Early Earth may have had two moons

NPR : To explain why the two sides of the Moon are so different from each other—the near side is low and flat and the far side is mountainous and deeply cratered—two researchers have made a novel proposition. They propose that Earth originally had two moons, the one familiar to us now and a smaller, sister moon. At some point before life on Earth began, the smaller moon collided with the larger one at a relatively low speed, which caused the smaller one to break up and spread out over the Moon’s surface, forming mountains on the far side. Erik Asphaug and Martin Jutzi of the University of California, Santa Cruz, who conducted computer simulations to test their theory, have published their results in Nature. “I think this idea is going to get a lot of attention because it’s very novel, it’s very clever, and people are going to be interested in testing to see whether it’s right or wrong,” said Maria Zuber from MIT.