This is a drawing of the interior of Jupiter's moon Ganymede.
Click on image for full size
NASA
How do we know what the inside of a Planet or Moon is like?
You may wonder how it is that scientists know what the inside of a planet is like.
The way the mass of a body is spread inside the body affects how the body spins in space. If the body has a large core, it will spin with a certain speed, if it has no core it will spin at another rate of speed. The affect is like that of an iceskater who spins faster when her arms are close to the body than when the arms are spread apart.
By studying the rate of spin of a body, as well as determining the mass of the body, scientists can figure out if the body must have a core and how large that core must be.
The picture shown here is that of Jupiter's moon Ganymede, and illustrates that spacecraft measurements were able to determine that the moon has at least two layers inside, besides the surface crust (scientists think that there are really three layers). Scientists must still use theories to estimate exactly what the layers are made of.
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