Exploratour - Evolution of the Solar System
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NASA/Hubble Space Telescope
The cooler temperatures also influenced why the icy moons may be different at the different planets as the planets get further from the sun.
For example, Titan is the only icy moon to have an atmosphere that is big enough to be compared to a planets atmosphere. 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? (Why doesn't Ganymede also have an atmosphere? Or Triton, a moon of Neptune?) The answer is that the nebula was colder in the vicinity of Saturn, than in the vicinity of the Galilean satellites. Evidently the nebula near Titan was cold enough to allow the nitrogen to condense to solid form.
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, causing Titan to be the only icy moon to have an atmosphere.
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