There are two groups of Saturn’s moons that share their orbits three at a time. On the one hand we have Tethys , accompanied by Telesto and Calypso and on the other hand we have Dione , accompanied by Helene and Polydeuces . These moons orbit at exactly the same distance from Saturn and with the same orbital period (that is, they take the same time to complete an orbit) but separated by an angle of 60º . This is so because two moons of each trio orbit at the fourth and fifth Lagrange points of the other moon. But let’s get to know them a little better.
Tethys and Dione are two of the gas giant’s main moons. Dione is its fourth largest moon , with a diameter of about 1,120 kilometers, while Tethys is its fifth largest moon , with a diameter of about 1,060 kilometers. These diameters, although considerably smaller than that of our own Moon, which reaches more than 3,500 kilometers in diameter, give Tethys and Dione larger than, for example, the dwarf planet Ceres .
This size is more than enough for both moons to have a roughly spherical shape (deformed by interaction with the gravity of Saturn and its other moons). Tethys is considerably less dense (and less massive) than Dione , with a density just below that of water. This tells us that this satellite is made up mostly of water ice , with perhaps a tiny rocky core inside. We think it is unlikely that it contains an ocean of liquid water, unlike Dione , which could. This moon also has an outer layer of water ice, although its rocky core could make up a good percentage of its mass .
Both moons were discovered in 1684 by the Italian mathematician, astronomer, and engineer Giovanni Domenico Cassini using the Paris Observatory telescope. In his honor , the NASA Cassini probe that would visit Saturn and its system of moons and that would give us the best images we have today of both Tethys and Dione was named.
The four moons that accompany Tethys and Dione two by two are considerably smaller and consequently we know much less about them. Telesto , the first of Tethys’s companions, is irregularly shaped with a size of about 25 kilometers and a very smooth surface, devoid of major craters . Calypso , on the other hand , has a much more elongated shape , with its longest side reaching 30 kilometers while its shortest side barely exceeds 14 kilometers . It also has a relatively smooth surface , due to the dust and regolith that covers it, although it does show several impacts from smaller bodies on its surface.
Dione is accompanied by Helene and Polydeuces. Helene is the largest of these four satellites, with dimensions of around 30 kilometers and its surface is the best studied of the set. Remains of several important collisions have been observed in it, as well as signs of geological processes and erosion , probably promoted by Saturn’s gravity and its magnetosphere. Polydeuces is by far the smallest of these moons, just 3 kilometers across. This made it the last to be discovered, in 2004 by the Cassini probe. Telesto, Calypso and Helene were discovered in 1980 with observations made from Earth. Their names come from Greek mythology and their myths and legends.
These four moons are probably asteroids trapped by Saturn’s gravity sometime after its formation, in the early solar system . These asteroids would be among the few that would have survived a process that started hundreds or thousands of other objects. It is very difficult to know exactly how they ended up in their current configuration, but the result of all this was that they ended up inhabiting the fourth and fifth Lagrange points of the moons Tethys and Dione (Telesto occupies the L 4 of Tethys and Calypso its L 5 , while that Helene occupies Dione’s L4 and Polydeuces her L5 ). These Lagrange points are regions of space in which the gravity of two larger bodies (in this case Saturn and Tethys and Dione separately) compensate to create a certain balance.
In the case of the first three Lagrange points , located on the very line that joins the two main bodies, this balance is unstable , so that any deviation leads the third object to move away from that point. On the other hand , the fourth and fifth points lead to a stable equilibrium , in which the satellites located there oscillate around these points, sharing their orbit with the larger moons.
These objects of course have the same behavior as Jupiter’s Trojan asteroids , which we believe could number more than a million and inhabit the vicinity of their L4 and L5 points. This is why these four minor moons are known as Trojan satellites . They are one more example of the complexity and variety of our solar system.
