Where Did the Moon Come From?

Citations Over TimeTop 10% of 2005 papers

Abstract

The current standard theory of the origin of the moon is that the 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 1AU radius in the solar nebula while computer simulations are consistent with it approaching earth on a zero-energy parabolic trajectory. But how could such a large object form in the disk of planetesimals without already having collided with the earth at an earlier epoch before having a chance to grow large or before its or the earth's iron core had formed? We propose that the giant impactor could have formed in a stable orbit among debris at the earth's Lagrange point L4 (or L5). We show that such a configuration is stable, even for a Mars-sized impactor. It would grow gradually by accretion at L4(orL5), but eventually gravitational interactions with other growing planetesimals would kick it out into a chaotic creeping orbit which we show would likely cause it to hit the earth on a zero-energy parabolic trajectory.

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