Moons made of ice can still be very different from each other. For example, Titan seems like the other icy moons, but scientists want to know "how come Titan is the only one that has a big atmosphere?" The answer may be that where Titan formed (next to Saturn), the nebula was colder than it was near Jupiter. The nebula near Titan was so cold that even the molecule Nitrogen could become solid.
This means Titan is probably made of Nitrogen ice! Saturn is giving off heat, so it is warm enough for Nitrogen to evaporate and form the air of Titan.
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