Saturn's Moons Engage in Cosmic Paintball Fight

Five of Saturn's inner moons are engaged in a cosmic paintball fight, pelting each other with particles that leave bright, colorful splotches and may be the source of a Pac-man feature on one of the satellites.

NASA's Cassini spacecraft recently spotted reddish and blue splashes on the icy surfaces of Mimas, Enceladus, Tethys, Dione and Rhea. The colors are helping scientists map out how material travels between these moons of the ringed planet, and they're highlighting how "space weathering" can impact bodies in our solar system, researchers said. [New photo of colorful Saturn moon.]

The discovery may also help explain the mysterious Pac-Man heat pattern Cassini spotted on Mimas earlier this year, scientists said.

"The beauty of it all is how the satellites behave as a family, recording similar processes and events on their surfaces, each in its own unique way," said study lead author Paul Schenk, of the Lunar and Planetary Institute in Houston, in a statement. "I don't think anyone expected that electrons would leave such obvious fingerprints on planetary surfaces, but we see it on several moons, including Mimas, which was once thought to be rather bland."

The research is detailed in a recent online edition of the science journal Icarus.

Enceladus: The aggressor

In the new study, Schenk and his colleagues used raw images taken by Cassini from 2004 to 2009, to produce new, high-resolution global color maps of the five Saturn moons.

The new maps incorporate images shot through visible-light, ultraviolet and infrared filters. Scientists processed the photos to reveal details the human eye can't detect.

The chief aggressor in the Saturnian particle paint war appears to be icy Enceladus, researchers said.

Mysterious ice geysers blast from the south polar region of this small moon. Particles from these icy jets make up most of Saturn's misty E ring, and they also appear to splatter Tethys, Dione and Rhea.

These three moons run head-on into Enceladus' spray as they orbit Saturn, and the gunk leaves a coral-colored stain on their icy surfaces, researchers said.

Enceladus' spray also tags Mimas, but it hits the moon's trailing side. This probably occurs because Mimas orbits inside the path of Enceladus ? or closer to Saturn ? while Tethys, Dione and Rhea are on the outside, scientists said.

But Enceladus doesn't emerge from the paintball fight unscathed: Some of its own icy material blows back onto the moon, dyeing parts of its surface bright blue.

Scientists aren't sure why the frost stains the other moons pinkish and Enceladus blue, but they're working on figuring it out.

Mimas' Pac-Man pattern

On Tethys, Dione and Rhea, darker, rusty reds paint the entire trailing hemisphere ? the side that faces backward in the orbit around Saturn, scientists said. These colors are thought to be caused by tiny particle strikes from circulating plasma ? an electrically charged form of matter similar to gas ? in Saturn's magnetic environment.

In addition, Mimas and Tethys both sport a dark bluish band. The bands match patterns that might be produced if the surface were being irradiated by high-energy electrons drifting in a direction opposite to the flow of plasma in the magnetic bubble around Saturn, researchers said.

Scientists are still figuring out exactly what is happening, but the electrons appear to be zapping the Mimas surface in a way that matches the Pac-Man thermal pattern detected by Cassini's composite infrared spectrometer, Schenk said.

Schenk and his colleagues also found a unique chain of bluish splotches along the equator of Rhea that re-opens the question of whether Rhea ever had a ring around it. The splotches do not seem related to Enceladus, but rather appear where fresh, bluish ice has been exposed on older crater rims.

Though scientists recently reported that they did not see evidence in Cassini images of a ring around Rhea, the authors of the new study suggest the crash of orbiting material ? perhaps a ring ? to the surface of Rhea in the not-too-distant past could explain the bluish splotches.

"Analyzing the image color ratios is a great way to really enhance the otherwise subtle color variations and make apparent some of the processes at play in the Saturn system," said Amanda Hendrix, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "The Cassini images highlight the importance and potential effects of so-called 'space weathering' that occurs throughout the solar system ? on any surface that isn't protected by a thick atmosphere or magnetic field."

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