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
Whether an impact produced a moonlet depended not only on the size of the incoming object, but also whether it made a head-on or glancing collision, as well as its rate of spin. Nevertheless, Rufu believes approximately 20 of these small moons formed, eventually amalgamating to become the giant we see today.
The idea has one big advantage over the single-impact theory. Rufu's modeling shows that many of these smaller collisions would create debris rings predominately made of terrestrial material, with little content from the incoming object. On the other hand, most models of a single impact suggest that at least 70 percent of the Moon's material should have come from the impacting object. The other planets have distinctive isotopic signatures, for example having different ratios of titanium-50 to titanium-47 compared to the Earth. This makes it puzzling that an object such as the Moon could have such a similar signature to Earth if it was formed largely out of different material.
The implications would also be rather positive for the quest for intelligent life. The presence of a large Moon makes the Earth far more habitable by stabilizing our planet's axial tilt and thereby keeping the seasons relatively constant. Many astronomers hypothesize that the reason we have not encountered aliens is that the arrival of a single object of just the right size and at just the right angle to make a decent-sized moon is so rare that few habitable planets have it, and therefore never develop really advanced life.
Rufu's proposal would seem far more likely to be replicated on another world. Nevertheless, it leaves open the question of why Venus and Mercury, which should have experienced a similar bombardment, have no moons. Rufu told IFLScience both probably once had one or more moons, but lost them. "Mercury is too small and too close to the Sun, so it is not surprising that Mercury does not have moons," she said. Venus' slow rotation is indicative of a past moon that moved away until it was no longer trapped by its planet's gravity.