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Science 17 March 2006:
Vol. 311. no. 5767, p. 1517
DOI: 10.1126/science.311.5767.1517c
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This Week in Science

Dark radial streaks or spokes in Saturn's main B-ring were first seen with the Voyager space probes, and later by the Hubble Space Telescope. In 1998, they faded from view from the Earth as Saturn's rings became oriented edge on. Contrary to expectations, the spokes remained absent even when the Cassini spacecraft flew close to the rings in 2004 but then reappeared faintly in September 2005. These latter findings suggested that the spokes are intermittent features whose presence depends on the rings' angle to the Sun. Mitchell et al. (p. 1587) use Cassini data to model the formation of spokes as charged dust particles are lifted into the plasma above the ring plane by electrostatic forces. They find a sharp switch in the spokes' visibility, such that they disappear abruptly when the rings are open to the Sun, and also predict when the spokes are likely to appear clearly.




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Science. ISSN 0036-8075 (print), 1095-9203 (online)