New Theory That Many Collisions, Not One, Formed The Moon

The Moon looks great over the Atacama Desert, but why do we have such a large satellite? A new theory challenges the favored one. Skreidzeleu/Shutterstock

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 asĀ 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.

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