Phase III – Stealth Is.

The current best theory for the origin of the Moon is the “giant impact” model. This holds that a Mars-sized object, sometimes called Theia, once shared Earth’s orbit and collided with it over four billion years ago, and the remains of this cataclysm ultimately formed into the Moon. Now it appears that that cosmic drama repeated itself in miniature with the Moon itself. Scientists are rather awesomely calling this “The Big Splat.”

It’s not just that the far side of the Moon is mountainous whereas the near side is flat – it actually has a substantially thicker crust than its lowland counterpart. That’s hard to explain if the Moon formed all at once, as the formation processes tend to even out such substantial irregularities. But planetary scientists at UC Santa Cruz propose a novel solution: what if the giant impact between Earth and Theia created not one, but two moons?

This second satellite would have been considerably smaller than the Moon we know today. It probably would have only been about 1/30 the mass of the Moon, which is still decently sized by the satellite standards (that’s a lot bigger than Mars’s moons, for instance). Both satellites would likely have shared the same orbit around Earth, with the second moon situated at one of the two Trojan points of stability relative to our Moon.

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So how did the two collide? The second moon was able to remain stable in the early days of the Earth-Moon system, because the still forming larger Moon was initially much closer to Earth. As the initial chaos passed, the primary Moon moved out further into its current orbit. The second moon was no longer able to remain in a stable orbit, and it began a slow, gentle (collision?) with its larger companion – well, slow and gentle by the standards of two moons smashing into each other, that is.

Because the colliding object was moving at a relatively low velocity, crater formation and melting of the lunar surface was minimal. Instead, the entire second moon simple flattened and accumulated on one side of its former orbital partner, piling on tens of kilometers worth of new crust. This piling would not have been totally even, however, and that’s where you get the lunar highlands.

And it’s not just the elevation differences that this hypothesis accounts for. It also explains why the two sides of the Moon have such radically different compositions. The near side is rich in potassium, phosphorus, thorium, uranium, and various rare-earth elements, all of which were deposited on that side by the Moon’s primordial magma ocean. Now we know why they all ended up on side – the collision flattened the far side and pushed the resource-rich magma around to the other side of the satellite.