The Tethys Ocean?

The Saturn moon Tethys was discovered by Giovanni Cassini in 1684 and named by John Herschel (son of William) in 1847. Tethys was a Greek sea goddess and it may be that Herschel’s choice of name may have been quite appropriate as it may once have harboured a liquid ocean.

Tethys is a mid-sized satellite with a density close to that of pure ice. But a large valley system visible today must have formed when the crust was being heated and under great strain according to a presentation at the 39th Lunar and Planetary Science Conference in Houston.

Calculations by Erinna Chen and Francis Nimmo, from the University of California, Santa Cruz, show that tidal interactions were the only viable way of providing the amount of heat associated with the formation of Ithaca Chasma. They propose that Tethys' orbit around Saturn was once perturbed by gravitational interactions with another moon - Dione - which made Tethys' orbit more "eccentric". The resulting tidal forces caused frictional heating of Tethys' interior. But at some point, the orbital interaction between Tethys and Dione was broken, and Tethys fell back into a less eccentric orbit. As it did so, it began to cool. Freezing of a liquid ocean would have generated sufficient stresses in the crust to form Ithaca Chasma, the researchers said.

"We have a large rift system that brought water to the surface, so it seems likely that this happened," Ms Chen explained. She said that there was no way of knowing exactly how deep the ocean was, but speculated that it could have been 100km deep at some point in Tethys' past.

Tethys joins Europa and possibly Callisto in a small club of icy Solar System bodies thought either to have a subsurface ocean today, or to have had one in the distant past. Some researchers also think Saturn's moon Enceladus could harbour liquid water beneath the surface, although this idea has been called into question recently.