This drawing illustrates the general composition of icy moons.
The Composition of Icy Moons
Not only ice but several different kinds of snow (crushed ice) have been found on the surfaces of these moons.
Scientists can only guess what the inside is made of, but when the Voyager missions flew by some the icy moons, measurements were made that showed that the inside of the moons might also made of ice.
The fact that the major moons are made primarily of ice suggests a certain path for the evolution of these bodies.
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