Mysterious Patches of Color Mapped on Saturn's Moons
Maps of the colored patterns seen on Mimas, Enceladus, Tethys, Dione and Rhea by instruments on board NASA's Cassini spacecraft. The leading hemisphere is on the right half of each image and the trailing hemisphere is on the left half.
Credit: NASA/Paul Schenk

New maps reveal colorful patterns on the surfaces of Saturn's five innermost icy moons.

Some of the patterns have been seen before, but others took scientists by surprise, suggesting dynamic interactions between the moons and other particles orbiting around Saturn.

The maps of Mimas, Enceladus, Tethys, Dione, and Rhea were created from images taken by NASA's Cassini spacecraft and were presented by Paul Schenk of Houston?s Lunar and Planetary Institute at a recent meeting of the American Astronomical Society.

Surface redness

Some of the most striking patterns are revealed when the brightness of the surface in infrared light is divided by the brightness in the ultraviolet part of the spectrum, which gives the so-called redness of the surface. The maps reveal that except for Mimas, all the inner moons are redder on their trailing hemispheres.

This reddening is strongest at the centers of the trailing face (like the center of a bulls-eye). The leading faces of the moons also appear to be redder in their centers, though the redness is weaker than for the trailing hemisphere.

Seeing this pattern on both hemispheres is difficult to explain because most processes that would color the surface in this way would only affect one hemisphere or the other.

Possible explanations include bombardment by grains in Saturn's E ring, which would impact the leading hemisphere as the satellites over took the grains in their orbit, and radiation by charged particles. The latter explanation could favor the trailing hemispheres of the moons, as they are heavily bombarded by cold plasma, or the leading hemispheres, which are impacted by high-energy electrons.

Bands and splotches

One curious feature, seen before in Voyager observations nearly 30 years ago, was the dark equatorial lens-shaped band across the front-side of Tethys. This feature is very prominent in the new Cassini color maps and appears bright in the ultraviolet end of the spectrum, but dark at infrared wavelengths.

An unexpected discovery is that Mimas also has a prominent equatorial lens-shaped band about 175-kilometers wide stretching across its leading side as well.

Scientists are investigating the cause of these bands; one possible explanation is the impact of high-energy electrons, which is predicted to produce such a feature. A similar process has been investigated for Jupiter?s moon Europa, where the same energy particles preferentially impact the trailing hemisphere.

Another surprise was the discovery of a very narrow and straight band of discrete ultraviolet-bright features very close to Rhea?s equator. At higher resolution, the band appears as bluish splotch-marks that form a very narrow chain only a few kilometers wide across the center of the leading hemisphere of Rhea. These features are seen on no other icy satellite of Saturn.

The pattern of the splotches suggests they are the result of impacts from Rhea's dusty ring system, first proposed in 2008.

Researchers hope that these maps will help them better understand the dynamics of the particles in orbit around Saturn as the Cassini mission continues. Higher resolution maps could also help better understand how the particles might play a part in making the patterns revealed by the new maps.

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