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Saturn's largest ring


Most planetary rings in the Solar System lie within a few radii of their host body, because at these distances gravitational accelerations inhibit satellite formation. The best known exceptions are Jupiter’s gossamer rings1 and Saturn’s E ring, broad sheets of dust that extend outward until they fade from view at five to ten planetary radii. Source satellites continuously supply the dust, which is subsequently lost in collisions or by radial transport. Here we report that Saturn has an enormous ring associated with its outer moon Phoebe, extending from at least 128RS to 207RS (Saturn’s radius RS is 60,330 km). The ring’s vertical thickness of 40RS matches the range of vertical motion of Phoebe along its orbit. Dynamical considerations argue that these ring particles span the Saturnian system from the main rings to the edges of interplanetary space. The ring’s normal optical depth of 2 × 10-8 is comparable to that of Jupiter’s faintest gossamer ring, although its particle number density is several hundred times smaller. Repeated impacts on Phoebe, from both interplanetary and circumplanetary particle populations, probably keep the ring populated with material. Ring particles smaller than centimetres in size slowly migrate inward2,3 and many of them ultimately strike the dark leading face of Iapetus.

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Figure 1: Spitzer MIPS imaging in the vicinity of Saturn.
Figure 2: A subset of the Spitzer MIPS 24 μm Basic Calibrated Data images, comprising a MIPSON scan.
Figure 3: Vertical profiles through the Phoebe dust structure.
Figure 4: The orbital distribution of dust grains launched from Phoebe followed for 2,000 years.


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This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech.

Author Contributions All authors contributed substantially to this work. A.J.V. and M.F.S. did most of the observation planning, data reduction, and associated write-up. D.P.H. contributed most of the dynamical interpretations and write-up.

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Correspondence to Anne J. Verbiscer.

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Verbiscer, A., Skrutskie, M. & Hamilton, D. Saturn's largest ring. Nature 461, 1098–1100 (2009).

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