This is a cutaway drawing of the proposed structure of Ganymede's interior.
Click on image for full size
Also, prior to the official "beginning" of the Earth or start of the Archean age, the Earth turned inside out! That's right. Iron is heavy. As soon as the Earth became warm enough, the heavy iron which was part of the material which came together to form the Earth, fell to the center of the Earth and formed the Earth's core.
This means, in addition to forming a core, at this time in history, the surface of the Earth was very hot.
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