Where Did the Moon Come From?

The current standard theory of the origin of the Moon is that Earth was hit by a giant impactor the size of Mars, causing ejection of iron-poor impactor mantle debris that coalesced to form the Moon. But where did this Mars-sized impactor come from? Isotopic evidence suggests that it came from 1 AU radius in the solar nebula, and computer simulations are consistent with its approaching Earth on a zero-energy parabolic trajectory. But how could such a large object form in the disk of planetesimals at 1 AU without colliding with Earth early on, before having a chance to grow large or before its or Earth's iron core had formed? We propose that the giant impactor could have formed in a stable orbit among debris at Earth's L4 (or L5) Lagrange point. We show that such a configuration is stable, even for a Mars-sized impactor. It could grow gradually by accretion at L4 (or L5), but eventually gravitational interactions with other growing planetesimals could kick it out into a chaotic creeping orbit, which we show would likely cause it to hit Earth on a zero-energy parabolic trajectory. We argue that this scenario is possible and should be further studied.