Titan

"Although Titan is similar in terms of mass and size to Jupiter's moons, Ganymede and Callisto, it is the only one harboring a massive atmosphere. Moreover, unlike the Jovian system 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% less that 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 Jupiter's"--