March 19, 2010 | David F. Coppedge

No Clear Explanation for Saturn’s Rings After 6 Years of Cassini

Those who hoped Cassini would solve the puzzles of Saturn’s rings should read the paper today in Science.1  The state-of-the-rings report, authored by a Who’s Who of ring scientists, is filled with questions.  Titled “An Evolving View of Saturn’s Dynamic Rings,” the report cannot determine their origin, their age, or their composition.  List the points of doubt after the initial word understanding in the abstract:

We review our understanding of Saturn’s rings after nearly 6 years of observations by the Cassini spacecraft.  Saturn’s rings are composed mostly of water ice but also contain an undetermined reddish contaminant.  The rings exhibit a range of structure across many spatial scales; some of this involves the interplay of the fluid nature and the self-gravity of innumerable orbiting centimeter- to meter-sized particles, and the effects of several peripheral and embedded moonlets, but much remains unexplained.  A few aspects of ring structure change on time scales as short as days.  It remains unclear whether the vigorous evolutionary processes to which the rings are subject imply a much younger age than that of the solar system.  Processes on view at Saturn have parallels in circumstellar disks.

Note: “evolutionary processes” refers here not to any Darwinian-like theory, but only to physical changes over time, such as the grinding down of icy particles or spreading of the rings.
    It’s not all bad news; certainly much has been learned from Cassini’s ringside seat and unprecedented views.  Scientists have watched the moons Pan, Pandora, Daphnis, Prometheus and others tug on ring particles.  The rings have been photographed in multiple wavelengths from radio to ultraviolet through seasonal changes.  Waves, spokes and impacts have been observed.  Structures large and small have been watched as they change.  The data set is rich – but the explanation set is impoverished.
    Among the main surprises are the dynamism of the ring particles.  “The very short lifetimes of particles in this size range [centimeter size] to various evolutionary processes suggest that sizes are determined by an active accretion-destruction cycle and are not primordial; thus, any radial variations indicate ongoing dynamics,” they said.  In other words, if the centimeter-sized particles had formed when Saturn is believed to have formed 4.5 billion years ago, they would be gone long before now – unless some process of accretion and destruction cycles them in and out of that size range.  Ring scientists now believe that self-gravity and inelastic collisions take particles through repeated cycles of clumping and dispersal.  This is very different from the picture of the rings prior to Cassini.
    The particles are moving very fast relative to Saturn – on the order of 20 km/sec, but collisions between them are very slow (0.01 to 0.1 cm/sec).  Like fluffy cars gently tapping one another going at the same speed, they damp each other’s motions, spread and circularize their orbits.  “Meanwhile, these small random motions are replenished by collisions and gravitational encounters with large particles and clumps of particles, ultimately deriving energy from the overall orbital motion.”  For practical purposes, the rings can be treated like a dense gas or fluid with its own viscosity and pressure.  Unlike a gas, though, the ring particles can clump and produce “self-gravity wakes” that appear ubiquitous throughout the dense parts of the rings.  The rings are also perturbed by spiral bending waves and density waves caused by gravitational perturbations from moons orbiting right outside the rings (and some within the rings).  These give the rings a corrugated appearance that was seen clearly at equinox when transverse waves cast long shadows.  “Some faint rings have changed appreciably since Voyager’s visit,” the authors noted (some of whom have been studying rings since the Voyager flybys in 1981).  “Both the D ring and inner C ring display a vertical corrugation that may have been generated only 25 years ago” by a large impact whose imprint is winding up over time.  The waves contribute to clumping and spreading of material.  Also, since the particles move otherwise independently at speeds determined by Kepler’s laws of motion (faster closer in, slower farther out), there is a “Keplerian shear” effect from the faster-revolving inner particles compared to those outside a given radius.
    Particularly surprising are dynamical effects that take place in matters of days or hours.  These are especially visible in the F ring, a thin bundle of ringlets outside the main rings, where streamers of material get pulled out when the small moon Prometheus passes by.  The motions are quasi-periodic but also have a chaotic component.  This means the patterns cannot be explained by simple shepherding models.  “Occasionally, more extraordinary events are observed,” the authors said.  “Within a few days, a ring sector’s brightness can double or triple after a sudden injection of dust.”  That suggests that incoming material from micrometeoroids is making the ice dirty.  If injections of dust can be observed now, how much dust would have been added after billions of years?  There’s even a moonlet about 5km in diameter that passes through the F ring on a regular basis.  “The primary core of the F ring has an eccentric, inclined orbit that precesses smoothly, maintaining its integrity in seeming defiance of the large distortions and variations present, and, like Uranus’s rings, avoiding differential precession as well.”  A few embedded moons 30-1200km in size have been observed.  “These may be members of a previously unseen population of larger bodies that serve as dust sources and that provide the mass needed to stabilize the ring’s orbit.” One wonders how long these dynamical processes can be sustained in such a tenuous, chaotic environment: “The F ring dramatically documents the difficulty of living near the edge of the Roche zone, where accretion and disruption are in continual combat.”  There are also diffuse, faint rings: the G ring with its dense arc, the E ring (fed by the geysers of Enceladus), and the newly discovered Phoebe ring (see 10/07/2009).  These are subject to disruptive forces from the solar wind, gravity and collisions.
    Two observations suggest the rings are young (at least far younger than the assumed age of Saturn).  Due to exchange of angular momentum, Mimas and the other close-in moons could not have maintained their presently-observed proximity for billions of years.  The other is the purity of the rings.  They are 90 to 95% water ice, with some reddish impurities of unknown origin – despite ongoing pollution from incoming micrometeoroids.  The authors estimate these two factors put limits on ring ages to about 1/10 the age of Saturn.  “These short lifetimes are problematic because the generation of the entire ring through disruption of a Mimas-size (or larger) parent is unlikely on this time scale.”  Therefore, they needed to find “loopholes” in the “young-ring arguments.”  The resonances with ring moons have been confirmed – the puzzle remains.  “Some flexibility in their implications for ring age may emerge if ring-moons periodically interact and perhaps temporarily destroy each other or are held up by much-sought-for, but as-yet-unidentified, resonances with exterior massive moons.”  Those two loopholes are appeals to unobserved factors.  How about the problem with impurities?  A loophole could be found if the rings are denser than thought.  That would give them the ability to absorb (and hide) some of the incoming pollution from micrometeoroids.  Unfortunately, “Firm mass measurements from density waves now blanket most of the rings, but the murky depths of the B ring may contain considerably more material than previously believed.”  That’s another appeal to an unobserved factor.  Even if there is considerably more material, it is not enough to keep the B ring pristine for 4.5 billion years (12/13/2007).  The thinner rings have even less time to stay clean.
    Are ring scientists better off than they were before Cassini?  Yes and no; the measurements are better than ever; “Yet, much of the ring’s structure—the irregular structure covering the B ring; the crisp, symmetrical, banding in the C ring; and the Cassini division itself—remains unexplained…. Far more needs to be done.”  By mission’s end in 2017 there will be hundreds of times more data than Voyager.  Examination of the flood of data is still in the early stages, they claimed.  “Explanations for the origin of Saturn’s rings,” they said in conclusion, “will remain unconvincing until we have understood the powerful dynamical processes that have formed, and continue to shape, these elegant structures on time scales reaching from yesterday to billions of years.”  At least they have job security.
The April issue of the journal Icarus has a special section on Saturn’s rings and icy satellites.  And late last year, a thick technical book by the Cassini scientists, Saturn from Cassini-Huygens, was published by Springer Link, with five chapters about the rings of Saturn.  Some of the authors of the paper commented on the rings in a JPL press release March 18.  Cassini enters its second extended mission this September, hoping to last into 2017.


