Ring Around the Moons

51; Saturn is known for its rings, and some small moons have been found inside its rings.  But wouldn’t it be strange if some of its moons had rings of their own?  Such a thing had not been widely considered before 2007, when there was a tentative detection of a ring around Rhea (see 03/10/2008).  That ring has not been confirmed by subsequent observations.  The thinking now, however, is that it used to be there – and not only there, but there might have been one around Iapetus, too.

• Rhea ring tones:  Paul Schenk, planetologist at Lunar and Planetary Institute in Houston and stereo animation wizard, was glad to be able to share his secrets at his blog 3D House of Satellites now that his paper in Icarus was published.¹  The paper concerned color patterns on Saturn’s inner moons Mimas, Enceladus, Tethys, Dione, and Rhea.  All of them show evidence of plasma stains and electron discolorations from the magnetosphere and E-ring, some on the leading hemispheres and some on the trailing hemispheres, or both.  The last section of the paper is the intriguing one: “A ring at Rhea?”  The authors acknowledged that the cameras couldn’t see a ring the plasma and magnetospheric instruments thought they had detected earlier, but Schenk noticed a chain of blue splotches along the equator, only 20 km wide, going almost all the way around the moon.  Could these be the smoking gun of a ring that was there but partly collapsed onto the surface?  “Well,” he said on his blog, “we don’t need the ring to be present today to explain the ultraviolet splotches on the surface.  They could have formed a few thousand or million years ago and still exist on the surface today.  Probably not much longer than that but that’s [sic] very young for the Solar System.”
      To see these blue features that might mark impact points of ring boulders driven into the surface, look at the JPL news feature or fly over them on Schenk’s YouTube page.  Schenk’s StereoMoons blog entry from 02/25/2010 has the best close-up photos of the blue streaks.  Strangely, the orientation of the features suggest that the ring rotated in the wrong direction – retrograde to the orbit of the moon and Saturn.  Where did the particles come from?  The authors doubted that it could have been debris from one of Rhea’s largest and youngest impact craters.  Whatever the impactors were, “the strong bluish color indicates that formation occurred recently and these may be among the most recently formed features on the surface,” the paper said.  “On the other hand, the surface features we observe might date from an earlier time, as no bright rays have been observed in association with the equatorial deposits.  This would imply that sufficient time has lapsed for ray erasure to have occurred but not sufficient time to erase the blue crater rims associated with relatively young impact craters elsewhere on Rhea.”  In short, this means they don’t know.  The latter suggestion (of old age) sounds ad hoc; Schenk’s true feelings are probably reflected by his blog statement that they are “very young for the Solar System.”  Interesting how he can thrown “thousand or million” years around without blinking an eye.
• Iapetus ring bumps:  The detection of possible ring impact debris on one moon opened up a search for others.  (On Saturn, of course, ring debris that falls into the planet disappears into the fluid clouds).  None of the other major moons have an equatorial signature like Rhea’s, but Iapetus, that walnut mimic way out in the suburbs (picture), has its mysterious equatorial mountain range (see 01/07/2005, 09/13/2007).  Schenk thinks that the evidence at Rhea supports the theory that the mountain range is debris from a collapsed ring (see Geophysical Research Letters paper by W.-H Ip).  The idea gains a little support from a couple of small moons, Atlas and Pan, that have a pronounced saucer shape that might indicate deposition of ring material during their histories (see Cassini image).  The alternative, the spin-up theory (see 03/01/2006, bullet 2), got more press from National Geographic this week.  Both the exogenic and endogenic theories require highly special conditions (e.g., 07/18/2007) that are running rings around planetary scientists.

Speaking of moons, Cassini was a “weekend warrior” on a major moon hunt last week.  It observed nine moons in just 62 hours.  A few of the early unprocessed images were posted by the Cassini Imaging Team.  The new images of Dione revealed a major area that is remarkably smooth and devoid of large craters.  “The smooth plains must have been resurfaced at some point in Dione’s past,” the caption said.  Whatever caused that must remain a story to tell.

1.  Schenk, Hamilton, Johnson, McKinnon, Paranicas, Schmidt and Showalter, “Plasma, plumes and rings: Saturn system dynamics as recorded in global color patterns on its midsize icy satellites,” Icarus, doi:10.1016/j.icarus.2010.08.016.

Go, Cassini!  What a great collection of fantastic discoveries and surprises have come from this epic mission.  Schenk said, “Its [sic] times like these when I really enjoy my job.”  Keep those images and data flowing, Cassini.