The two sides of the Moon look nothing like each other: the near side is flat and low, while the far side is incredibly mountainous. We may now be able to solve this topographical mystery...and it involves a second moon.
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. We discussed Trojan points in some detail recently with the discovery of Earth's first Trojan asteroid - for a more detailed explanation, go here.
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 with its larger companion - well, slow and gentle by the standards of two moons smashing into each other, that is.
Erik Asphaug, one of the leaders of the new study, explains why this second moon hypothesis is so compelling:
"Our model works well with models of the moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the moon itself. It agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the moon, and the ages of lunar rocks. Of course, impact modelers try to explain everything with collisions. In this case, it requires an odd collision: being slow, it does not form a crater, but splats material onto one side. It is something new to think about."
As Asphaug indicates, this was no ordinary collision. 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.
Indeed, the collision explains why there's a far and near side at all. It made the Moon lopsided, causing the entire satellite to reorient itself so that the lighter side faced the Earth while the heavier side pointed away. All that said, there are still other possibilities to account for why the Moon is shaped the way it is. It's possible that tidal forces could account for the Moon's composition without invoking a second satellite.
But even one of the main proponents of the tidal force model, fellow UC Santa Cruz researcher Francis Nimmo, says there's a good chance that a second moon really did have a part to play in the Moon's current look:
"The fact that the near side of the moon looks so different to the far side has been a puzzle since the dawn of the space age, perhaps second only to the origin of the moon itself. One of the elegant aspects of Erik's article is that it links these two puzzles together: perhaps the giant collision that formed the moon also spalled off some smaller bodies, one of which later fell back to the Moon to cause the dichotomy that we see today."
So then, while we can't quite consider it confirmed, there's a very reasonable chance that, at some point in Earth's far distant past, you could have looked up at the night sky and seen two moons up there. (Well, assuming you had a time machine handy to get back to before the dawn of life, of course.) There's no two ways about it - that's pretty damn amazing.