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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 a High-Energy Astronomer
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