This drawing illustrates the general composition of icy moons.

The Composition of Icy Moons

When the Voyager missions flew by some the icy moons, they took the spectra of the surface. The spectra matched that of ice and also several different kinds of snow (crushed ice).

The Voyager spacecraft could not measure the interior of the moons, however. Instead they measured the density. The density of iron or rock is between 3 and 5. The density of ice is 1. Voyager measured the densities to be about 1.2 (refer to the reference information for exact measurements of the icy moons), which is closer to that of ice than rock, so most of these moons must be made out of ice.

The fact that they are made primarily of ice suggests a certain path for their evolution.

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