November 1, 2010 | David F. Coppedge

Are Saturn Rings Like Galaxies?

Dramatic photos of dynamic processes in Saturn’s rings have been released by the Cassini mission at Jet Propulsion Laboratory.  The photos, taken near Saturn’s equinox last year, show ruffled edges of the B ring made of particles lofted as high as 2 miles above the ring plane, casting long shadows across the rings.  By watching the action over time, scientists have noticed waves of particles reflecting off the ring edges and oscillating in waves and arcs.  This is helping explain some of the complexity noticed 29 years ago when Voyager 1 first showed more processes at work than could be explained by resonances with nearby moons.
    Some of the scientists are extrapolating the interpretations far beyond the Saturn ring system.  Imaging team lead Carolyn Porco, for instance, thinks the observations provide “visibility into the mechanisms that have sculpted not only Saturn’s rings, but celestial disks of a far grander scale, from solar systems, like our own, all the way to the giant spiral galaxies.”  One counter-intuitive process is the effect of viscosity.  “Normally viscosity, or resistance to flow, damps waves,” said Peter Goldreich (Caltech), “…But the new findings show that, in the densest parts of Saturn’s rings, viscosity actually amplifies waves, explaining mysterious grooves first seen in images taken by the Voyager spacecraft.”
    The press release, accompanied by three Quicktime video clips showing the action, agreed that these phenomena, “the first large-scale wave oscillations of this type in a broad disk of material anywhere in nature,” are applicable to “spiral disk galaxies and proto-planetary disks found around nearby stars.”  The popular press eagerly snatched up this idea with headlines like, “Cassini sees Saturn rings oscillate like mini-galaxy” (PhysOrg) and “Saturn’s Shimmying Rings May Be Imitating Galaxy” (
    Anecdote about the mission: It looks like Hollywood has a new movie coming out that will feature the Cassini mission in a prominent role.  An interview with the writer-director of Quantum Quest can be found at New Scientist.

Saturn’s rings may elucidate processes in circumstellar disks and spiral galaxies, but those notions need to be proved, not assumed.  A circumstellar disk is six orders of magnitude larger than Saturn; a spiral galaxy, 12 orders of magnitude.  The particle sizes involved are correspondingly larger.  What other forces and influences lacking in Saturn’s rings might come into play at larger scales?  Are gravitational, electrical, mechanical, and magnetic influences comparable?  Perhaps.  Quantum mechanics should have taught scientists that one cannot recklessly extrapolate phenomena at all scales.  Scale-invariant laws may indeed operate throughout the universe.  If a system incorporates some that are scale-independent and some that are not, the physics gets messy.  Whether processes can be applied at all scales is as much a philosophical question as an observational one.  The phenomena in a galaxy, for instance, may be analogous without being physically the same.  We have no overriding reason to doubt that applying Saturn processes over 12 orders of magnitude are legitimate, but would that science reporters would ask these kinds of questions instead of acting like toadies, barfing out whatever the mandarins say.  It’s a bad habit that spills over into other subjects.

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