The Moon may have formed from a series of large impacts instead of a single giant collision, according to a study published online in Nature Geoscience. This model explains why the Moon appears to be composed largely of Earth-like material, rather than a mix of Earth and another planet.
A giant impact between the proto-Earth and a Mars-sized body is the leading candidate to explain the Earth-Moon system. In a successful giant-impact scenario, either most of the material that forms the Moon comes from Earth (not the impactor), or the impactor is compositionally identical to Earth. However, both are possible, but unlikely, scenarios.
Raluca Rufu and colleagues conducted numerical simulations of large planetary bodies (but not giant ones) impacting the proto-Earth. In the simulations, the impacts produce disks of debris, many of which are composed of mostly Earth, not impactor, material. After each impact, the debris disks accrete to form a moonlet that the authors suggest eventually migrates outwards and coalesces with a growing Moon. It would take about 20 of these moonlet-forming collisions to assemble the Moon.
The authors suggest moonlet-forming impacts between the proto-Earth and large bodies were sufficiently common in the early inner Solar System to produce the Moon and match observational constraints. A Moon assembled through multiple impacts implies that the Moon formed over many millions of years, rather than in a geologic instant, and that the Earth and Moon’s interiors may be less well mixed than in a giant-impact scenario, potentially preserving a record of this period of bombardment.
In an accompanying News & Views article, Gareth Collins writes that the study revives “the hitherto largely discarded scenario that a series of smaller and more common impacts, rather than a single giant punch, formed the Moon.”