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 how the inside of an icy moon comes together. As the moons drew material
from the nebula
around them, bits of heavy rock gathered inside the forming moon, as illustrated in figure A.
Once the moon finished forming, these heavy, solid bits of rock fell into the center, as illustrated in figure B. Eventually, the rock 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 ice, shown in purple in figure B, to completely frozen ice, as shown in figure C.
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