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 Tethys
Tethys' low density indicates that it is almost completely composed of water ice. The western hemisphere is dominated by a huge impact crater whose 400 km diameter is nearly 2/5 of that of Tethys itself. That such an impact didn't shatter Tethys completely indicates that it may have been liquid or at least not very solid at the time. The crater is now quite flat (or more precisely, it conforms to Tethys' spherical shape), like the craters on Callisto, without the high ring mountains and central peaks commonly seen on the Moon and Mercury. The second major feature seen on Tethys is a huge valley 100 km wide and 3 to 5 km deep which runs 2000 km or 3/4 of the way around Tethys' circumference . Clearly then, Tethys has not always been frozen solid. At some point in its past it was probably liquid. The impact craters from that era have been smoothed out. As it froze and expanded, the surface must have cracked to accommodate the extra volume. The smaller impact craters we see today are more recent.
Telesto
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