15 October 2010

Colors Published

Our major paper on the colors of Saturn's moons was published last week online at the journal Icarus.  I have indeed posted on this before (see last October and February), but as always I have a few new graphics to share.

One of the more interesting features are the broad equatorial blue bands on Mimas (discovered here first) and on Tethys (discovered by Voyager first), which are in fact due to the impact of lots of high energy MeV or greater electrons that travel more slowly in Saturn's magnetic field and appear to be spiraling in "retrograde" into the front side of these satellites.  (This is opposite of what happens on Europa due to the different energies involved).  What is so amazing is that tiny electrons have the power to alter the surfaces of these satellites.  Why the blue (really ultraviolet) brightening is unclear but the subsequent observation that these same areas are colder in the daytime than they should be (observed by my friends on the Cassini CIRS team) adds a key tot he puzzle and suggests that the surface structure is being altered on the microscopic level enough to change the thermal inertia (ability to conduct heat) of the upper centimeter of the surface!  Who would have thought.  Scientists are looking at this now with new data expected over the next few months.

Enhanced color view of the leading hemisphere of Mimas, showing the large crater Herschel and the broad ultraviolet band across the equator (shown in blue in this RGB rendering)

The second excitement is from Rhea.  First the plasma teams observed very odd signatures around Rhea in 2007 which looked rather like the telltale signs of a thin ring around this otherwise heavily cratered satellite about the size of Alaska.  Then, looking at stereo images of the craters I noticed an odd blue (really ultraviolet) patch that seemed to be right on the equator.  "What a minute," I says to myself  "Thats odd."  So I made a global map and sure enough the spots went almost all the way around!  Only a ring could do that!  But then, when the imaging camera was trained to look specifically for a Rhea ring and the next close pass for the plasma instruments happened in 2009, neither time was a ring observed.   Hmmm . . . Here we had direct evidence for surface impact onto the surface of small bits from a ring around Rhea (disturbing the dusty coating on the icy surface) and yet the ring turns out to be some sort of as yet unexplained phantom.  Well, 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 thats very young for the Solar System.  Its times like these when I really enjoy my job.

Enhanced color medium resolution (2009) view of equatorial region of Rhea from Cassini orbiter.   The ring deposits are the dark splotches running east-west along center frame.  Turns out these patches don't have the same color shift as those on the leading hemisphere, perhaps due to the presence of E-ring dust on that hemisphere.