05 October 2009

The Colors of Saturn's Mid-Sized Icy Satellites

October 5  -  Fajardo, Puerto Rico

Okay, this isn't stereo or topography but it is some cool stuff.  Research can take some interesting twists, and these maps are a case in point.  In the course of examining some of the high resolution images from Cassini's Rhea pass in 2007, searching for stereo images, I found some were in color.  In the process of registering these images I produced a color mosaic and noticed some odd bluish spots (it turns out the Cassini team had already seen a few of these odd spots, but I didn't know that at the time).  Well, my natural curiosity was aroused by their location very close to the equator, so, with the report by Jones and colleagues of ring around Rhea, it was obvious that a global color map was necessary.  That proved to be an effort but this "feature" was very obvious in this new global map, and so were some other interesting patterns too.  This lead to a third bout of curiosity: why not map all the icy moons?  Do they show any similar patterns?  These color maps (of Mimas, Enceladus, Tethys, Dione, and Rhea) are what I show here.  The maps contain more than a few surprises: these moons are not so bland after all!  To understand these patterns I have called on the aid of several of my colleagues, and the discussion below represents some of our early conclusions.  We will have much more to say over the next few months.

The Data
Voyager first mapped the icy saturnian satellites from a distance (something my friend Bonnie Buratti did much of the work on), but Cassini has two advantages: it can map each moon at full resolution (400 to 750 meters) and over a broader spectral range: from UV (0.38 microns) to IR (0.93 microns).  This allows us to map out global and local patterns with confidence.  The most dramatic way to map out these patterns is to ratio the infrared reflectance by the ultraviolet reflectance (what we call the IR/UV ratio).  I show both versions.

The Maps
Each map is in cylindrical projection.  They extend from pole-to-pole and 0° to 360° (from right to left).  Thus the leading hemisphere is on the right half and the trailing hemisphere on the left half.

The Patterns

Global Asymmetries
Several curious features are apparent in the new color maps.  First is the basic color asymmetry apparent on Tethys, Dione and Rhea.  This is an enhancement in the redness of the surface on the trailing hemisphere of each of these satellites (visible as a brightness in the IR/UV ratio maps).  There is also a subtle enhancement on the leading hemisphere.  This pattern suggests that each satellite is getting bombarded by particles and charged plasma on both sides.  One candidate for the front-side color pattern is E-ring particles, which are suspected to coat (or blast) the surfaces of these moons.  It turns out that the Enceladus pattern may be related to the fallout of plume material back onto the surface.

The Mimas Band
The second  unusual feature are the narrow lens-shaped IR-dark, UV-bright features across the equator of both Mimas and Tethys.  The Tethys feature was known from Voyager days, but the Mimas feature has not been recognized previously.  It isn't new, but Voyager did not have the spectral range to detect it and it does not contrast as strongly on Mimas as on Tethys.  High-energy charged particles in saturn's magnetosphere can make a very similar pattern on the surfaces of these two inner moons, and we are currently testing this model with numerical predictions tuned to each moon.  We should have an answer soon.

The equatorial Line on Rhea  
(I forgot to note initially that the above image is taken from the IR/UV ratio map.   It shows the leading hemisphere only.  The features thenselves are not dark, but rather more blue than regular terrains.  In fact they are not distinctive in ordinary images of the surface.)
A third unusual feature is a very narrow set of small UV-bright spots on Rhea.  Normally this is not a cause for excitement, as fresh crater rims have this signature, but these are lined up along a great circle trace very close to Rhea's equator.  This alignment is not a random coincidence.  No other satellite has comparable features.  (The Cassini team is planning higher resolution observations during next year's Rhea flyby).  This feature is only a few kilometers across, but its linear pattern across nearly 2/3rds of Rhea's circumference and alignment within 2 degrees of the equator indicate it is quite plausibly material from Rhea's proposed ring system that has struck the surface of Rhea.  A higher resolution color observation (the one that started this entire project) suggests that this material would be composed of discrete but incoherent packets of ring material that this the surface at scattered intervals along the equator, and I should be posting that image here later.  These and the other observations make an intriguing story but one that requires a lot more work to fully understand. 

email:  schenk@lpi.usra.edu

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