Origin Of Saturn’s Rings Via Tidal Stripping From A Primordial Massive Companion To Titan
Robin M. Canup
Southwest Research Institute.
Saturn’s main rings are > 90% water ice by mass. Because bombardment of the rings by micrometeoroids increases their rock content over time, the rings’ current composition implies that they were essentially pure ice when they formed, a much different composition than the roughly half-rock, half-ice mixture expected for a solar abundance of solids. The two leading ring origin theories involve the collisional disruption of a small moon, or the tidal disruption of a comet during a close passage by Saturn. However, disruption of a small moon would generally lead to a mixed rock-ice ring, while tidal disruptions of comets would occur much more often at Jupiter, Uranus and Neptune than at Saturn (Charnoz et al. 2009). I here explore a new alternative. Saturn’s sole large satellite contrasts notably with Jupiter’s four Galilean satellites. Canup & Ward (2006) propose that Saturn has only one large satellite because large satellites interior to Titan spiraled into Saturn due to density wave interactions with the gaseous protosatellite disk at the end of the satellite formation era. As a large, Titan-sized satellite approached Saturn, it would likely be differentiated due to the combination of the energy of its formation and strong tidal heating. Planetary tidal forces then preferentially strip mass from the satellite’s outer layers prior to its collision with Saturn, leading to the production of a massive, pure ice ring. Over time the ring viscously spreads, its mass decreases, and icy moons are spawned from its outer edge. In this way, ice rings and ice-enhanced inner moons originate as a primordial byproduct of the same process that produces Saturn’s regular satellite system, removing the need to invoke a later, and potentially low-probability, ring forming event.
Support from NASA’s Outer Planets Program is gratefully acknowledged.
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