Arcos Rojos

Enigmatic red arcs on Tethys. Credit: NASA/JPL-Caltech/Space Science Institute.

Enigmatic red arcs on Tethys. Credit: NASA/JPL-Caltech/Space Science Institute.


Mar 3, 2016

What causes colorful streaks on Tethys?

As reported in previous Pictures of the Day, maps of Mimas, Enceladus, Rhea, and Dione show that they are red to some degree. All the latest maps indicate that most of them are redder on their trailing hemispheres. No one is quite sure why, since meteor bombardment, or particles from Saturn’s rings, should accumulate on the leading hemispheres.

Recent images from the Cassini orbiter now reveal that Tethys, a medium-sized moon of Saturn, is also marked by red streaks within the wider swathes of color on its surface. A dark equatorial stripe is faintly visible across the leading face of Tethys. The stripe is bright at ultraviolet wavelengths, but dark in the infrared. Within the band, reddish colored arcs are seen.

As the NASA press release states, “The origin of the features and their reddish color is a mystery to Cassini scientists. Possibilities being studied include ideas that the reddish material is exposed ice with chemical impurities, or the result of outgassing from inside Tethys. They could also be associated with features like fractures that are below the resolution of the available images.”

A clue to the formation of red streaks on Tethys might be found on Jupiter’s moon, Europa, whose red channels are thought by some to be caused by oxygen atoms from water ice combining to form a sulfur atom. Galileo mission images of Europa reveal dark deposits that trace the paths of subsurface electric discharges. It is those discharges that most likely altered Europa’s surface composition, transforming the water ice into other materials.

Saturn’s moons generate ionic plumes because they occupy positions in an electrical circuit with Saturn. Therefore, it is probably a plasma, in the form of high energy ions flowing into Tethys, that is contributing to the coloration. Electrical theorists argue that Saturn moves within the Sun’s heliosphere, so it interacts with the Sun’s electric field. Since planets and moons in the Solar System are charged bodies that are not isolated in space, it is to be expected that they are connected with one another. Larger swathes of red and smaller streaks of the same color might indicate the strength, diffusion, and direction of electric discharges within the Saturnian system.

Stephen Smith

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