Tethys

Tethys, major regular moon of Saturn, remarkable for a fissure that wraps around the greater part of its circumference. It was discovered in 1684 by the Italian-born French astronomer Gian Domenico Cassini and named for a Titan in Greek mythology.

Tethys has a diameter of 1,066 km (662 miles), and its density of about 1.0 grams per cubic cm—the same as that of water—indicates that it is composed essentially of pure water ice. It revolves around Saturn in a prograde, circular orbit at a distance of 294,660 km (183,090 miles), which is within the planet’s broad, tenuous E ring. It is involved in an orbital resonance with the nearer moon Mimas such that Tethys completes one 45-hour orbit for every two of Mimas. Tethys rotates synchronously with its orbital period, keeping the same face toward Saturn and the same face forward in its orbit. It is accompanied by two tiny moons, Telesto and Calypso (named for daughters of Titans), that maintain gravitationally stable positions along its orbit, analogous to Jupiter’s Trojan asteroids. Telesto precedes Tethys by 60°, and Calypso follows by 60°. (For comparative data about Tethys, its companions, and other Saturnian satellites, see the table.)

Tethys’s most impressive feature is Ithaca Chasma, a giant crack several kilometres deep that extends along three-quarters of the moon’s circumference and accounts for 5–10 percent of its surface. Because the ridges around the feature are heavily cratered, scientists have theorized that the chasm was produced early in the moon’s geologic history, when the water that composes its interior froze and expanded. A second notable feature is the crater Odysseus, which measures 400 km (250 miles) across and has a large central peak. The density of impact craters on Tethys is high, suggesting that the surface is ancient. Nevertheless, the surface is very bright, particularly on Tethys’s leading face, and reflects nearly all incident visible sunlight, which is not typical of geologically old surfaces. Planetary scientists suspect that this distribution of surface brightness is affected by the deposition of micrometre-sized ice particles from Saturn’s E ring, in which Tethys is well-embedded. Cited as evidence is the observation that many of the craters on Tethys have bright floors, whereas the craters on Saturn’s moon Hyperion, which orbits relatively far from Tethys and the E ring, tend to have dark floors. Tethys has several well-defined darker patches, including one near its equator on the leading side and one centred on the trailing side, which is expected to be the region least coated by the E ring. Some low level of geologic activity on Tethys is suggested by the effect the moon has on the charged particles associated with Saturn’s magnetic field.