March 10, 2008 | David F. Coppedge

Saturn Moons Continue to Surprise Scientists

Just days before a long-awaited dive into the plume of Enceladus (see PhysOrg and JPL press release, flyby stats and news release), Cassini found another surprise in the Saturn system: a moon with rings.
    A Jet Propulsion Lab press release on March 6 reported that the large moon Rhea may have rings – the first ring system detected around a satellite of a planet.  The rings, composed of particles up to a meter in size, were not detected visually, but are inferred from electromagnetic effects detected by Cassini’s instruments.  Dips in electron density were seen on both sides of the moon, suggesting the presence of rings.  They apparently extend out eight moon diameters from Rhea.  See also the National Geographic story.  More detail is available at the Planetary Society blog, where Emily said the discovery was unexpected.
    The debris orbiting Rhea could have resulted from a comet impact in the moon’s past.  It appears to be organized into three ringlets and is composed of pebble to boulder-sized particles, based on inference of what material could cause the dips in electron density.  No small dust has yet been detected.  What is most mysterious is how the debris could remain there for millions or billions of years.  The original paper in Science1 said that boulders could remain in orbit for 70 million years, but that would only amount to 1/64 the assumed age of the solar system.  Richard Kerr commented on the discovery in the same issue of Science.2  He brought up some reasons why the rings shouldn’t exist:

But ring specialists still have their reservations.  Such rings, they say, are possible but improbable.  First, just the right sort of impact would probably have been required to blast material off the icy moon and into orbit.  Then the ring particles would have had to survive millions if not billions of years being torn apart by the tidal pull of Saturn and worn down to dust by eroding small impacts.  Most constraining, perhaps, is the “incredibly low” limit on dust around Rhea set by Cassini’s camera, says Joseph Burns of Cornell University, who is on the imaging team.  Ring boulders must shed some dust, and even tiny amounts of dust show up when backlit by the sun.

If the rings survive further observational scrutiny, Cassini scientists will have another age problem on their hands.  In addition to the geysers on Enceladus, the Rhea rings look like young phenomena that are supposed to be billions of years old, according to the scientific consensus.
    To prepare for the historic close encounter with Enceladus, watch the Enceladus Virtual Tour.  A map of the encounter is available, and a graph of the plume trajectory.  For more about the moon Enceladus, see the Enceladus main page and our reports from 02/17/2007, 07/11/2006 and 11/28/2005.  Geeks will drool over the Mission Description (PDF).  For latest news, see the new Enceladus blog begun on March 10 by the Cassini team.


1.  Jones et al, “The Dust Halo of Saturn’s Largest Icy Moon, Rhea,” Science, 7 March 2008: Vol. 319. no. 5868, pp. 1380-1384, DOI: 10.1126/science.1151524.
2.  Richard A. Kerr, “Electron Shadow Hints at Invisible Rings Around a Moon,” Science, 7 March 2008: Vol. 319. no. 5868, p. 1325, DOI: 10.1126/science.319.5868.1325.

Let’s all sing together:
My Rhea lies over at Saturn,
My Rhea has rings that can’t be;
Their real lies tell us it’s old stuff,
O bring back some Rheality.

    You Rhealize what’s going on, don’t you?  All observations must be forced into the old-age assumption, whether they fit or not.  This is known as science.
    Enceladus is the moon to watch for a reality check.  Come back Wednesday and Thursday this week for late breaking news about the dramatic flyby as soon as it is made public.  If you downloaded CASSIE 3D tour imager, you can zoom in on the encounter to visualize the history about to be made.  Keep up on the Enceladus blog, too.

Categories: Solar System

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