Saturn's moon Titan belongs to a very select club within the solar system. It is one of only four "terrestrial" planets or moons -- those with solid bodies, as opposed to those made largely of gas, like Jupiter and Saturn -- that has a substantial atmosphere. The other three that wear blankets of gas are Venus, Mars, and our own Earth. Why just these four? Why not also Mercury, or Jupiter's biggest moons, or our own moon? How did those lucky four come by their atmospheres? Find out more in this article from NOVA Online.