You may wonder how it is that scientists know what the inside of a planet is like.

The interior of a moon or planet can be closely determined from spacecraft navigation data when a spacecraft passes by or goes into orbit around a planet or moon. When a spacecraft goes into orbit, the planet or moon's gravity helps to pull it into a certain trajectory around the body. The trajectory of the spacecraft helps scientists determine the mass of the planet or moon.

The way the mass of a body is distributed 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 somewhat like that of an iceskater who spins faster when her arms are close to the body than when the arms are spread apart. The mass of an iceskater who's arms are spread apart is distributed differently than the mass of an iceskater who's arms are close to the body.

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.