Tidal stripping of a pre-existing moon explains the origin of Saturn’s rings and planetocentric impactor population

The origin of Saturn’s rings, which could be as young as ~100 Myr and composed of nearly pure ice, has been debated. One hypothesis is that the rings formed through the recent tidal disruption of a pre-existing Iapetus-mass moon, Chrysalis, after a close encounter with Saturn. However, the mechanism by which this encounter would have formed the rings remains largely unexplored, in particular, whether Chrysalis could supply ring material of the desired mass and composition. We performed SPH simulations to investigate the tidal response of Chrysalis during close encounters with Saturn. Our results demonstrate that preferential tidal stripping of the ice mantle from a differentiated moon can produce rings with both mass and composition resembling the present rings—provided that the closest encounter occurs between the parabolic Roche limits for ice ~1.53Rs and rock ~1.07Rs. Following close encounters, the rocky remnant of Chrysalis would have been removed either by collision with Saturn or ejection onto a hyperbolic orbit. We then performed N-body simulations to investigate the long-term dynamical evolution of the tidal debris. The debris initially occupies eccentric and inclined orbits, which undergo rapid precession and disperse into a torus-like structure in ~0.1 kyr, and finally damp their eccentricities and inclinations by mutual collisions over a few kyr. As the system evolves, some debris would cross the orbits of the regular moons and collide with them at relatively low velocities as ~5 km/s. Compared with heliocentric impactors, these low-velocity impacts are more likely to produce the unusual elliptical craters which have been observed on Mimas, Tethys, and Dione. These findings support the hypothesis that Saturn’s rings could have originated from a recent lost moon, and imply a highly dynamical evolution of the Saturnian system over the past few hundred Myr.