Tethys the fifth moon of Saturn in size. In our entire Solar System, it ranks 16th among planetary satellites. Its surface is very similar to the nearby Dione, which I have described to you lately. The landmark of this moon is Odysseus impact crater with a diameter of more than 400 kilometers visible in the picture below.
14-frame clear filter mosaic of Tethys from a distance of 42,000 km
By NASA / JPL / SSI / Justin Cowart link [CC BY-SA 2.0 license]
The satellite was discovered in March 1684 by Giovanni Cassini. An Italian astronomer discovered the neighboring moon of Saturn at the same time - Dione. Tethys circles Saturn in an orbit with a diameter of about 589 thousand kilometers, and thus is closer to the home planet than the Dione satellite. One lap of Saturn takes him 45 hours and 18 minutes. The diameter of the Tethys is 1060 kilometers, which is a much smaller satellite than our Earth's Moon. The density of Tethys is about 0.98 kg / dm3, which indicates that the moon is completely made of ice. Like Dione, Tethys has a common orbit with Trojan moons: Telesto and Calypso. These are the tiny moons of Saturn, with a diameter of less than 3 kilometers. They follow before and after Tethys. In the picture below, this satellite is on the background of the Saturn ring system.
Tethys and the rings of Saturn
By NASA/JPL-Caltech/Space Science Institute link [Public domain]
Tethys probably doesn't have an ocean of water beneath its surface, which occurs on larger, ice-rock moons. The surface of this satellite is very bright and reflects much of the sunlight. Impact craters dominate on Tethys, and their number is huge. The most known of them is Odysseus with a diameter of over 400 km, or nearly 40% of the Tethys diameter! It is amazing that the impact of such a huge object didn't destroy this satellite. The second most famous creation of the surface of this moon is the great valley of Ithaca Chasma. This huge canyon is about 2000 kilometers long, 100 km wide and more than 3 km deep! We see it well in the photo below.
Tethys' massive canyon, Ithaca Chasma (near the terminator, at right)
By NASA/JPL-Caltech/Space Science Institute link [Public domain]
The research on this satellite began with the Pioneer 11 probe, followed by Voyager probes in the vicinity of Tethys. However, they didn't give much information. The revolution didn't take place until after the Cassini mission in 2004, just like the rest of Saturn's satellites. Since then, the probe has taken many photos of this ice moon, covering its physical properties and surface details. Saturn's moons hide many secrets and aren't as well known as satellites of planets closer to us. In the following articles I will describe even less known, distant worlds.