1.  Cuzzi, Burns, Porco, Esposito, Spilker et al, “An Evolving View of Saturn’s Dynamic Rings,” Science, 19 March 2010: Vol. 327. no. 5972, pp. 1470-1475, DOI: 10.1126/science.1179118.

Today’s secular scientists just love the E-word.  The have to bring up evolution in everything.  Most of the problems these authors have with the rings is that they are married to billions-of-years thinking and cannot even begin to visualize thinking outside that rigid box.  While we are all fascinated by Saturn’s beautiful rings and see great value in trying to understand them (and the observational parts of this paper were terrific), there was a subtext evident that the authors were struggling to find ways to str-r-r-r-r-etch the rings into the moyboy cartoon (moyboy: millions of years, billions of years).  Many of these problems would evaporate if they thought outside the box.  The rings don’t just look young; they are young!  Why not?  Isn’t science supposed to follow the evidence where it leads?  No – they don’t need evidence any more.  They can postulate unobservable entities: hidden mass in the B-ring, “much-sought-for, but as-yet-unidentified, resonances with exterior massive moons” and clumps of ice that hide the pollution on their insides, where it cannot be observed.  Post-Voyager scientists were eager to find embedded moons that would supply the rings endlessly.  Except for a few examples at specific radii, these have not turned up.  The destructive processes that are observed (which they euphemize as “evolutionary processes” and “dynamical processes”) put serious upper limits on the age of the rings.
    One would think that these unmet expectations and stark realities would humble the scientists from stepping out on flimsier limbs, but no—one would think incorrectly.  One must learn the character of the Scientist.  The Scientist must be perceived as a Knower of the Deep Secrets.  The authors, therefore, did not hesitate to speculate about the origin of planets and stars.  Saturn has a disk of orbiting material; what can that tell us about how planets and galaxies evolved?  One would think very little, but again, we are dealing with Scientists.  They love to extrapolate far beyond their knowledge – and beyond their ability to observe, because they see it as their duty to tell the common folk how the universe, life and ultimate things came to be.  Do we see ring particles accreting into planets?  No.  Do we see them evolving little people on them?  No.  But while they’re speculating, they notice that some of the gaps in the rings have little bitty moons inside them.  Why, this must mean that an accreted planet can clear a gap for itself to avoid the giant sucking sound at the center (08/21/2009, 05/21/2009).  They can see that other embedded moons in Saturn’s rings do not have the mass to clear a gap.  This “holds the promise of directly observing processes analogous to the complex evolution of a protoplanet through a circumstellar disk,” they clucked.  Does one directly observe an analogy?  That’s a bizarre scientific logic.  They directly observe the rings, but nobody could watch the “complex evolution of a protoplanet.”  It would take too long, for one thing, and there aren’t any protoplanets in our solar system – just planets.  The scale between ring particles and planets differs by orders of magnitude.  If anything, the lack of ring particles clumping into larger and larger objects over time should put a stop to speculations that protoplanets (imaginary entities) would ever form in their fictional creative circumstellar disks, where destructive processes are observed – not acts of creation.  A reasonable scientific conclusion would be that circumstellar disk particles might also undergo continual processes of clumping and disruption, and planets would never form.  But saying that Saturn’s ring dynamics have “parallels with the processes active in protoplanetary disks” (with emphasis on the suggestive term protoplanet) is like saying that the processes at work in a Piper cub have parallels with the city-sized UFOs in Independence Day.  Maybe in Hollywood they do.  Scientists: Get real.  Stay real.

(Visited 12 times, 1 visits today)
Categories: Physics, Solar System

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.