This is a cutaway of Ganymede's interior.
Click on image for full size
Interior of Ganymede
The diagram to the left shows a cutaway of the possible interior structure of Ganymede, based on recent measurements by the Galileo spacecraft. It shows a small core of metal, overlain with some rocky material, overlain still further with ice of various phases. Further measurements by Galileo suggest that, in addition, Ganymede may have a thin, salty-slushy layer near the surface, where electric currents are found. Ice cannot support electric current, but water can. (That is why people don’t put electric appliances into the bathtub!) An instrument on the spacecraft that is sensitive to current measured something that, like on Ganymede, suggested a watery layer was present on Callisto and Europa as well.
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! This provides further evidence that Ganymede is not completely frozen, but has a warm iron core. Over its history, Ganymede may have gone through a late period of warming, rather like Mars, and experienced some icy surface tectonism as a result.
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