Titan has always stood out as the only moon in the solar system with a substantial atmosphere. In fact, the surface pressure on Titan is 50 percent greater than the pressure on Earth. [Photos: The Rings and Moons of Saturn]
The main ingredient of Titan's atmosphere is nitrogen, just as it is on Earth. Where this nitrogen came from has long been debated. For instance, it could be primordial, accumulating as Titan formed, or it could have originated later.