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Learn About Saturn's Moon, Tethys

written by: Rod Martin, Jr.•edited by: RC Davison•updated: 8/12/2011

Saturn has many fascinating moons and Tethys is no exception. Two massive surface features, a body of nearly pure water ice and two Trojan companions make it unique amongst the solar family of celestial bodies.

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    Saturn and its Family

    At last count, Saturn holds reign over more than sixty moons. Most of them are tiny, irregular-shaped rocks only a few kilometers long or smaller. Far more stunning, Saturn's complex system of rings are made of countless particles, each of which might arguably be considered a natural satellite, but this pushes the word "moon" to an unusable extreme.

    Tethys is one of eight moons with a diameter greater than 250 kilometers. Titan, with a diameter of 5,151 kilometers, is the largest of these and claims the title as the second largest moon in the Solar System, after Jupiter's Ganymede, and the only one with an appreciable atmosphere. Hyperion is the smallest of the eight with dimensions of 360 × 266 × 205 kilometers.

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    Tethys, moon of Saturn (NASA). Tethys, moon of Saturn, is appropriately named after the Greek titan and wife of Oceanus. The Greeks also considered her to be a sea goddess and mother to the chief rivers of the known world. All of this association with water gives us a happy coincidence in that Tethys is thought to be almost entirely water ice.

    Originally discovered by Giovanni Cassini in 1684, the moon did not receive its mythological name until 1847, when John Herschel (son of the famous astronomer, William Herschel) proposed its name and that of the six other, then-known Saturn moons. Until that time, it was known as "Saturn III."

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    Surface Features

    Like most other moons in the Solar System, the face of Tethys is pocked with craters. The largest such crater (450 kilometers across) is named Odysseus, after the hero of Greek myth. The other mega-feature of Tethys is the vast chasm called Ithaca Chasma, after the island home of Odysseus in Greek myth. This massive valley stands 100 kilometers wide by 2000 kilometers long and 3–5 kilometers deep.

    Both features are very old, owing to the large number of smaller craters which have littered the surface since those features were created.

    Though Tethys remains one of the brightest (most reflective) bodies in the Solar System, it does possess some surface discoloration. On the leading edge of Tethys in its motion in orbiting Saturn, there is a slight reddening of the surface. On the trailing anti-apex of motion, Tethys is not only reddened, but somewhat darkened.

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    Orbit and Environment

    Saturn, its rings and inner moons (NASA). Tethys orbits about 234,000 kilometers above Saturn's equator and 113,650 beyond the outer edge of Saturn's rings. A day on Tethys is about 1.89 Earth days (45 hours, 20 minutes). Its orbit is almost perfectly circular and inclined to the equator of Saturn by about only one degree.

    Because it is embedded deep within Saturn's magnetosphere, Tethys receives a good helping of plasma and other energetic particles. Standing on Tethys for very long without adequate protection would thus be hazardous to your health.

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    Lagrangian points (Wikipedia). In astronomical terms, a "Trojan" is a body which co-orbits with a larger body at the same distance from their primary. Thus, they share the same orbital period as the larger body. Trojans can persist at two locations in a larger body's orbit, called Lagrangian points. These are gravitationally stable locations, so the bodies never run into each other. The co-orbital Lagrangian points are L4 and L5 (leading and trailing, respectively). These two co-orbital locations are at a sixty degree angle from the larger body as measured from the primary around which they orbit.

    Tethys possesses two Trojans—Telesto (average 24.8 kilometers across) and Calypso (average 21.4 kilometers across). Telesto leads Tethys, while Calypso trails the larger moon. What is interesting is that Tethys' orbital neighbor, Dione, also has a pair of Trojans sharing her orbit—Helene, leading (average 35.2 kilometers) and Polydeuces, trailing (average 2.6 kilometers). Equally interesting is that Saturn possesses the only known Trojan moons in the entire Solar System.

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    Tethys Water

    Sometimes it proves interesting to speculate about the future. What if we had more advanced technology, like that found in science fiction? If Tethys is indeed almost entirely water ice, what could it be used for?

    One fascinating possibility is that of terraforming Mars. If tomorrow's "warp drive" could envelope Tethys and whisk it away from Saturn, all that water might be used to build an ocean on Sol's fourth planet. Thus, Mars might become a second blue planet. Of course, this would have to come after beefing up the Martian atmosphere.

    How much ocean could Tethys offer of her body? Simple volume calculations yield a depth of nearly eight and a half kilometers on a perfectly spherical Mars. Of course, the red planet is not a perfect sphere, so there would be at least one island, if all the ice were used for this one purpose.

    There are two obvious problems with this wild idea directly related to Tethys. How do you warm up all that ice to make it a liquid ocean? And how do you placate the Preserve Tethys Society, which would inevitably organize against such an effort? The first we would have to leave to that future science, along with all the other problems involved with terraforming Mars. For the second, perhaps one could offer the Society far easier visits to Tethys on nearby Mars. One could even name the ocean after that moon. Those with knowledge of Earth's past might appreciate the link to our world's own Tethys Ocean, 250 million years ago—a new bond, forever linking the three worlds.

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    Image Credits

    Tethys color photo, provided by NASA (public domain).

    Saturn, its Rings and Inner Moons, provided by NASA (public domain).

    Lagrangian points diagram, provided by Wikipedia.