Saturn's rings: compositional and dynamical evolutions as markers of their young or old age.

The age of Saturn's rings is subject of a vivid debate, with arguments based on their almost pure water ice composition arguing in favour of young rings (100-400 Myrs) and arguments based on dynamics favouring old, primordial rings (4.5 Gyrs) (see Crida et al., 2025, for a review).

Here, we will present the work from Ricerchi and Crida (2026) which addresses this issue. Our fundamental rework of the gravitational focusing reveals that it had been over-estimated by a factor 5 in the literature. Moreover, we introduce the uncertainty on key parameters of the dust deposition (fraction accreted, vaporisation at impact, space weathering). This yields to the conclusion that any exposure age is virtually possible making the argument in favour of young rings from their composition irrelevant.

Furthermore, we use the A-ring to study if its current shape can live over few hundred millions years under viscosity, the bombardment flux, the Yarkovsky effect introduced by Zhou et al. (2026), and resonances. We find that the A-ring drifts inwards in a timescale much shorter than a hundred million years. In contrast, its present confinement by the 7 : 6 Lindblad resonance with Janus shows the ring material wants to drift outwards, at least in the outer part.

This suggests that the bombardment is over estimated, or that its present value is not representative of the bombardment history of the Solar System. The determination of physical parameters using the present state of the A-ring opens the way for a global modelisation of the rings over billions of years.