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The recent formation of Saturn's moonlets from viscous spreading of the main rings


The regular satellites of the giant planets are believed to have finished their accretion concurrent with the planets, about 4.5 Gyr ago1,2,3,4. A population of Saturn’s small moons orbiting just outside the main rings are dynamically young5,6 (less than 107 yr old), which is inconsistent with the formation timescale for the regular satellites. They are also underdense7 (600 kg m−3) and show spectral characteristics similar to those of the main rings8,9. It has been suggested that they accreted at the rings’ edge7,10,11, but hitherto it has been impossible to model the formation process fully owing to a lack of computational power. Here we report a hybrid simulation in which the viscous spreading of Saturn’s rings beyond the Roche limit (the distance beyond which the rings are gravitationally unstable) gives rise to the small moons. The moonlets’ mass distribution and orbital architecture are reproduced. The current confinement of the main rings and the existence of the dusty F ring are shown to be direct consequences of the coupling of viscous evolution and satellite formation. Saturn’s rings, like a mini protoplanetary disk, may be the last place where accretion was recently active in the Solar System, some 106–107 yr ago.

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Figure 1: Mass of Saturn’s inner moons versus distance.
Figure 2: Time evolution of our model with σ0 = 400 kg m−2.
Figure 3: Comparing the mass distribution of the moonlets obtained in our simulation with observations.


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This work was funded by Université Paris Diderot and CEA/IRFU/SAp. The authors thank F. Bournaud, J. Burns, L. Dones, Z. Leinhardt and H. Throop.

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S.C. and J.S. designed the code and analysed the results, and A.C. was involved in the analysis of the results and provided critical contributions.

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Correspondence to Sébastien Charnoz.

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The authors declare no competing financial interests.

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Charnoz, S., Salmon, J. & Crida, A. The recent formation of Saturn's moonlets from viscous spreading of the main rings. Nature 465, 752–754 (2010).

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