Did Earth once have two moons?

Earth may have once had two moons that collided, leaving an asymmetry in our companion's shape that has long puzzled scientists, a U.S. researcher says. In an article published this week in Nature, Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, suggests the two bodies merged in a slow-motion collision lasting many hours that created a mysterious mismatch between the moon's visible side and its remote far side. The moon's visible side is dominated by low-lying lava plains, while its far side is composed of highlands. The differences, however, go below the surface, as the thickness of the moon's crust and the minerals found below the surface vary between the two regions, Asphaug and co-author Martin Jutzi of the University of Bern said. Asphaug suggests something squeezed some of the moon's layers to one side, and that a collision with a smaller moon about 600 miles in diameter is the most likely explanation. "By definition, a big collision occurs only on one side," he says, "and unless it globally melts the planet, it creates an asymmetry." Following the slow-motion impact, Asphaug says, gravity would have squeezed the material of the smaller body to a relatively thin layer on top of the moon's existing crust.