There are many instances in planetary science where the same material can come in different forms. An example of one such instance is shown in this picture. A diamond is one form of pure carbon, and graphite (what goes into pencil lead) is another form of the same material. They are both made of the identical substance (carbon), but they have a very different size and structure. The difference in molecular construction is referred to as the "phase" of carbon. Graphite is a different phase of carbon than that of diamond.
One might be able to imagine two different moons made out of the same amount of carbon, but one moon made of graphite and the other made of diamond. The two imaginary moons would have very different sizes! Then one might imagine one moon evolving into the other, and having to change sizes as it does so. This phenomena explains the evolution of the icy moons. Ice comes in nine different phases, and all of them have a different molecular construction. In it's evolution, an icy moon may change many times from one phase to another, or include many different phases in it's interior.
Two other materials, from the earth's interior, which exhibit similar phase differences are olivine and spinel.
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