This is a drawing of the evolution of the interior of an icy moon.
Image from: The New Solar System

How Titan Formed

Titan is similar to the other icy moons, but Titan is the only icy moon to have a big atmosphere. It is natural to ask how is this possible?

The nebula was colder near Saturn, than near Jupiter. The nebula near Titan was cold enough to allow the molecule Nitrogen to become solid.

Thus Titan is probably made of Nitrogen ice, and, because Saturn is giving off heat, it is warm enough for Nitrogen to evaporate to form an atmosphere.

The difference between Ganymede and Titan, is that Titan formed in a region cold enough for solid Nitrogen to form. The difference between Titan and Triton, is that Titan is presently in a region that is warm enough for the slow evaporation of Nitrogen.

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