A collision might explain the moon's asymmetry
(NEWSER) - 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 . More»
