Charged Particle and Ring Environment Discolors Saturn's Inner Icy Moons

Press Release From: Lunar and Planetary Institute
Posted: Tuesday, October 6, 2009

image New global color maps reveal provocative patterns on the surfaces of Saturn's five innermost large icy satellites: Mimas, Enceladus, Tethys, Dione, and Rhea. These new maps are being presented in a report by Dr. Paul Schenk of Houston's Lunar and Planetary Institute at the AAS Division for Planetary Sciences meeting in Puerto Rico today. The new maps reveal complex global color patterns on each of these satellites, including color asymmetries on four of these moons and equatorial banding on three.

The patterns indicate that particles within the Saturn system have significant effects on the surfaces of these icy bodies. These new maps will ultimately provide new constraints on the dynamics of particles and grains within the Saturn system, including the apparent impact of Rhea ring material directly onto its surface.

These new maps represent the first comprehensive color mapping of all of Saturn's mid-sized icy moons together. They were created by Dr. Schenk from images acquired by Cassini's solid state imager over the first 4 years of the mission and have resolutions of between 400 and 750 meters. These maps cover the infrared to the ultraviolet spectral range, which is beyond the range of human sight, and show distinct patterns on each moon.

Dr. Schenk has been working with a team of researchers from across the nation, including Robert Johnson of the University of Virginia, William McKinnon of Washington University in Saint Louis, Chris Paranicas of the Johns Hopkins University Applied Physics Laboratory in Maryland, Doug Hamilton of the University of Maryland, and Mark Showalter of the NASA Ames Research Center in California to determine the origin of these color patterns.

Some of the patterns observed in these new maps were first seen by the Voyagers nearly 30 years ago, but there are several surprises as well. The most striking patterns are revealed when we compare colors by dividing the infrared (IR) brightness by the ultraviolet (UV) brightness map. These IR/UV ratio maps provides a measure of the so-called redness of the surface, defined by enhanced absorption at shorter wavelengths. The maps reveal that except for Mimas, all the inner moons have strong IR/UV ratio enhancements (that is they are redder) on their trailing hemispheres. This reddening is strongest at the center of the trailing hemisphere, and is also seen by the Cassini VIMS instrument.

A similar though weaker asymmetry is also centered on the leading hemisphere, however. This simultaneous reddening toward the centers of both the leading and trailing hemispheres is difficult to explain because most impact processes associated with surface weathering preferentially favor one hemisphere or the other.

Potential explanations that are currently being investigated include bombardment by E ring grains on the leading hemisphere as the satellites overtake these grains in their orbit, and radiation by charged particles. This latter can favor the trailing hemispheres, which are heavily bombarded by the cold plasma or the leading hemispheres, which are preferentially impacted by > 1 MeV electrons. One surprise is that the pattern for Enceladus is offset 20 degrees to the west. The origin of this offset is unclear but could relate to reimpact of plume material.

A curiosity from Voyager observations was the dark equatorial lens-shaped band across the front-side of Tethys. This feature is very prominent in the new Cassini color maps as an IR-dark, UV-bright feature. An unexpected discovery is that long-neglected Mimas also has a prominent equatorial lens-shaped band about 175-kilometers wide stretching across its leading side as well. On Mimas, this band is bright in the ultraviolet, which may explain why Voyager did not see it. Knowing now that there are two of these features gives us an important clue that this is a phenomenon not limited to Tethys.

The origin of these bands is also being studied, but impact of high-energy electrons is predicted to produce very similar features on the front sides of Mimas and Tethys and less so on Dione and Rhea. 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 UV-bright features very close to (within 2 degrees of) Rhea's equator. These features are resolved at higher resolution 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 absence of tectonism in association with these clumps and the close proximity to the equator points to the impact of debris from Rhea's dusty ring system, a system that was first proposed by Jones et al. in Science in 2008.

The color patterns observed on the five inner icy moons of Saturn reveal a wealth of phenomena, including the likely impact of ring material (whether from the diffuse E-ring or from the possible dust ring around Rhea itself), as well as the charged particle environment surrounding Saturn. Work continues to test these hypotheses and perhaps some new ideas as well. These new maps will be very important in constraining the dynamics of particles in the Saturn system as the Cassini mission continues.

Further work is needed in integrating the maps with observations acquired by Cassini's fields and particles instruments as well as other remote sensing instruments as they pass these moons. Higher resolution color images may provide insight into the nature of the particles hitting these satellites, including Rhea.

The global color maps of Saturn's large inner icy moons will be available on-line at the afternoon of October 5, shortly after the presentation.

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