This drawing shows the possible interior of Callisto.
NASA
Interior of Callisto
When the Galileo spacecraft flew by Callisto, it made measurements which showed that Callisto was made almost entirely of the same thing. This means that Callisto never separated into layers, but is probably formed of a unusual material which is made of rock and ice combined.
At the same time, there were some measurements that suggested that there might be a thin salty-slushy layer of watery-ice right under the solid surface of Callisto that creates an electric current. This watery-ice layer is thought to be on Ganymede and Europa as well. This is a picture that shows the interiors of Io, Europa, Ganymede and Callisto together.
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