This is a drawing of the evolution of moons.
Image from: The New Solar System

How Titan Formed

In many respects Titan is similar to the other icy moons, but Titan is the only icy moon to have an atmosphere that can be compared to other planets'. It is natural to ask how is this possible since Titan sits in the same position relative to Saturn that Ganymede sits relative to Jupiter?

Just as the planets became more and more icy the further from the sun, moons seem to be more and more icy the further from the planet they orbit. The nebula was colder in the vicinity of Saturn, than in the vicinity of the Gailiean satellites. Evidently the nebula near Titan was cold enough to allow the molecule Nitrogen to condense to solid form.

Thus Titan is probably made of Nitrogen ice. Because Titan is near Saturn however, which is giving off heat, it is warm enough for the evaporation of Nitrogen to form an atmosphere.

The difference between Ganymede and Titan, is that Titan formed in a region cold enough for the condensation of Nitrogen. 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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