Tethys is the fifth-largest moon of Saturn. Its low density and a very bright surface indicates that it is made of water ice with only a small fraction of rock.

Crater

The western hemisphere dominates a huge impact crater, Odysseus, with nearly 2/5 the size of the entire moon. It's not known why this impact didn't shatter the moon but it is theoretized Tethys may have been in a semi-liquid state at the time of the impact.