A challenge has been thrown down to the dominant theory of the Moon's formation. For years, we have believed the Moon was the result of a single, enormous collision that threw vast quantities of material into orbit, which eventually aggregated into the Earth's companion. However, a paper in Nature Geoscience argues instead for a string of smaller impacts, the result of which gradually accumulated into the body we see today.
The Moon is so large, relative to the Earth, that it needs a lot of explaining. Although Charon is even closer to Pluto's size, the other true planets have either no moons at all or satellites that are proportionally far smaller. Heated debate on how we came to have such a large companion eventually gave way to a near consensus: Early in its formation, the Earth was struck by a Mars-sized object, and the impact threw enough matter into space to coalesce into the Moon, even after much of it had rained back down.
Raluca Rufu of the Weizmann Institute of Science, Israel, is challenging that belief. The problem, she argues, is that the Moon is formed of material too much like the Earth. If the collision came from a single planet-sized object, some of it should have been blasted into space and incorporated into the Moon, giving it a distinctly different composition.
Along with her co-authors, Rufu simulated what would happen if the Earth had instead been struck by a series of objects, all 1-10 percent of the mass of the Earth. Depending on their speed, these are large enough to have ejected considerable material into orbit. After each collision, a debris disk would have formed in orbit, which would gradually coalesce to become a modest-sized moon.
The theory of how the Moon formed from multiple impacts. Rufu et al/Nature Geoscience