Late origin of the Saturn system

Saturn is orbited by a half dozen ice rich middle-sized moons (MSMs) of diverse geology and composition. These 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, 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 our 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 collisional mergers around Saturn, while Jupiter’s original satellites stayed locked in resonance.