New images taken from NASA's Cassini spacecraft appear to show unusual redish arcs marking the surface of the Saturn's icy moon Tethys. The strange features cover significant stretches of the moon's surface, and have left astronomers baffled as to their origins.

First discovered in 1684 by Italian mathematician and astronomer Giovanni Domenico Cassini, the icy moon Tethys orbits Saturn at a distance of roughly 295,000 km (183,000 mi), and is believed to consist predominantly of water ice.

The newest shots of the enigmatic moon were created by combining a number of images taken using clear, green, infrared and violet spectral filters from Cassini's onboard camera. The resulting views emphasize a series of strange red arcs, some of which stretch for several hundred miles across the barren landscape, dulling the moon's otherwise highly reflective surface.

The images captured by Cassini in April are the first to clearly feature large sections of Tethys' northern hemisphere, which has become better lit as the region transitioned into a "summer phase". Judging by the way that the arcs seem to traverse older surface features, such as impact craters, astronomers are working on the assumption that the arcs are relatively young.

Saturn's moon Tethys as seen from the Cassini spacecraft in April(Credit: NASA/JPL-Caltech/Space Science Institute)

Another fascinating aspect of the arcs is their reddish tinge, as this hue is extremely rare amongst Saturn and her 53 confirmed satellites. There is currently no consensus as to the origins of the arcs, however leading theories attribute their existence to chemical impurities in the icy surface of the moon, or possibly a bi-product of outgassing, similar to the activity observed to be taking place on the comet 67P/Churyumov–Gerasimenko.

Should the marks turn out to be a shallow stain on the surface of the moon, it is expected that exposure to the harsh space environment should diminish their presence in a fairly short period of time.

"After 11 years in orbit, Cassini continues to make surprising discoveries," states Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory. "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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