The leading theory of how the moon was created--a powerful cosmic collision dubbed the "Big Whack"-- has gotten an important boost. The idea is that 4.5 billion years ago, an object more massive than Mars slammed into the infant Earth so hard that its iron core plunged to the center of our planet. Some of this rock, the theory goes, went skittering into orbit as extremely hot vapor and other debris. Eventually, the vapor cooled and condensed into a spinning disk, like Saturn's rings.
The theory says this disk eventually clumped up to form the moon. Astronomer Robin Canup of the University of Colorado and her colleagues report in today's issue of Nature that computer modeling shows that the disk of debris could have coalesced into the moon in as little as a year--strong confirmation that the proposed scenario could have happened.