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. The fact that Callisto never
separated, early in its
history, probably means that Callisto never warmed up after formation.
At the same time, there were some measurements that suggested that there might be a thin salty-slushy layer of watery-ice that creates current. Water supports current. That is why people don’t drop electric appliances into the bathtub! An instrument on the spacecraft that is sensitive to current measured something that, like on Callisto, suggested a watery layer was present 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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