![Image of Tethys, showing Ithaca Chasma, from the Cassini-Huygens spacecraft.[Credits : NASA/JPL/Space Science Institute]](http://media-2.web.britannica.com/eb-media/10/96610-003-F827CEA4.gif)
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 1,060 km (659 miles), and its density of 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°; Calypso follows by 60°. (For comparative data about Tethys, its companions, and other Saturnian satellites, see the table.)
| Moons of Saturn{1} | |||||||||||
| name | numerical designation | ****mean distance from ***centre of planet ***(orbital radius) | orbital period (sidereal period; Earth days){5} | inclination of orbit to planet’s equator (degrees) | eccentricity of orbit | rotation period (Earth days){7} | radius or radial dimensions (km) | mass (kg) | mean* density (g/cm3) | ||
| 00000km | Saturn radii0 | ||||||||||
| Pan | XVIII | 133,583 | 2.216 | 0.575 | about 0 | about 0 | 10 | ||||
| Atlas | XV | 137,666 | 2.284 | 0.602 | about 0 | about 0 | 19 × 17 × 14 | ||||
| Prometheus | XVI | 139,377 | 2.313 | 0.613 | 0 | 0.0024 | 70 × 50 × 34 | ||||
| Pandora | XVII | 141,713 | 2.351 | 0.629 | 0 | 0.0042 | 55 × 44 × 31 | ||||
| Epimetheus{2} | XI | 151,415 | 2.512 | 0.694 | 0.34 | 0.009 | sync. | 69 × 55 × 55 | |||
| Janus{2} | X | 151,417 | 2.512 | 0.695 | 0.14 | 0.007 | sync. | 99 × 96 × 76 | |||
| Mimas | I | 185,520 | 3.078 | 0.942 | 1.53 | 0.0202 | sync. | 199 | 3.60 × 1019 | 1.10 | |
| Enceladus | II | 238,020 | 3.949 | 1.370 | 0.02 | 0.0045 | sync. | 249 | 8.53 × 1019 | 1.30 | |
| Telesto{3} | XIII | 294,600 | 4.888 | 1.888 | about 0 | about 0 | 15 × 13 × 8 | ||||
| Calypso{3} | XIV | 294,600 | 4.888 | 1.888 | about 0 | about 0 | 15 × 8 × 8 | ||||
| Tethys | III | 294,660 | 4.889 | 1.888 | 1.09 | 0.0000 | sync. | 530 | 6.03 × 1020 | 1.00 | |
| Dione | IV | 377,400 | 6.262 | 2.737 | 0.02 | 0.0022 | sync. | 560 | 1.12 × 1021 | 1.50 | |
| Helene{4} | XII | 377,400 | 6.262 | 2.737 | 0.2 | 0.005 | 16 | ||||
| Polydeuces{4} | XXXIV | 377,400 | 6.262 | 2.737 | 6.5 | ||||||
| ------------------------------------------------------------------------------------------------ known outer extent of ring system ------------------------------------------------------------------------------------------------ | |||||||||||
| Rhea | V | 527,040 | 8.745 | 4.518 | 0.35 | 0.0010 | sync. | 764 | 2.46 × 1021 | 1.30 | |
| Titan | VI | 1,221,850 | 20.274 | 15.940 | 0.33 | 0.0292 | sync. | 2,575 | 1.35 × 1023 | 1.88 | |
| Hyperion | VII | 1,481,100 | 24.575 | 21.280 | 0.43 | 0.1042 | chaotic | 185 × 140 × 113 | 5.5 × 1018 | 0.50 | |
| Iapetus | VIII | 3,561,300 | 59.091 | 79.330 | 15{6} | 0.0283 | sync. | 718 | 1.59 × 1021 | 1.00 | |
| Phoebe | IX | 12,952,000 | 214.907 | 550.4 R0 | 175.3 | 0.163 | 0.4 | 110 | 8.3 × 1018 | 1.60 | |
| {1}Beginning in 2000, about 40 additional moons were discovered with electronic detectors in Earth-based observations or in spacecraft images. Most have large orbital radii, eccentricities, and inclinations, and more than half have retrograde orbits. A small number orbit in a prograde direction within a few Saturn radii of the planet. Rough size estimates based on brightness place them between 3 and 20 km in radius. They were assigned provisional numerical designations on discovery; many have received official names. {2}Co-orbital moons; certain values are variable owing to orbital exchange. {3}"Trojan" moons: Telesto precedes Tethys in its orbit by 60°; Calypso follows Tethys by 60°. {4}"Trojan" moons: Helene precedes Dione in its orbit by 60°; Polydeuces follows Dione by 60° on average, but with wide variations. {5}R following the quantity indicates a retrograde orbit. {6}Average value. The inclination oscillates about this value by 7.5° (plus or minus) over a 3,000-year period. {7}Sync. = synchronous rotation; the rotation and orbital periods are the same. |
|||||||||||
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.
Link to this article and share the full text with the readers of your Web site or blog-post.
If you think a reference to this article on "Tethys" will enhance your Web site,
blog-post, or any other web-content, then feel free to link to this article,
and your readers will gain full access to the full article, even if they do not subscribe to our service.
You may want to use the HTML code fragment provided below.
We welcome your comments. Any revisions or updates suggested for this article will be reviewed by our editorial staff. Contact us here.
Regular users of Britannica may notice that this comments feature is less robust than in the past. This is only temporary, while we make the transition to a dramatically new and richer site. The functionality of the system will be restored soon.