Giant Backward Ring Found Around Saturn

Saturn has a newly-discovered ring to add to its decor – the largest of all.  It’s so big, it makes Saturn look like a speck in the middle of it.  The ring, located at the orbit of the small outer moon Phoebe, is inclined 27 degrees and revolves backwards around Saturn.  This was announced today by Jet Propulsion Laboratory based on an advance-release paper in Nature.¹  The story was picked up by New Scientist, National Geographic, Science Daily and other popular news sources.
    The ring is very wide, but very sparse.  It was discovered by the Spitzer Space Telescope.  It would not be visible in normal light; that’s why the Cassini spacecraft did not observe it.  “The particles are so far apart that if you were to stand in the ring, you wouldn’t even know it,” said co-discoverer Anne Verbiscer.  The very cold “heat” emitted by the dust particles was detected by Spitzer’s infrared camera (see image).  A diagram shows the extent of the ring and its effects on neighboring moons.
    The team believes this dust ring could explain the dark side of Iapetus.  The Phoebe ring is formed by collisions.  Particles large enough to escape immediate ejection by the solar wind slowly spiral in toward Saturn.  Some hit Iapetus like bugs on a windshield.  Without knowing the particle size distribution precisely, the team could not know for sure how long it would take to coat the leading hemisphere of Iapetus with dark material.  They did some calculations:

Assuming (1) that Iapetus intercepts all this material and (2) that the ring population is currently near its long-term average, the accumulation rate is about 40 [micrometers per] Myr^-1.  Over the age of the Solar System,² deposition at this rate would bury the leading side of Iapetus to a depth of 20 cm.  The population of distant satellites, however, was probably much higher in the past, leading to more collisions, more debris and a cumulative thickness of material deposited on Iapetus that is probably measured in metres.

Photographs of Iapetus by Cassini showed a bright surface under the dark material that was easily excavated by small impacts.  The team believes smaller amounts of the material hit Hyperion (the next moon in) and even Titan.
    According to the paper in Nature, dust motes smaller than 3.5 micrometers will strike Saturn or its rings within 15 years, while those smaller than 1.5 micrometers will be rapidly ejected from the Saturnian system.  Larger particles 40 micrometers and up will begin to precess and form a torus around Saturn within a few thousand years at the orbit of Phoebe comparable to the one seen today.  Dust particles smaller than a centimeter will absorb sunlight and re-radiate it asymmetrically (the Poynting-Robertson effect), causing them to lose energy and spiral into Saturn in about 100,000 years (or less, depending on mutual collisions).  To maintain the ring, the researchers used a working average of 10 micrometers for particle sizes and estimated enough ring material to fill a 1 km diameter crater on Phoebe.  That is probably a lower limit.  Phoebe was seen by Cassini to be pockmarked with craters, some more like 60 km in diameter.
    The scientists compared Phoebe’s new ring with the gossamer rings around the inner moons of Jupiter, Thebe and Amalthea.  The Phoebe ring probably contains thousands of times more mass of material, they estimated.  It is also unique in that it is inclined to the orbit of Saturn and most likely shares Phoebe’s retrograde orbit.  “Although these exotic properties as well as its sheer size make the Phoebe ring unique among known planetary rings,” they said, “similar structures should also adorn the other gas giant planets.”

1.  Verbiscer, Skrutskie and Hamilton, “Saturn’s largest ring,” Nature advance online publication 7 October 2009 | doi:10.1038/nature08515.
2.  Assumed to be 4.5 billion years (4.5 x 10⁹ yr or 4.5Gyr).

This would make a good research project for someone not enslaved by the received wisdom on the age of the solar system.  Can this ring be sustained for 4.5 billion years?  Some uncertainties make it hard to constrain a definitive answer: the particle size distribution and impact rate among them.  Still, it should be possible to make a case for whether or not the Phoebe ring is a transient phenomenon we are “lucky” to observe now, or is sustainable in steady state for billions of years.  One must consider sources (factors that add material) and sinks (factors that remove material).
    The mass of dark material on Iapetus can provide additional information.  It should be possible to estimate the amount of dark material that would accumulate on Iapetus over 4.5 billion years given the ring mass as it is now.  Unfortunately, not knowing the particle size distribution allows for large differences in the answer.  Small particles are eliminated quickly.  Presumably, centimeter size particles and larger ones could orbit in the ring much longer before spiraling in.  One would also have to estimate the rate of collisions within the ring.  These would fragment the particles and eliminate them more rapidly.  It might be possible to work out upper and lower limits for the accumulated deposits with reasonable assumptions.  It would seem that much more material would have accumulated over billions of years than is observed now on Iapetus.
    Another source of information may come in 2014 when the James Webb Space Telescope is launched.  Its infrared cameras could follow up on Spitzer’s observations to determine if there is any short-term variability in the ring.  With the data currently available, and given other age-shattering discoveries in the Saturn system thus far (e.g., 03/26/2008, 02/02/2009) would anyone like to predict that the Phoebe ring is unsustainable for billions of years without ample application of theory-rescuing devices? (e.g., 02/06/2006, 12/13/2007).