Tilting Saturn. II. Numerical Model

We argue that the gas giants Jupiter and Saturn were both formed with their rotation axes nearly perpendicular to their orbital planes, and that the large current tilt of the ringed planet was acquired in a post-formation event. We identify the responsible mechanism as trapping into a secular spin-orbit resonance that couples the angular momentum of Saturn's rotation to that of Neptune's orbit. Strong support for this model comes from (1) a near-match between the precession frequencies of Saturn's pole and the orbital pole of Neptune, and (2) the current directions that these poles point in space. We show, with direct numerical integrations, that trapping into the spin-orbit resonance and the associated growth in Saturn's obliquity are not disrupted by other planetary perturbations.