December 8, 2003 | David F. Coppedge

Keeping Planetary Rings Going for Eons

It’s common knowledge that planetary rings, like those at Saturn, don’t last forever (see 02/12/2002 headline), so scientists either have to find a way to keep them going, or admit that we live in a special period in the lifetime of the solar system to see them now.  That latter option is “philosophically unappealing” to most ring experts, like Larry Esposito of the University of Colorado, principal investigator for the Ultraviolet Imaging Spectrometer (UVIS) on the Saturn-bound Cassini spacecraft.
    Esposito and colleague Joshua Colwell have been thinking of ways to stretch out the admittedly short lifetime of rings (“a few hundred million years” as an upper limit, “the blink of an eye compared to the planets”), says EurekAlert.  They have a new theory: recycling.  They know that ring particles tend to degrade into a cascade of smaller and smaller particles, until as dust they get blown away.  But what if they might re-accrete?  Then “the lifetime of the ring system may be longer than we initially thought.”  He admits, “Without this recycling, the rings and moons are soon gone,” but their computer models show it might prolong the rings for billions of years.  Otherwise, their existence is a puzzle: “The question naturally arises why rings still exist, to be photographed in such glory by visiting human spacecraft that have arrived lately on the scene.”  For another review of this hypothesis, see Nature Science Update.
    The Cassini Imaging Team recently released a beautiful new picture of Saturn from 69 million miles away taken November 9, as the bus-sized spacecraft continues to close in on the ringed planet at over 36,000 mph.  The Cassini Portal website now has a countdown timer, ticking off the seconds till the big day of “Saturn Orbit Insertion” (SOI) on July 1, 2004.

Well, good luck with this model.  Even with the overtime, sooner or later you’ve got to end the game.  Much of the dust gets swept into the planet by gas drag, and those particles are not coming back.  Other dust is knocked out of the ring plane and out of orbit entirely by the impact of incoming micrometeoroids (which may be frequent, as shown by the near-continual phenomena of the spokes).  It is clear that much of the lost material is never coming back.  Rings are not forever.
    Cassini’s high approach over the ring plane next July 1 should provide some spectacular new data that may either support or weaken the new computer models.  Until then, this hypothesis appears to be little more than ad hoc fudging to get away from “philosophically unappealing” observations.

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Categories: Solar System

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