This is a drawing of the evolution of the interior of an icy moon.
Click on image for full size
Image from: The New Solar System
How the Interior of Icy Moons Formed
The drawing shows a possible path in the evolution of the interior of icy moons. As the moons drew material
from the nebula
around them, bits of heavy, solid material accumulated inside the forming moon, as illustrated in figure A.
Once the moon finished forming, these heavy, solid bits of material fell into the center, as illustrated in figure B. Eventually, the heavy, solid material at the center solidified into a core, as illustrated in figure C.
The ice on the outside of the core gradually changed from warm, slushy material, shown in purple in figure B, to frozen ices of various phases, as shown in figure C.
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