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By Keith Wagstaff

NASA's Cassini spacecraft has discovered strange red lines stretching across the surface of Saturn's icy moon Tethys.

What are they? Scientists aren't sure. They could be areas of exposed ice with chemical impurities, according to NASA, or the result of gas being released from within the moon.

The view from Saturn

July 19, 201300:30

"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 Cassini imaging scientist Paul Helfenstein. "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."

Cassini has been orbiting Saturn since 2004. Over the last few years, Tethys' northern hemisphere has been experiencing its summer, allowing the sun to illuminate formerly dark areas of the moon's surface.

A wider view of Saturn's icy moon Tethys taken by the Cassini orbiter.NASA/JPL-Caltech/Space Science Institute

In April of this year, Cassini took this image, and NASA has since used clear, green, infrared and ultraviolet spectral filters to enhance the colors to make them visible to the human eye. Red features are rare on Saturn's 62 moons, NASA said, except for a few areas on crater-covered Dione.