Late Origin of the Saturn System

Saturn is orbited by a half dozen ice rich middle-sized moons (MSMs) of diverse geology and composition (e.g. Smith et al. , 1981; Thomas, 2010; Schenk et al. , 2011) that comprise ∼4.4% of Saturn’s satellite mass. The rest is Titan, more massive per planet than Jupiter’s satellites combined. Jupiter has no MSMs. Disk-based models to explain these differences exist (e.g. Sasaki et al. , 2010; Canup, 2010; Mosqueira et al. , 2010a; Charnoz et al. , 2011) but have various challenges and assumptions. We introduce the hypothesis that Saturn originally had a ‘galilean’ system of moons, comparable to Jupiter’s, that collided and merged, ultimately forming Titan. Mergers liberate ice-rich spiral arms, in simulations, that self-gravitate into escaping clumps resembling Saturn’s MSMs in size and compositional diversity. We reason that MSMs were spawned in a few such collisions around Saturn, while Jupiter’s original satellites stayed locked in resonance. The dynamical validity of our scenario depends on whether some MSMs can be scattered or otherwise migrated to stable orbits following each collision, before they are accreted. If satellite formation concludes with a ‘late stage’ of giant impacts (e.g. Ogihara & Ida, 2012) then MSMs could have formed originally by this mechanism. More speculatively, solar-system-wide dynamical upheaval (e.g. Tsiganis et al. , 2005; Morbidelli et al. , 2009)might have triggered final mergers, leaving behind young MSMs and a dynamically excited Titan.