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NASA/JPL-Caltech

One of the striking features of Earth (besides the living things upon it) is that it possesses a relatively large moon. While our Moon is not the largest in the solar system, it is large for such a small planet. The mass of the Moon is more than 1% that of Earth, compared to Jupiter’s largest moon Ganymede, which is just 0.008% of Jupiter’s mass. The origin of our large moon has been a matter of some debate.

The most popular model is the collision model. In this model, the young proto-Earth collided with a Mars-sized planetoid sometimes named Theia. Part of the mass of Theia was captured to become Earth, and the remains formed a debris ring around Earth, much of which coalesced to form the Moon. This model has a lot going for it. For example, the Moon has a density about 60% that of Earth, which is exactly what is predicted by the collision model, where the lighter outer layers of Theia and proto-Earth are scattered to the debris disk, while the heavy core of proto-Earth remains.

The main downside of this model is the fact that the Earth and Moon have a very similar chemical composition. The only way to explain that fact with the collision model is to assume that Theia and Earth had a very similar composition, which would seem unlikely. Now new research published in Nature has found that similarities between colliding bodies aren’t as unlikely as we thought.¹

The team ran computer simulations of young solar systems, and looked at how chemically similar large bodies were with their last major impactors. What they found was that 20% – 40% of the time they were similar enough to account for the Earth-Moon compositions. So it’s not so unusual that the Earth and Moon are chemically similar.

There’s only one other body in the solar system with such a relatively large moon as ours, and that’s Pluto’s moon Charon. Charon’s mass is 12% that of Pluto. Charon is also thought to be the result of a collision, so when New Horizons makes its flyby of the planet in July, we find even more clues about planet-shattering collisions.