This is a cutaway drawing of the proposed structure of Ganymede's interior.
Click on image for full size
Differentiation is a scientific term which really means "to separate". In their earliest history, elements which comprised the planets and moons would part into separate regions, if the body was warm enough. This is the akin to the process whereby an oil & vinegar salad dressing will part into regions made solely of oil and solely of vinegar.
Planetary elements which separate include the heavy iron and the lighter silicate rock. Iron falls to the center of a planet and forms a core, silicate material might separate further into silicate-rich and silicate-poor regions.
Earth/Mars are examples of contiguous planets which did/did not differentiate early in their histories. Ganymede/Callisto are examples of contiguous moons which did/did not differentiate early in their histories. The lack of differentiation may say something about how warm these planets were upon formation.
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