July 1, 2004 | David F. Coppedge

Saturn Runs Rings Around Cassini

“Shocked” was how Carolyn Porco, lead Cassini imaging scientist, described her initial reaction to new pictures of Saturn’s rings.  Precious images began to pour in early July 1 from science observations right after the previous night’s perfect orbit insertion maneuver (see 06/30/2004 headline).  Even though the imaging team had been confident in the capabilities of Cassini’s cameras, Porco said she was surprised to be so surprised at the clarity and beauty of the results.  The spacecraft was as stable as a tripod as the narrow-angle camera snapped 61 sharp photos of the rings at closer range than ever before.  Portions of all the major rings – C, B, and A, along with the F ring and the Encke, Keeler and Cassini divisions – were photographed.  The highest resolution images were taken on the backlit side of the rings.  After the second ring plane crossing, Cassini turned and imaged the sunlit side for a few frames from a greater distance.  Though unable to resolve individual particles, Cassini’s CCD cameras detected remarkable structural detail:

  • The C ring was the most featureless of the rings, as expected.  (Note: horizontal lines are artifacts of noise not yet removed from the raw images.)
  • The B ring was the most opaque, also as expected, but showed vivid bands and fine ringlets.
  • The Cassini Division is filled with material, not an empty gap as seen in amateur telescopes.  Infrared measurements showed this material to contain more dirt than the main rings, which are composed almost completely of water ice; a press release states that the dirt resembles material seen on Phoebe.
  • The A ring had many classic examples of waves.  Density waves are longitudinal waves caused by orbital resonances with moons; they showed damping away from Saturn.  Spiral bending waves are transverse waves generated by interactions with the inclined moon Mimas; these showed damping toward Saturn.  One image showed both wave types in the same frame.  Another image showed fine structure that Porco described as “straw” in appearance; as yet, she could offer no explanation for what must be fine-scale clumping.
  • The Encke gap revealed the previously-imaged scalloped edge in beautiful detail.  The scalloping is a wake induced by the embedded moonlet Pan.  The image was so surreal that Dr. Porco thought at first her team must have been tricking her with a simulation.  Ringmaster Jeff Cuzzi also was stunned and taken by the beauty and clarity of this image, and how it so perfectly demonstrated the wave processes theorists had predicted.
  • The F ring showed several strands, and streaks of material being pulled toward the shepherd moon Prometheus.  The sunlit side of the F ring showed several bright strands with diffuse material between them.
  • Contrary to expectations, no ring “spokes” were yet seen by Cassini like the Voyagers saw 23 years ago.  JPL scientists are speculating the spokes are a seasonal phenomenon that appear when the sunlight hits the rings at a steeper angle.  The cosmic dust analyzer instrument did detect dust particles coming from the rings, however, which might be related to the spoke phenomenon.
  • Ultraviolet measurements showed that the Cassini Division and Encke Gap both have dirtier material than the main rings (see Space.Com).  This unknown material resembles the spectral characteristics of Phoebe.  The UV images were color-coded in dazzling patterns of maroon and turquoise like black-light posters.

Infrared measurements revealed that the rings are almost 99% water ice, higher than the 98% previously thought.  This argues against the rings having formed from the breakup of a rocky body.  Dr. Roger Clark (USGS), in answer to a reporter’s query whether ring particles might have rocky cores, answered in the negative; repeated collisions would have uncovered any rock remnants that the Cassini VIMS instrument would have detected.  The ring particles are almost pure water ice.  Differences in colors of the main rings come from a small proportion of contaminants.  Each main ring also has a distinct particle size distribution, temperature, density and thickness.
    Additional findings from other instruments such as the ion and neutral mass spectrometer and plasma spectrometer should be announced soon.  At a lecture a few hours after the images came down, ringmaster Andre Brahic explained that a few of these features match predictions from known physical processes exactly, but there is more structural detail seen than scientists can explain.  There are gaps without known moons to create them, for instance, and some predicted gaps cannot be found.
    Disk features like Saturn’s rings also show up on larger scales, like dust disks around stars, and the star-studded disks of spiral galaxies.  Understanding the rings of Saturn, therefore, can shed light on the dynamics of stars and galaxies.  Brahic cautioned against simplistic extrapolation of ring physics to large-scale disks, however, since there are significant differences between them, such as mass, viscosity, magnetic field effects, charge, velocity, gravitational influences, external influences, and composition.
    Many more ring observations will be taken over the next four years, including occultations by the radar and ultraviolet instruments that will provide even finer detail on the density and structure of the rings.  It’s clear already, however, that one day’s worth of data will keep scientists busy for years getting their models to match these exciting new observations from the ringed planet.

Brahic also stated that most ring scientists accept the notion that the rings are far younger than the solar system (assuming the rarely-challenged date of 4.5 billion years).  He said he preferred not to believe that for personal, not scientific, reasons.  He finds it hard to believe that the rings just appeared in time for humans to observe them.  Most of his energetic, humorous and informative talk concerned how resonances, collisions and interactions could induce the structures seen – which is fine, because those are all ongoing effects of present-tense, observable processes.  But how can these delicate features remain stable for long ages?  A shepherd can keep the sheep in line only so long if wolves are routinely picking them off and no new sheep are being added.  Brahic did not dwell on how ring material could be replenished during billions of years of gas drag, light pressure, collisional spreading, micrometeorites and ionic sputtering, though he acknowledged these destructive processes are at work in the rings.  This is a clear case of wanting to believe something without evidence, even in spite of the evidence.  Enjoy the rings in the age of man, because they are going, going, gone.

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