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Mystery "graffiti" on Saturn moon has experts stumped

It's as if someone took a red marker to Saturn's icy moon Tethys.

In new images from NASA's Cassini spacecraft, red arcs are clearly visible on the moon's surface, and are among the most unusual color features on Saturn's moons to be revealed by Cassini's cameras.

The images, obtained in April, are the first to show large northern areas of Tethys with such clarity. It also has helped that the Saturn system moved into its northern hemisphere summer over the past few years, meaning northern latitudes have become increasingly well illuminated.

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This enhanced-color mosaic of Saturn's icy moon Tethys shows a range of features on the moon's trailing hemisphere. Tethys is tidally locked to Saturn, so the trailing hemisphere is the side of the moon that always faces opposite its direction of motion as it orbits the planet. NASA/JPL-Caltech/Space Science Institute

"The red arcs really popped out when we saw the new images," Cassini participating scientist Paul Schenk, of the Lunar and Planetary Institute in Houston, said. "It's surprising how extensive these features are."

Scientists can't say what caused the reddish lines. Among the possibilities being studied are that the reddish material is exposed ice with chemical impurities, or the result of outgassing from inside Tethys. They could also be associated with features like fractures that are below the resolution of the available images.

Many reddish features occur on the geologically young surface of Jupiter's moon Europa but the phenomena is relatively rare with moons of Saturn - with the exception of reddish-tinted features found on a few small craters on Saturn's moon, Dione.

"The red arcs must be geologically young because they cut across older features like impact craters, but we don't know their age in years," said Paul Helfenstein, a Cassini imaging scientist at Cornell University, Ithaca, New York, who helped plan the observations. "If the stain is only a thin, colored veneer on the icy soil, exposure to the space environment at Tethys' surface might erase them on relatively short time scales."

The next step is following up the observations of the features, at higher resolution, later this year.

"After 11 years in orbit, Cassini continues to make surprising discoveries," Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, said. "We are planning an even closer look at one of the Tethys red arcs in November to see if we can tease out the source and composition of these unusual markings."

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