Huge red scars spotted on the surface of Saturn's icy moon Tethys are leaving Nasa scientist baffled as to what they are or where they came from.
The images of the mysterious streaks were taken using clear, green, infrared and ultraviolet spectral filters to create enhanced-colour views of the surface of the moon.
Several of the arc-shaped scars had been observed previously, but the latest images are the first to show them clearly and in colour.
Their origin remains a mystery to scientists monitoring information sent back to Earth from the Cassini-Huygens probe, although it has been suggested the red material is exposed ice with chemical impurities from gas being released from inside Tethys. Another suggestion is that they are the result of fractures that cannot be seen with the current resolution of the images.
Reddish features are very rare on Saturn's moons, with the only exception being a few small craters on Dione. Other red features have previously been seen on Jupiter's moon Europa, the surface of which is geologically young.
Paul Helfenstein, a Cassini imaging scientist at Cornell University, New York, said: "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. If the stain is only a thin, coloured veneer on the icy soil, exposure to the space environment at Tethys' surface might erase them on relatively short time scales."
Linda Spilker, Cassini project scientist at Nasa's Jet Propulsion Laboratory, said even after 11 years in orbit, the spacecraft continue to make surprising discoveries and that they are now planning to look even closer at the red arcs to see if they can work out the source and composition of the mystery lines.