Accretion of Saturn's mid-sized moons during the viscous spreading of young massive rings: solving the paradox of silicate-poor rings versus silicate-rich moons

TL;DR

This paper investigates how Saturn's mid-sized moons could have formed from the viscous spreading of massive, icy rings, explaining their current positions, compositions, and geological activity through a new accretion model.

Contribution

It introduces a hybrid model demonstrating that mid-sized moons formed from icy ring debris, accounting for their locations and silicate content, and links their origin to ring formation over 2.5 billion years ago.

Findings

Moons can form from massive icy rings within a hybrid accretion model.

High dissipation within Saturn is needed to match current moon positions.

Silicate chunks could be embedded in moons, explaining compositional differences.

Abstract

The origin of Saturn's inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) and Saturn's rings is debated. Charnoz et al. (2010) introduced the idea that the smallest inner moons could form from the spreading of the rings' edge while Salmon et al. (2010) showed that the rings could have been initially massive, and so was the ring's progenitor itself. One may wonder if the mid-sized moons may have formed also from the debris of a massive ring progenitor, as also suggested in Canup (2010). However, the process driving mid-sized moons accretion from the icy debris disks has not been investigated in details. In particular, this process does not seem able to explain the varying silicate contents of the mid-sized moons (from 6% to 57% in mass). Here, we explore the formation of large objects from a massive ice-rich ring (a few times Rhea's mass) and describe the fundamental properties and implications of this new process. Using a hybrid computer model, we show that accretion within massive icy rings can form all mid-sized moons from Mimas to Rhea. However in order to explain their current locations, intense dissipation within Saturn (with Qp<2000) would be required. Our results are consistent with a satellite origin tied to the rings formation at least 2.5 Gy ago, both compatible with either a formation concurrent to Saturn or during the Late Heavy Bombardment. Tidal heating related to high-eccentricity post-accretional episodes may induce early geological activity. If some massive irregular chunks of silicates were initially present within the rings, they would be present today inside the satellites' cores which would have accreted icy shells while being tidally expelled from the rings (via a heterogeneous accretion process)while those still present in the rings are interpreted as today Saturn's rings' propellers and ring-moons (like Pan or Daphnis).