Saturn has a large number of moons, some of which are locked in orbital resonances with each other. Resonant moons closest to the rings include the Janus-Epimetheus pair that are in the 1:1 coorbital resonance of the horseshoe type, and Mimas, which is in 4:2 inclination-type resonance with larger moon Tethys. The orbits of Saturn’s moons are expected to evolve due to tides and ring torques, with Mimas and Janus expected to be among the fastest-evolving moons. Here we use numerical simulations to explore how these resonant pairs may have evolved in the geologically recent past, measured in millions rather than billions of years.
Mimas-Tethys resonance has a large libration amplitude, which is usually taken as evidence of a large pre-resonance inclination of Mimas. However, this scenario makes the capture of Mimas into the present resonance unlikely. Here we show that the amplitude of libration in the Mimas-Tethys resonance can be greatly affected by divergent passage of Pandora and Mimas through their 3:2 corotation-type resonance. We tentatively conclude that this resonance happened only a few million years ago, and that tidal evolution of Mimas must dominate over ring-torque-driven migration of Pandora.
The coorbital moons Janus and Epimetheus are located between 2:1 resonance with Enceladus and 4:3 resonance with Mimas. Many hypotheses of the past evolution of Mimas and Enceladus propose them crossing their 3:2 resonance in the past, which would require the coorbitals to cross a first-order resonance with either Mimas or Enceladus. We find that either resonance would destroy the coorbital configuration (both Trojan or horseshoe), and that a single progenitor would not be able to get past 2:1 resonance with Enceladus. Our current hypothesis is that the progenitor of the coorbitals and Mimas divergently crossed their 4:3 resonance, after which the single progenitor split into two coorbital bodies. In this view, the horseshoe configuration is only a couple of tens of Myr old. We will present our ongoing research at the meeting.