Looking up at the moon on clear cold nights such as these I don’t often stop to think about how it was formed. The life cycle of stars gets a lot of attention in science class, as do the geological ages of the earth, but what about the poor old moon? If I did ever wonder about its birth, I suppose I just assumed it was leftover material from the Big Bang or some other celestial event, and that it eventually got caught in the Earth’s gravitation net. It turns out that this idea is in fact one of four long-debated explanations. It even has a name: The Capture Theory. The problem with this, according to those who study such things, is that captured bodies (cooled and hardened long ago) aren’t usually as spherical as our moon–they’re just hunks of rock, like those found in the Asteroid Belt. A second theory, called Co-formation, posits that the moon and the earth both coalesced at roughly the same time, of roughly the same material, and that the moon formed close enough to stick around. Co-formation is problematic, however, because we know the earth is far denser than the moon. A third and less-often discussed theory, called the Fission Hypothesis, suggests a naturally occurring “georeactor” went nuclear during the formation of the Earth and produced enough energy to spit out a blob that eventually cooled into the moon. This isn’t a new theory, in fact its origins trace all the way back to George Darwin, son of you know who. And amazingly enough, evidence from Oklo, in West Africa, provides some precedent and proof that uranium-fed natural fission events have actually occurred. Nobody, however, has been able to put forth a robust enough model of this explosion to satisfy the scientific community. This leaves the fourth and prevailing theory about the moon’s creation, which is much more violent and scary than the other three. The Giant Impact Hypothesis (aka The Big Splash), says that sometime in the early life of our planet another Mars-sized body (dubbed “Theia” after the mythical Greek Titan and mother of the moon goddess, Selene) collided with Earth with a force great enough to return the entire planet to a sea of molten lava. Parts of Theia were then subsumed by this soup and the rest spun off to form the moon. As crazy as this idea sounds, it has by far the most scientific support. Early last year, a research team from Harvard took the theory one step further by identifying isotopic anomalies beneath the Earth’s crust which suggest that the entire planet didn’t melt, as long thought, and that remnants of the first Earth are still there, deep down. “This implies that the last giant impact did not completely mix the mantle and there was not a whole mantle magma ocean,” say lead researcher Sujoy Mukhopadhyay. Good to know, I suppose, that there might be a rock or two to surf on in case it ever happens again.