Ithaca Chasma

Ithaca Chasma is a large valley, specifically a graben, located on Saturn’s moon Tethys. Its name comes from the Greek island of Ithaca.

Geology

The valley measures up to 100 kilometers (62 miles) wide, between 3 and 5 kilometers (1.9–3.1 miles) deep, and stretches 2,000 kilometers (1,200 miles) in length, covering roughly three-quarters of Tethys’s circumference. This makes it one of the longest valleys in the Solar System. The chasma is roughly concentric with the Odysseus crater.

Origin

One theory for its formation suggests that a global saltwater ocean inside Tethys froze, causing the moon to expand and the surface to crack as it accommodated the increased volume. Another hypothesis proposes that Ithaca Chasma is the outermost ring of the Odysseus impact basin. However, age estimates range from 4 billion to 0.4 billion years old, indicating that the chasma is slightly older than Odysseus.

Discovery

The chasma was first spotted by the Voyager 1 spacecraft on November 12, 1980, during its flyby of Saturn, though its full extent was not understood until the Voyager 2 flyby in 1981. It was then named after Ithaca, Greece.

Geologically, Ithaca Chasma is a giant trough system about 3 kilometers deep, roughly following a great circle that passes through Tethys’s poles. It is approximately concentric with the Odysseus impact crater—one pole of the chasma lies only about 20 degrees from it. The structure is complex, with two narrow branches toward the south. Its exterior walls consist of multiple sub-parallel scarps and terraces.

In some places, the chasma’s rim rises as high as 0.5 kilometers above the surrounding cratered plains. Its width varies from just a few kilometers in some spots to over 100 kilometers in others. Crater counts suggest the chasma is slightly older than Odysseus but much younger than the cratered plains.

Regarding its origin, one hypothesis holds that the chasma formed when Tethys’s internal global liquid saltwater ocean solidified, causing the moon to expand and crack its surface. Earlier craters from before this solidification were likely erased by geological activity. This subsurface ocean may have resulted from a 2:3 orbital resonance between Tethys and Dione early in Solar System history, which would have caused orbital eccentricity and tidal heating, warming Tethys’s interior enough to form the ocean.