Tethys is one of Saturn’s moons. It was discovered in 1684 by the Italian astronomer Giovanni Domenico Cassini.

Diameter: 1.06 x 106 m
Mass: 6.17 x 1020 kg
Composition: Water ice with minor amounts of rock
Orbit: 2.94 x 105 m from Saturn

Tethys is the fifth largest of Saturn’s moon and the smoothest of its photographed satellites. Saturn has 62 moons with definite orbits, 53 of which have been named. 24 of these are regular satellites that orbit Saturn with only small inclinations to the planet’s equatorial plane. 37 of the remaining 38 are irregular satellites that orbit at high inclinations and a much greater distance from Saturn. These are probably minor planets that have been captured by the planet or else debris from their break-up during the process. The final irregular satellite is unclear.

Saturn was the Roman god of agriculture and harvest and he had a number of siblings, called Titans and Titanesses. Their names were used for Saturn’s moons, but when astronomers ran out of names, other ones were taken from additional characters in Greek and Roman mythology or from notable figures from the mythology of other cultures. The irregular satellites are named after characters in Inuit, Gallic and Norse mythology. Tethys is named after a Titaness and sea goddess who was the daughter of Uranus and Gaia, the mother of all the heavenly gods. Tethys was mother to the main rivers known to the Greeks, such as the Nile, Alpheus and Maeander. The name was first suggested 1847 by William Herschel’s son John.

The composition of Tethys is almost entirely water-ice. Astronomers suggest that Tethys was once fluid that later froze. The crust froze before the moon’s interior. Its surface is dominated by a gigantic canyon system, Ithaca Chasma. This is 100 km wide, 2,000 km long and 8 km deep. It is dotted with smaller, younger impact craters. The western hemisphere is notable for a 400 km wide impact crater, called Odysseus. This crater looks like a smooth basin and has no central mountain.

Astronomers had assumed that Tethys had no geological activity. In 2007, Cassini spacecraft flybys detected plasma – charged atoms – plumes. Later research showed these plumes were associated with geologically active moons such as Enceladus and Io, rather than Tethys.

Tethys takes 1.89 Earth days to orbit Saturn in the planet’s E ring. The orbit coincides with the rotation of the moon on its own axis. Along with Saturn and its other moons, it takes 29.4 Earth years to orbit the Sun.

Spacecraft missions Pioneer 11 in 1979, Voyager 1 in 1980 and Voyager 2 in 1981 took the first photographs of Tethys. The Cassini spacecraft mission to Saturn provided detailed images of the moon. The closest Cassini flyby was 1,500 km from Tethys, providing images of 1.63 km resolution.

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