This enhanced-color mosaic of Tethys shows a range of features including mysterious arc-shaped streaks.
This mosaic of Tethys is an orthographic projection constructed from 52 images obtained on April 11, 2015 with Cassini’s narrow-angle camera. The images were obtained at a distance of approximately 33,000 miles (53,000 km) from the moon. The color of the surface can be seen to change across the disk, from yellowish hues to nearly white. These broad color changes are affected by a number of external processes. First, Saturn’s diffuse E-ring preferentially bombards Tethys’ leading hemisphere, toward the right side in this image, with ice bright ice grains. At the same time, charged particles from Saturn’s radiation belt bombard the surface on the trailing side, causing color changes due to chemical alteration of the materials there. The albedo drops by 10 to 15% from the moon’s leading side to the trailing side. Similar global color patterns exist on other Saturnian moons. On a much smaller scale, enigmatic, arc-shaped reddish streaks are faintly visible across the heavily-cratered surface, particularly in the upper right quarter of the image. The origin of this localized color contrast is not yet understood. Image credit: NASA / JPL-Caltech / Space Science Institute.
The red ‘arcs’ are narrow, curved lines on Tethys’ surface. They are among the most unusual color features on Saturn’s moons to be revealed by the NASA/ESA Cassini spacecraft.
Images taken using clear, green, IR and UV spectral filters were combined to create this enhanced-color mosaic, which highlights subtle color differences across the moon’s surface at wavelengths not visible to human eyes.
The origin of the features and their reddish color is a mystery to scientists with the Cassini mission.
“The red arcs really popped out when we saw the new images. It’s surprising how extensive these features are,” said Dr Paul Schenk from the Lunar and Planetary Institute in Houston, a scientist for the Cassini mission.
Possibilities being studied include ideas that the reddish material is exposed ice with chemical impurities, or the result of outgassing from inside the icy moon.
They could also be associated with features like fractures that are below the resolution of the available images.
Except for a few small craters on Saturn’s moon Dione, reddish-tinted features are rare on other moons of Saturn.
Reddish streaks cut across the surface of Tethys in this image. The streaks are narrow, curved lines on the moon’s surface, only a few miles wide but several hundred miles long. The yellowish tones on the left side of the view are a result of alteration of the moon’s surface by high-energy particles from Saturn’s magnetosphere. This particle radiation slams into the moon’s trailing hemisphere, modifying it chemically and changing its appearance in enhanced-color views like this one. The area of Tethys shown here is centered on 30 degrees north latitude, 187 degrees west longitude, and measures 305 by 258 miles (490 by 415 km) across. The original color images were obtained at a resolution of about 2,300 feet (700 m) per pixel on April 11, 2015. Image credit: NASA / JPL-Caltech / Space Science Institute.
Many reddish features do occur, however, on the surface of Jupiter’s moon Europa.
“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 Dr Paul Helfenstein of Cornell University in Ithaca, NY.
“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,” he said.
