Tethys (astronomy)

Tethys (astronomy), large moon of the planet Saturn. Tethys is spherical, measuring about 1,060 km (about 636 mi) across. Its diameter is about one-third the diameter of Earth’s moon. Tethys orbits Saturn at a distance of about 295,000 km (about 177,000 mi), completing an orbit about once every two Earth days. Tethys’s orbit parallels Saturn’s equator and is only slightly elliptical. Two tiny moons, Telesto and Calypso, move around Saturn at 60 degrees ahead and 60 degrees behind Tethys in the same orbital path as the large moon.

Tethys’s internal structure and composition are unknown, but measurements of its density cause planetary scientists to theorize that it is made almost entirely of water ice, with only a small rocky core. Data gathered by the Cassini space probe provide evidence of possible geological activity on Tethys according to a report published in 2007. Some of the charged particles trapped in Saturn’s magnetic field have been traced to the direction of Tethys, indicating that the moon is likely ejecting material into space. Saturn’s rapid rotation spins the electrically charged gas particles (called plasma) into a disk around the planet. Other moons of Saturn known to be geologically active include Titan, Enceladus, and Dione.

Tethys’s surface is heavily cratered and covered with cracks. The largest crater on the moon is Odysseus, a 400-km (240-mi) asteroid impact crater spanning two-fifths of Tethys’s diameter. Planetary scientists estimate that the blow that created Odysseus would have split the moon apart if Tethys had been solid when the asteroid hit it. Scientists theorize that Odysseus was blasted out long ago, when Tethys was still almost entirely liquid and could absorb the force of a strong shock.

The largest crack in the moon’s surface is Ithaca Chasma, an enormous trench 100 km (60 mi) wide and 3 to 5 km (2 to 3 mi) deep that runs for 2000 km (1200 mi) around Tethys. The length of Ithaca Chasma corresponds to about three-quarters of Tethys’s circumference. Scientists theorize that Ithaca Chasma formed as Tethys froze. The surface probably froze before the interior. The interior would have expanded as it froze, causing Tethys’s surface to shift and split, forming the enormous crack. There are also flat plains with few craters, indicating that Tethys was geologically active after it froze billions of years ago. Ice volcanoes may have erupted and squirted water onto the surface, covering up old craters.

Tethys was discovered in 1684 by Italian-French astronomer Giovanni Cassini, who named his find for a Titan and sea goddess from Greek mythology. Individual features on Tethys are named for persons and places from Homer’s epic poem the Odyssey.