New Theory: Earth Once Had 2 Moons

The sky above Earth may once have been a little more crowded. The planet originally had two moons that collided into one, according to a new theory proposed in Nature. It's widely believed that the early Earth collided with a Mars-sized proto-planet, and that debris coalesced into the moon. But the new theory says the debris created a pair of moons in close orbit, and their slow-motion collision is the cause for the moon's asymmetry. The second moon existed for tens of millions of years before the collision about 4.5 billion years ago, according to the theory, meaning no human would have seen it, notes Scientific American.

The differences between the near and far sides of the moon have long baffled scientists. The side we see is mostly lowlands lava plains, but the dark side is mostly highlands with crust that is much thicker and has a different mix of elements, all of which could be explained by the collision of the second moon. One researcher with a competing theory says, "All this is great fun and tells us that there are very fundamental questions that remain about the Moon." (More moon stories.)