Orbiting around Saturn at a distance of about 295,000 kilometers (183,000 miles), the moon Tethys is a cold, icy place. The thick coating of ice covering the moon, and the sandblasting it gets from the rings of Saturn, means it is one of the most reflective objects in the Solar System. This makes it all the more bizarre that images taken from the Cassini spacecraft show that its surface is not uniformly white, but has odd red streaks across it.

“The red arcs really popped out when we saw the new images,” said Paul Schenk of the Lunar and Planetary Institute in Houston, who has contributed to the Cassini mission. “It's surprising how extensive these features are.”

The images, which were taken in April, are the first to show the northern area of Tethys in light. This is because the Saturn system has moved into its northern hemisphere summer, illuminating the region and allowing scientists to see these odd patterns clearly for the first time.  

^{An enhanced-color mosaic image of Tethys, showing a range of features on the surface of the moon. Credit: NASA}

Taken using clear, green, infrared and ultraviolet filters, the researchers have combined the images to create the enhanced color view seen in the photo. What has caused these red lines remains a bit of a mystery, made even more intriguing considering only one other of Saturn’s 62 moons has any notable red features on its surface. It has been suggested that they could be the result of chemical impurities contained within exposed ice, the consequence of outgassing from inside the moon, or that they could be linked to fractures in the ice that are below the resolution of the pictures.

“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,” explained Paul Helfenstein, a Cassini imaging scientist at Cornell University. “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.”

^{Tethys passing behind the rings of Saturn. Credit: NASA}

The spacecraft Cassini has been conducting flybys and taking pictures of Saturn and its moons for the last 11 years, and is the first probe to have ever entered the planet's orbit. During this time, it’s made some incredible discoveries, including finding seven new moons, and even photographing the potential formation of a new one. The researchers are planning an even closer look at Tethys' red arcs later this year to see if they can figure out once and for all what’s causing the distinctive patterns to form.