Moonfalls: Collisions between the Earth and its past moons

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 (and also the final Moon) are naturally expected to form through/as-part-of 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 possiblywith 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 smooth particle hydrodynamical 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 bound debris that may later form new moonlets. Rarer 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 per cent where most of the material is unbound. Retrograde collisions produce more debris than prograde collisions, depending on the proto-Earth's initial rotation rate, which changes slightly as a result of the impacts. We also show that accreted impactor material is highly localized on the Earth's surface, potentially contributing to observed isotopic heterogeneities in highly siderophile elements in terrestrial rocks.