The birth of Saturn's baby moons

Abstract

Simulations show that Saturn's nearby moons, after forming on the outskirts of the planet's main rings, get pushed clear of them. This model reproduces the moons' orbital locations and remarkably low densities. A population of Saturn's smaller moons contrasts dramatically with the regular moons of the giant planets. Moons like Saturn's Enceladus and Jupiter's Europa orbit in the equatorial plane of their host planet and are thought to have finished their accretion at about the same time as the planets, 4.5 billion years ago. Saturn's icy moonlets are much younger, formed less than 10 million years ago, and as their spectra resemble those of the main rings it has been suggested that they formed by accretion at the rings' edges. A new hybrid numerical simulation of the Saturn system supports this idea, suggesting that the moons formed via viscous spreading of Saturn's main rings beyond the Roche limit (the distance beyond which rings are gravitationally unstable, about 140,000 km from Saturn). After the moons' formation, the edge of the ring migrated inward.