This is an artist's conception of the magnetosphere of Ganymede.
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Magnetosphere of Ganymede
When the Galileo spacecraft flew by Ganymede, it found something that was a big surprise to everyone -- a very strong magnetic field. This is the first time a strong magnetic field had been found 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. This means that Ganymede is not completely frozen, but has either a warm iron core or a salty, hideen ocean. This is completely new idea for an icy moon, and makes Ganymede unique.
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