Origin
There are two basic hypothesis as to how Ithaca Chasma formed. One of them is that it formed as Tethys' internal liquid water ocean solidified, causing the moon to expand and cracking its surface to accommodate the extra volume within. Earlier craters made before Tethys solidified were probably all erased by geological activity before then.
Tethys' subsurface ocean may have resulted from a 2:3 orbital resonance between Dione and Tethys early in the solar system's history. The resonance would have led to orbital eccentricity and tidal heating that may have warmed Tethys' interior enough to form the ocean. Subsequent freezing of the 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.
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