Janus and Tethys demonstrate the main difference between small moons and large ones; it’s all about their shape. Moons like Tethys are large enough that their own gravity is sufficient to overcome the material strength of the substances they are made of and mould them into spherical shapes, but small moons like Janus are not massive enough for their gravity to form them into a sphere.
Like graffiti sprayed by an unknown artist, unexplained narrow, arc-shaped, reddish streaks are visible on the surface of Saturn’s icy moon Tethys in new, enhanced-colour images from NASA’s Cassini spacecraft. The red arcs are among the most unusual colour features on Saturn’s moons to be revealed by Cassini’s cameras.
In this NASA/ESA Cassini mission image of Saturn’s 660-mile-wide moon Tethys, the giant impact basin Odysseus stands out brightly from the rest of the illuminated icy crescent. Some 280 miles across, Odysseus is one of the largest impact craters on Saturn’s icy moons, and may have significantly altered the geologic history of Tethys.