Ring formation around giant planets by tidal disruption of a single passing large Kuiper belt object II: The dynamical fate of tidal fragments

Abstract

Planetary rings are ubiquitous structure in our Solar System, but their formation mechanisms remain under debate. One of the proposed scenarios is the tidal disruption of a nearby passing body that enters within a planet's Roche limit, producing fragments that are gravitationally captured and finally form the rings. In this study, we investigate the detailed dynamical path and fate of such tidally captured fragments using direct Nbody simulations including collisional fragmentation with analytical arguments. Focusing on Saturn as a representative case, we explore how the inclination iTD and pericenter distance qTD of the orbit of the passing body control the subsequent orbital evolution, collisional grinding, and the survival of fragments mass. Our simulations show that initially highly eccentric and inclined fragments experience differential precession driven by the planet's J2 potential, followed by destructive high-velocity collisions that damp their eccentricities and inclinations. The timing and pathway of this evolution strongly depend on iTD, modifying the dynamical picture proposed in the previous work. For low to moderate iTD, a narrow, circular and equatorial rings finally form whose orbital radius is well predicted by an analytically derived equivalent circular radius based on the conservation of the vertical component of angular momentum. In contrast, for high iTD, collisional damping causes a substantial fraction of the material to fall onto the planet, preventing the formation of a massive ring. We compile our results of Nbody simulations with the analytical predictions on (qTD, iTD) parameter space and specify the parameter region where sufficient mass to form Saturn's present rings and inner satellites survives.