ScienceWeek

PLANETARY SCIENCE: ON SATURN'S RINGS

The following points are made by F. Spahn and J. Schmidt (Nature 2006 440:614):

1) The question of where Saturn's magnificent system of rings came from has intrigued planetary scientists for centuries. A currently favored thesis is that the flat disk of the main rings, which girdle the planet's equator, originated in the dispersion of material from the disruption of an icy satellite following the impact of a comet or asteroid[1,2]. Such a giant impact would have left behind debris in a broad range of sizes. But apart from two moons of kilometer size, only a main population of ice particles from a few centimeters to a few meters across has so far been deduced from remote sensing[3]. The detection of propeller-shaped brightness undulations in the rings, reported by Tiscareno et al[4], supplies the first evidence for large ring particles of between 40 and 120 meters in diameter. Their discovery bridges the size gap between the main population and the embedded moons.

2) The images on which Tiscareno et al[4] base their analysis were taken by the Cassini spacecraft, which is currently investigating the Saturn system. Two fundamental physical processes within Saturn's rings allow an embedded large boulder (or moonlet) to generate the kind of structure that the authors detect: gravity and collisions. Moonlet and ring particles both orbit in the strong gravity field of Saturn, so their mutual gravitational attraction will, contrary to intuition, act to scatter the particles away from the moonlet. So gravity tends to clear a gap around the orbit of the moonlet, and the width of this gap is proportional to the moonlet's size.

3) This process is, however, counteracted by frequent collisions among ring particles -- typically 10 to 100 per orbital revolution of the rings, lasting about 10 hours -- that jostle particles from high-density regions to the gravitationally depleted gaps. The stationary pattern that emerges between these two processes will depend on the size of the moonlet and the number density of the ring particles. If a body embedded in Saturn's A ring (the outer of the planet's two brightest rings, A and B) is larger than about 1 kilometer in diameter, its gravity will be strong enough to keep open a directly detectable gap around the ring's entire circumference. But for smaller moonlets, diffusion of particles as a result of collisions will close the gap at some distance from the moonlet. An incomplete, asymmetric gap, flanked by density enhancements, forms. This is the origin of the propeller pattern observed by Tiscareno et al[4,5].

References (abridged):

1. Dones, L. Icarus 92, 194-203 (1991)

2. Colwell, J. E. Planet. Space Sci. 42, 1139-1149 (1994)

3. French, R. G. & Nicholson, P. D. Icarus 145, 502-523 (2000)

4. Tiscareno, M. S. et al. Nature 440, 648-650 (2006)

5. Showalter, M. R. Nature 351, 709-713 (1991)

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