Tethys and Dione, side by side

Dione [dy-OH-nee] is the second densest moon of Saturn, after Titan. Dione is probably composed of a rocky core making up one-third of the moon's mass, and the rest is composed of water-ice. Dione is an icy body that is similar to Tethys.

Dione's icy surface includes heavily cratered terrain, with moderately and lightly cratered plains, as well as some severely cracked areas, with very bright material on the walls of the fractures. The heavily cratered terrain has numerous craters greater than 100 kilometers (about 62 miles) in diameter.

Tethys [TEE-thiss] lies in a part of the Saturnian system that was most likely once crowded with debris created by collisions between smaller and larger moons. Its icy surface is heavily cratered and contains faults in the ice that are probably a result of Tethys quakes at the times of some major impacts.

Credit: NASA/JPL/Space Science Institute