Many ideas have been put forth over the centuries to explain the origin of our moon. In the late 19th century, the son of Charles Darwin suggested that the moon might have spun away from the earth, leaving the Pacific Ocean as a scar, but this theory went out of fashion after the Apollo landings, and instead, the leading theory is now that a collision between the Earth and a body called Theia led to the formation of the moon. A high-speed collision is good at explaining the high angular momentum of the earth-moon system—the fact that they orbit quite rapidly, but it has had difficulties explaining why the composition of the earth and moon are so similar. If the collision was high-speed, the thinking goes, the moon should be mostly made of material from Theia, making the compositions different.
Now, an international group of researchers led by Natsuki Hosono of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the RIKEN Center for Computational Science (R-CCS) have used simulations on the powerful K computer to show that the moon may have formed through a collision that occurred at a time when the Earth was still covered with oceans of magma. Essentially, the liquid magma would be heated by the shock wave more efficiently than the solid mass of Theia and thus vaporize more quickly, leading it to be preferentially slung off to create the moon. This finding can explain why the composition of the moon is so similar to the earth and not to the hypothetical foreign object that collided with the earth. For more information on the study, see the press release by Yale University or the paper in Nature Geoscience.