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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