LAUSR

The origin of Saturn’s ~26.7° obliquity and ~100-million-year-old rings is unknown. The observed rapid outward migration of Saturn’s largest satellite, Titan, could have raised Saturn’s obliquity through a spin-orbit precession resonance with Neptune. We use Cassini data to refine estimates of Saturn’s moment of inertia, finding that it is just outside the range required for the resonance. We propose that Saturn previously had an additional satellite, which we name Chrysalis, that caused Saturn’s obliquity to increase through the Neptune resonance. Destabilization of Chrysalis’s orbit ~100 million years ago can then explain the proximity of the system to the resonance and the formation of the rings through a grazing encounter with Saturn. Description How did Saturn get its rings? Saturn’s rings are about 100 million years old, but it is unclear how they could have formed so recently. Wisdom et al. propose that the Saturn system previously contained an additional moon, the orbit of which was perturbed by the orbital migration of Titan, Saturn’s largest moon (see the Perspective by El Moutamid). The authors used numerical simulations to show that the perturbation would have eventually destabilized the system, scattering the additional moon. If it came too close to Saturn, the moon would have been ripped apart by tidal forces, forming the rings. The scenario explains several otherwise puzzling properties of Saturn’s tilt and the orbits of its other moons and is consistent with the measured age and mass of the rings. —KTS Saturn’s rings could have formed by ripping apart a former moon of the giant planet, scattered by orbital resonances.