Tethys

Tethys is 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 1,060 km (659 miles), and its density of 1.0 g/cm3 (0.58 oz/in3)—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 gravi-tationally stable positions along its orbit, analogous to Jupiter's Trojan asteroids.

Image of Tethys, showing Ithaca Chasma, from the Cassini-Huygens spacecraft. NASA/JPL/ Space Science Institute

Telesto precedes Tethys by 60°; Calypso follows by 60°.

Tethys shows little evidence of internal activity. 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 highly reflective, especially on Tethys's leading face, 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.

DlONE

Dione was discovered by the Italian-born French astronomer Gian Domenico Cassini in 1684 and named for a daughter of the Titan Oceanus in Greek mythology.

Dione has a diameter of 1,120 km (696 miles) and revolves around Saturn in a prograde, nearly circular orbit at a mean distance of 377,400 km (234,500 miles), which is within the outer part of Saturn's tenuous E ring. It is accompanied in its orbit by two much smaller moons, Helene and Polydeuces (also named for Greek mythological figures). Helene, which has a diameter of about 30 km (20 miles), maintains a gravitationally stable position 60° ahead of Dione. Polydeuces has less than half the diameter of Helene and follows Dione by 60°, though with large deviations from its mean position. The orbits of these tiny companion moons can be compared to those of Jupiter's Trojan asteroids.

Tidal interactions with Saturn have slowed Dione's rotation so that it now turns synchronously with its orbital motion, always keeping the same hemisphere toward the planet and always leading with the same hemisphere in orbit. The surface of Dione shows great brightness contrasts, with the trailing hemisphere generally darker than the leading one. On average, however, Dione is highly reflective, which indicates a surface composed of large amounts of geologically fresh water ice. The moon's low density, which is 1.5 times that of water, is consistent with a bulk composition of about equal amounts of ice and rock. Dione is heavily covered in places with impact craters; their density and distribution of sizes suggests a geological age close to four billion years. In other areas, however, the crater density is lower, which suggests that the moon may have experienced substantial ice melting and resurfacing. It is also possible that Dione's surface is continually coated by new ice particles deposited from the E ring.

Most of Dione's craters are found on the bright, leading hemisphere. The opposite hemisphere is broken by many bright linear features that form a polygonal network. Some of these bright features are associated with linear troughs and ridges and may have been caused by episodic tectonic activity. Wispy features seen in Voyager spacecraft images had been thought to be deposits of recondensed volatile material that erupted from Dione's interior along linear fractures. Higher-resolution images from the Cassini spacecraft, however, show no evidence of such activity. The asymmetry of the moon's surface is not understood, although there is evidence of a major impact near the centre of the linear network on the trailing hemisphere.

Like many objects in orbit around Saturn, Dione is involved in an orbital resonance; i.e., its 66-hour trip around Saturn is twice that of the nearer moon Enceladus. This relationship has been proposed as a source of the dramatic tidal heating seen in Enceladus, but the details of this mechanism have not been worked out.

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