Ringworlds: under the hood

Posted by Francis Reddy
on Tuesday, December 12, 2006

Larry Esposito knows planetary rings. In 1979, as a member of the imaging team for the Pioneer 11 Saturn flyby, he discovered the planet’s kinked and braided F ring. He’s also a member of the science team for the Cassini probe, now orbiting Saturn. Who better, then, to serve as guide on a tour of ringed planets?

Esposito’s Planetary Rings is not a book aimed at a broad audience — its $110 list price alone prevents that — but much of the text is accessible to general readers, and it is certainly worth a look. The first three chapters present a historical overview of the quest to explain ring systems and showcase the differences between the rings of Jupiter, Saturn, Uranus, and Neptune. Another chapter presents initial results from Cassini.

You can’t have rings without small moons, which slowly grind away or suddenly disrupt. They give rise to the dust-sized and larger particles that constitute rings. Observations of Jupiter’s main ring suggest it formed from a shattered satellite as recently as 1,000 years ago.

And then there’s Neptune, which sports not only rings, but incomplete ring arcs. The 1989 Voyager 2 flyby revealed four bright, discontinuous arcs embedded in the planet’s faint Adams ring. How can this arrangement be stable?“ In 1998, the situation seemed hopeless: Our best models were contradicted by Hubble and ground-based telescopes,” Esposito says. New theories since have appeared, but scientists will be unable to decide which one works without close-up observations. A return to Neptune is high on scientists’ short list of space missions to the outer solar system.

The particles in a planet’s ring are buffeted by collisions, electrostatic forces, tugs from gravitational resonances with large moons, density and bending waves, warps, chaotic interactions, and the gravity of embedded satellites. Such things must occur on a grander scale as planets grow around a star. Esposito notes that the computational methods brought to bear in understanding ringed planets has been extended to other cosmic systems.

Like many fascinated with space — myself included — it was Esposito’s first glimpse of Saturn through a telescope that sparked his lifelong interest. “What a wonderful surprise that the ringed planets are just as beautiful and scientifically compelling seen close up,” he says.

Wonderful, indeed.

The book in brief: Planetary Rings, Larry Esposito, 214 pages, Cambridge University Press, 2006; hardcover, $110.00

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