This is a cutaway drawing of the proposed structure of Ganymede's interior.
Click on image for full size
As the formation of the planets came to an end, the interiors of some planets as well as some moons chemically separated into layers. Warming of the entire body is possible either with energy from all the bombardment of the surface by leftover debris, or by way of radioactive
elements within the planet or moon itself. If the body is warm enough, elements which comprise the planets and moons will part into separate regions, thus forming layers in the interior of a planet as shown in this picture. 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. The term for this process is differentiation
Differentiation is a scientific term which really means "to separate into layers". 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.
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