It is a large satellite of Saturn, the ninth furthest from the planet, around which it revolves at a distance of about 295,000 kilometers. It takes about two Earth days to complete its orbit, which is in the same plane as the Saturn equator and is only slightly elliptical. Two very small moons, Telesto and Calypso, revolve around Saturn in the same orbit.
It was discovered in 1684 by astronomer Gian Domenico Cassini and watched closely by Voyager automatic probes.
Thetis is spherical and measures about 1,060 km in diameter, approximately one third of that of the Earth's moon. It is the fifth satellite of Saturn because of its size. Its internal structure and composition are unknown, but by measuring its density it is believed that it is composed almost entirely of ice water, with only a small rocky core.
Its surface is covered with cracks and has numerous craters, among which the largest is Odysseus, which, with 400 km in width, covers two fifths of the diameter of Tethys and is probably due to the impact of an asteroid. The biggest geographical accident of its surface is a huge pit of 100 km wide and between 3 and 5 km deep that extends along 2,000 km, almost three quarters of the circumference of the satellite.
There are also flat areas with few craters, indicating that Tethys had geological activity after freezing billions of years ago. It is possible that ice volcanoes threw water to the surface covering the ancient craters.
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