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A belt of moonlets in Saturn’s A ring


The origin and evolution of planetary rings is one of the prominent unsolved problems of planetary sciences, with direct implications for planet-forming processes in pre-planetary disks1. The recent detection of four propeller-shaped features in Saturn’s A ring2 proved the presence of large boulder-sized moonlets in the rings3,4,5. Their existence favours ring creation in a catastrophic disruption of an icy satellite rather than a co-genetic origin with Saturn, because bodies of this size are unlikely to have accreted inside the rings. Here we report the detection of eight new propeller features in an image sequence that covers the complete A ring, indicating embedded moonlets with radii between 30 m and 70 m. We show that the moonlets found are concentrated in a narrow 3,000-km-wide annulus 130,000 km from Saturn. Compared to the main population of ring particles6,7,8 (radius s < 10 m), such embedded moonlets have a short lifetime9 with respect to meteoroid impacts. Therefore, they are probably the remnants of a shattered ring-moon of Pan size or larger2, locally contributing new material to the older ring. This supports the theory of catastrophic ring creation in a collisional cascade9,10,11,12.

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Figure 1: Four new propellers in Saturn’s A ring seen by Cassini.
Figure 2: Close-up view and re-projection of two propeller structures.
Figure 3: Nearly linear spatial scaling of the features points to moonlet wakes.
Figure 4: Cumulative size distribution of particles in the moonlet belt region.


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We acknowledge the efforts of the Cassini ISS team in the design and operation of the ISS instrument. This work was supported by the Cassini project, Deutsches Zentrum für Luft und Raumfahrt, Deutsche Forschungsgemeinschaft and the Academy of Finland.

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Correspondence to Miodrag Sremčević.

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The file contains Supplementary Methods with additional references, Supplementary Tables S1-S3 and Supplementary Figures S1-S16 with Legends. (PDF 4445 kb)

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Sremčević, M., Schmidt, J., Salo, H. et al. A belt of moonlets in Saturn’s A ring. Nature 449, 1019–1021 (2007).

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