A new perspective on the irregular satellites of Saturn - I. Dynamical and collisional history

The dynamical features of the irregular satellites of the giant planets argue against an in situ formation and are strongly suggestive of a capture origin. Since the last detailed investigations of their dynamics, the total number of satellites has doubled, increasing from 50 to 109, and almost tripled in the case of Saturn system. We have performed a new dynamical exploration of Saturn system to test whether the larger sample of bodies could improve our understanding of which dynamical features are primordial and which are the outcome of the secular evolution of the system. We have performed detailed N-body simulations using the best orbital data available and analysed the frequencies of motion to search for resonances and other possible perturbing effects. We took advantage of the hierarchical Jacobian symplectic algorithm to include in the dynamical model of the system also the gravitational effects of the two outermost massive satellites, Titan and Iapetus. Our results suggest that Saturn's irregular satellites have been significantly altered and shaped by the gravitational perturbations of Jupiter, Titan, Iapetus and the Sun and by the collisional sweeping effect of Phoebe. In particular, the effects on the dynamical evolution of the system of the two massive satellites appear to be non-negligible. Jupiter perturbs the satellites through its direct gravitational pull and, indirectly, via the effects of the Great Inequality, i.e. its near-resonance with Saturn. Finally, by using the hierarchical clustering method we found hints to the existence of collisional families and compared them with the available observational data.