Formation of satellites from rings : the beautiful case of Saturn, and beyond

Aurelien Crida (Universite Nice Sophia Antipolis)

July 04, 2016

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Abstract: Saturn”s rings, like any astrophysical disc, spread. When the ice blocs that constitute the rings reach the Roche limit(*), they must aggregate into moonlets by self-gravity. These moonlets then migrate outwards, repelled by tidal interactions with the rings and Saturn. We showed with numerical simulations that the small and mid-sized moons of Saturn formed this way, which explains their composition and young age (Charnoz et al. 2010, 2011). Then, analytical calculations show this process generates satellites with a precise mass-distance relation, which matches that of the Saturnian system, confirming that it formed this way and explaining why Saturn”s satellites are further apart and larger, as one moves away from the rings (Crida & Charnoz, 2012). This relation also surprisingly fits the Uranian and Neptunian systems, suggesting that these planets used to have massive rings that gave birth to their regular satellites, and vanished. (*) The Roche radius is defined as the distance from a planet beyond which self-gravity is stronger than tidal forces, allowing gravitationally bound aggregates to form, and satellites to exist. Saturn’s rings are inside the Roche radius, which is why they don’t coalesce into one single object.