Letter | Published:

100-metre-diameter moonlets in Saturn's A ring from observations of 'propeller' structures

Naturevolume 440pages648650 (2006) | Download Citation



Saturn's main rings are composed predominantly of water-ice particles ranging between about 1 centimetre and 10 metres in radius. Above this size range, the number of particles drops sharply, according to the interpretation of spacecraft1 and stellar2 occultations. Other than the gap moons Pan and Daphnis (the provisional name of S/2005 S1), which have sizes of several kilometres, no individual bodies in the rings have been directly observed, and the population of ring particles larger than ten metres has been essentially unknown. Here we report the observation of four longitudinal double-streaks in an otherwise bland part of the mid-A ring. We infer that these ‘propeller’-shaped perturbations3,4,5 arise from the effects of embedded moonlets approximately 40 to 120 m in diameter. Direct observation of this phenomenon validates models of proto-planetary disks in which similar processes are posited4,6. A population of moonlets, as implied by the size distribution that we find, could help explain gaps in the more tenuous regions of the Cassini division and the C ring7. The existence of such large embedded moonlets is most naturally compatible with a ring originating in the break-up of a larger body8,9,10,11, but accretion from a circumplanetary disk12 is also plausible if subsequent growth onto large particles occurs after the primary accretion phase has concluded13,14.

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We thank E. Baker for help with data reduction, and P. Nicholson for discussions. We acknowledge support from JPL, the Cassini project, NASA's Planetary Geology and Geophysics program, and the UK Particle Physics and Astronomy Research Council.

Author information


  1. Department of Astronomy, Cornell University

    • Matthew S. Tiscareno
    • , Joseph A. Burns
    •  & Matthew M. Hedman
  2. Department of Theoretical and Applied Mechanics, Cornell University, New York, 14853, Ithaca, USA

    • Joseph A. Burns
  3. CICLOPS, Space Science Institute, 4750 Walnut Street, Colorado, 80301, Boulder, USA

    • Carolyn C. Porco
    •  & John W. Weiss
  4. Southwest Research Institute, 1050 Walnut Street, Colorado, 80302, Boulder, USA

    • Luke Dones
  5. Department of Astronomy, University of Maryland, Maryland, 20742, College Park, USA

    • Derek C. Richardson
  6. Astronomy Unit, Queen Mary, University of London, Mile End Road, E1 4NS, UK

    • Carl D. Murray


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Competing interests

Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Corresponding author

Correspondence to Matthew S. Tiscareno.

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    This file contains Supplementary Figures 1–4 and their legends. (PDF 2642 kb)

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