This is an artist's conception of the magnetosphere of Ganymede. The trapping region or radiation belts are featured in orange.
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Magnetosphere of Ganymede
When the Galileo spacecraft flew by Ganymede, to everyone's surprise, it measured a very strong magnetic field for the first time near any moon.
To generate a magnetic field, there must be conducting material inside. This may be provided either by iron, or by salt water. The conducting material must also be in motion. Motions are usually provided by hot convective motions of the conducting material itself.
The existence of a magnetic field provides further evidence that Ganymede is not completely frozen, but has either a warm iron core or a salty, subterranian ocean. This is completely uncharacteristic of an icy moon, and makes Ganymede unique.
The presence of a magnetic field provides for a different kind of interaction with the charged particles in the magnetic environment of Jupiter. Such an interaction might even include aurora.
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