Dynamical Evidence for a Late Formation of Saturn's Moons

TL;DR

This paper suggests Saturn's moons are much younger than the planet, likely formed about 100 million years ago from a debris disk, challenging the idea that they are primordial and explaining their current orbital configurations.

Contribution

It introduces a new formation scenario for Saturn's moons involving recent re-accretion from a debris disk triggered by orbital instability.

Findings

The Tethys-Dione 3:2 resonance likely did not occur in the past.

The moons' orbital inclinations indicate crossing the Dione-Rhea resonance.

Most craters on interior moons are from planetocentric impactors.

Abstract

We explore the past evolution of Saturn's moons using direct numerical integrations. We find that the past Tethys-Dione 3:2 orbital resonance predicted in standard models likely did not occur, implying that the system is less evolved than previously thought. On the other hand, the orbital inclinations of Tethys, Dione and Rhea suggest that the system did cross the Dione-Rhea 5:3 resonance, which is closely followed by a Tethys-Dione secular resonance. A clear implication is that either the moons are significantly younger than the planet, or that their tidal evolution must be extremely slow (Q > 80,000). As an extremely slow-evolving system is incompatible with intense tidal heating of Enceladus, we conclude that the moons interior to Titan are not primordial, and we present a plausible scenario for the system's recent formation. We propose that the mid-sized moons re-accreted from a disk about 100 Myr ago, during which time Titan acquired its significant orbital eccentricity. We speculate that this disk has formed through orbital instability and massive collisions involving the previous generation of Saturn's mid-sized moons. We identify the solar evection resonance perturbing a pair of mid-sized moons as the most likely trigger of such an instability. This scenario implies that most craters on the moons interior to Titan must have been formed by planetocentric impactors.