to form the ocean. Subsequent freezing of the internal ocean after the moons escaped from the resonance may have generated the extensional stresses that created Ithaca Chasma. An alternative hypothesis is that it was formed at the same time as the large crater Odysseus which lies near a pole of the Ithaca Chasma. When the impact that created Odysseus occurred, the shockwave may have traveled through Tethys and produced a circumcircular fracture analogous to outer ring graben of multiring impact basins. However, age determination based on crater counts in high resolution Cassini images showed that Ithaca Chasma is older than Odysseus, making the impact hypothesis unlikely. Citations References External links Extraterrestrial valleys Surface features of Tethys (moon)