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Origin of Saturn’s rings and inner moons by mass removal from a lost Titan-sized satellite


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The origin of Saturn’s rings has not been adequately explained. The current rings are more than 90 to 95 per cent water ice1, which implies that initially they were almost pure ice because they are continually polluted by rocky meteoroids2. In contrast, a half-rock, half-ice mixture (similar to the composition of many of the satellites in the outer Solar System) would generally be expected. Previous ring origin theories invoke the collisional disruption of a small moon3,4, or the tidal disruption of a comet during a close passage by Saturn5. These models are improbable and/or struggle to account for basic properties of the rings, including their icy composition. Saturn has only one large satellite, Titan, whereas Jupiter has four large satellites; additional large satellites probably existed originally but were lost as they spiralled into Saturn6. Here I report numerical simulations of the tidal removal of mass from a differentiated, Titan-sized satellite as it migrates inward towards Saturn. Planetary tidal forces preferentially strip material from the satellite’s outer icy layers, while its rocky core remains intact and is lost to collision with the planet. The result is a pure ice ring much more massive than Saturn’s current rings. As the ring evolves, its mass decreases and icy moons are spawned from its outer edge7 with estimated masses consistent with Saturn’s ice-rich moons interior to and including Tethys.

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Figure 1: Results of a satellite accretion simulation 6 that produced a Saturn-like system of satellites.
Figure 2: SPH simulation showing the tidal removal of ice from a differentiated, Titan-mass satellite.

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I thank W. R. Ward and L. Dones for detailed comments. Support from NASA’s Outer Planets Research Program is gratefully acknowledged.

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Correspondence to Robin M. Canup.

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Canup, R. Origin of Saturn’s rings and inner moons by mass removal from a lost Titan-sized satellite. Nature 468, 943–946 (2010).

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