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The Question

(Submitted March 10, 1998)

I am in third grade. I would like to know what the core of the moon is like. Is it magma like the Earth?

The Answer

We know that the Moon has a very weak magnetic field. Oddly enough, this is fairly strong evidence that it does not have a molten core. Rotating planets or moons with molten cores will produce magnetic fields through "the dynamo effect." In a planet like the Earth, the molten core can flow freely in a process called convection. In addition, the Earth rotates, adding to the movement of the molten core. The flowing molten iron-nickel material can produce electrical current, which, in turn produces a magnetic field that surrounds the Earth. If the Moons core were molten , then it would have a field too, though it would be weaker. We only a detect a very weak field, much weaker than that expected from the dynamo effect.

Since small "moonquakes" have been measured, which probably originate in the core of the Moon, it could be partially molten. But for the reason described above, it can not be totally molten.

Much of the information in this reply can be found in Michael Seeds' _Foundations of Astronomy_, and probably several other astronomy and geology books.

Steve Bloom
for Ask an Astrophysicist

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