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Saturn’s F ring and shepherd satellites a natural outcome of satellite system formation


Saturn’s F ring is a narrow ring of icy particles, located 3,400 km beyond the outer edge of the main ring system. Enigmatically, the F ring is accompanied on either side by two small satellites, Prometheus and Pandora, which are called shepherd satellites1,2,3. The inner regular satellites of giant planets are thought to form by the accretion of particles from an ancient massive ring and subsequent outward migration4,5,6,7. However, the origin of a system consisting of a narrow ring and shepherd satellites remains poorly understood. Here we present N-body numerical simulations to show that a collision of two of the small satellites that are thought to accumulate near the main ring’s outer edge can produce a system similar to the F ring and its shepherd satellites. We find that if the two rubble-pile satellites have denser cores, such an impact results in only partial disruption of the satellites and the formation of a narrow ring of particles between two remnant satellites. Our simulations suggest that the seemingly unusual F ring system is a natural outcome at the final stage of the formation process of the ring–satellite system of giant planets.

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Figure 1: Formation process of Saturn’s ring–satellite system.
Figure 2: Time series of the spatial distribution of particles in a case of partial disruption of colliding satellites.
Figure 3: Mass distribution in the quasi-steady state as a function of the difference in the initial semimajor axes of the colliding satellites.


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This work was supported by JSPS Grants-in-Aid for JSPS Fellows (15J02110) and for Scientific Research (15H03716). R.H. is grateful for support from the UnivEarthS Labex program at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02) and IPGP. Part of the numerical simulations were performed using the GRAPE system at the Center for Computational Astrophysics of the National Astronomical Observatory of Japan, and visualization of simulation results was performed in part using Zindaiji 3. We thank L. W. Esposito for comments on the manuscript and H. Daisaka for support in simulations.

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Both R.H. and K.O. designed the study, discussed the results, and wrote the paper; R.H. performed simulations, analysed results, and produced the figures.

Corresponding authors

Correspondence to Ryuki Hyodo or Keiji Ohtsuki.

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The authors declare no competing financial interests.

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Hyodo, R., Ohtsuki, K. Saturn’s F ring and shepherd satellites a natural outcome of satellite system formation. Nature Geosci 8, 686–689 (2015).

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