1.1 Introduction
Although Titan is similar in terms of mass and size to Jupiter's moons Ganymede and Callisto, it is different in that it is the only one harboring a massive atmosphere. Moreover, unlike the Jovian system, which is populated with four large moons, Titan is the only large moon around Saturn. The other Saturnian moons are much smaller and have an average density at least 25 percent less than Titan's uncompressed density and much below the density expected for a solar composition (Johnson and Lunine, 2005), although with a large variation from satellite to satellite. Both Jupiter's and Saturn's moon systems are thought to have formed in a disk around the growing giant planet. However, the difference in architecture between the two systems probably reflects different disk characteristics and evolution (e.g., Sasaki et al., 2010), and, in the case of Saturn, possibly the catastrophic loss of one or more Titan-sized moons (Canup, 2010). Moreover, the presence of a massive atmosphere on Titan, as well as the emission of gases from Enceladus' active south polar region (Waite et al., 2009), suggest that the primordial building blocks that comprise the Saturnian system were probably more volatile-rich than those of Jupiter.
The composition of the present-day atmosphere, dominated by nitrogen, with a few percent methane and lesser amounts of other species, probably does not directly reflect the composition of the primordial building blocks and is rather the result of complex evolutionary processes involving internal chemistry and outgassing, impact cratering, photochemistry, escape, crustal storage and recycling, and other processes.