Ithaca Chasma
A giant valley on Saturn's moon Tethys, one of the Solar System's longest.
Ithaca Chasma is a large valley, specifically a graben, located on Saturn’s moon Tethys. Its name comes from the Greek island of Ithaca. 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.
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.
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.
- Width
- up to 100 km (62 mi)
- Depth
- 3–5 km (1.9–3.1 mi)
- Length
- 2,000 km (1,200 mi)
- Discovery date
- 12 November 1980
- Discovered by
- Voyager 1 spacecraft
- Age estimate
- 4.0 or 0.4–3.3 billion years
Lore & Background
Ithaca Chasma was discovered by the Voyager 1 spacecraft on 12 November 1980 during its flyby of Saturn, though its full extent was realized only in 1981 after the Voyager 2 flyby. It is a giant trough system about 3 km deep, approximately confined to a great circle running through the poles of Tethys. The chasma is approximately concentric with Odysseus crater—a pole of Ithaca Chasma lies only approximately 20° from it. Its structure is complex, consisting of two narrow branches towards the south, with exterior walls made of multiple sub-parallel scarps and terraces. At some places the chasma has a rim standing as high as 0.5 km above the surrounding cratered plains, and its width varies from only a few kilometers to more than 100 km. The age of Ithaca Chasma is estimated to be either 4.0 or 0.4–3.3 billion years depending on the chosen impact chronology, and crater counts indicate it is slightly older than Odysseus crater but much younger than the cratered plains.
Reader's Guide
Ithaca Chasma's significance lies in its origin hypotheses, which reflect key processes in the history of Tethys. One hypothesis proposes that it formed when Tethys's internal global liquid salt water ocean solidified, causing the moon to expand and crack its surface to accommodate the extra volume. This subsurface ocean may have resulted from a 2:3 orbital resonance between Tethys and Dione early in the Solar System's history, which led to orbital eccentricity and tidal heating that warmed Tethys's interior. Subsequent freezing after the moons escaped the resonance may have generated the extensional stresses that created Ithaca Chasma. An alternative hypothesis suggests it formed at the same time as the large crater Odysseus, as a circumcircular fracture from the impact shockwave. 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. The chasma thus provides evidence for a past subsurface ocean and the geological evolution of Tethys.
Did You Know?
- Ithaca Chasma runs approximately three-quarters of the way around Tethys's circumference.
- It was discovered by Voyager 1 on 12 November 1980.
- The chasma is up to 100 km wide and 3–5 km deep.
More in Moons of Saturn, Part 2 1-24
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