What magnetized the moon?

The earth has naturally occurring magnetized rocks appearing, in part, because of its moving internal core which gives it a massive magnetic field. The moon has no such core, but it too has magnetized rocks. Planetary scientists believe, though, that the moon has its own ways of creating magnets.

The internal motion of the earth's core stirs up the iron in the mantle of the earth, swirling it around enough that the magnetic fields it creates not only protect us from harmful radiation from space, but magnetizes rocks on the surface of the earth. The moon, considerably smaller than the earth, with no rotation of its own, should not have magnetized rock on its surface. And yet it does. Why?

The obvious answer, to me at least, is Moon Man and His Evil Magnetic Moon Mission, but since no one seems interested in the film rights to that, I suppose its acceptable to kick the question over to scientists. The scientists at UC Santa Cruz believe they have an answer. Since the moon first split from the earth, both are undergoing a long period of gradual estrangement, with the moon slowly moving away from the earth over billions of years, and still slipping away today. When the moon was close to the earth, its orbit was faster.

The core of the moon is filled with liquid. If the moon spun completely smoothly, this wouldn't cause a problem, but the rotation wasn't smooth. Instead, the moon wobbled a bit in its spin - a phenomenon known as precession. Thought the liquid spins with the moon, it doesn't precess in the same way. As the moon precessed, the liquid core sloshed just slightly inside of it, spinning out of the sync with the outer layers. This constant motion of solid outer surface and inner liquid could cause a magnetic field on the moon that could have lingered for a billion years before the moon slipped farther away, the rotation slowed down, and the motion shut off.

But if that's the case, how would we add in a comely Moon Maiden with her Lunar Lady Liberation Army? Science doesn't have the answers to everything.

Image: Lawrie Cate

Via Nature.