Yesterday, news suggesting that the Earth once had a second, smaller moon, erupted onto the interwebs. The model is based on the fact that the moon has two sides - a dark, basalt covered side that faces Earth, and a lighter, more mountainous side made of anorthosite. How the moon obtained its dual sides is unknown, and a number of hypotheses have been put forth. This latest model is the result of computer simulations of the moon's formation.
Based on available data, it is very likely that the moon originated when a Mars-sized body collided with Earth. This collision would have happened very early on in Earth's history; it can be thought of as the final major coalescence of planetesimals that built up the Earth. During that collision, which is sometimes referred to as the Big Whack, the moon would have formed from the debris ejected out into space. Some models of this collision result in additional, smaller satellites forming along with the moon. If a second, smaller moon were to have formed, it would have only been gravitationally stable for a short period of time, geologically speaking. If it were to have slowly collided with the moon, that collision could potentially explain why the moon has its two different sides. It will be interesting to see how this hypothesis stands up to testing in the coming years.
And now to a related question: how would two moons have affected Archean Werewolves?