation rate. Moonfalls can
slightly change the rotation-rate of the proto-Earth. Accreted moonfall
material is highly localized, potentially explaining the isotopic
heterogeneities in highly siderophile elements in terrestrial rocks, and
possibly forming primordial super-continent topographic features. Our results
can be used for simple scaling laws and applied to n-body studies of the
formation of the Earth and Moon.
