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. This is a drawing of the evolution of the interior of an icy moon.
Titan is like other icy moons .
Titan formed the same way other planets did, as shown in this picture. Where Titan formed, the nebula was colder than it was near Jupiter. The nebula near Titan was so cold that even the molecule Nitrogen could become solid. Saturn is giving off heat, so it is warm enough for Nitrogen to evaporate and form the air of Titan. This is a drawing of the history of the inside of an icy moon.