Titan and Other Saturn’s Moons

Although Saturn is most famous for its rings, it also has plenty of interesting moons, with Titan standing out for a couple of reasons. There are 62 moons around Saturn, 24 of which are regular satellites that have prograde, nearly circular orbit and are not greatly inclined with respect to Saturn’s equatorial plane. Other 38 moons are mostly small  most likely captured minor planets, with irregular orbits that are greatly inclined and both prograde and retrograde. Prograde orbit means they revolve around Saturn in the same direction that Saturn rotates.

At first, Saturn’s moons were named after Titans and giants from Greek mythology, but as they ran out of names, astronomers gave them different names from Greco-Roman mythology, as well as Inuit, Gallic and Norse mythologies for respective groups of irregular moons.

Titan

Titan is the second largest moon in the Solar System with the diameter of 5,150 km, after Jupiter’s Ganymede. Titan too is larger than the planet Mercury, but since it’s mainly composed of water ice and rocks, it has only 40% of Mercury’s mass. It orbits Saturn at the distance of 1,221,870 km and it takes 16 days to complete its orbit around the planet.

Titan is the only natural satellite with atmosphere worth mentioning and, other than Earth, only known object with stable bodies of surface liquid. It has winds and rains, rivers and lakes made most likely of ethane and methane. It’s atmosphere is made of 98.4% nitrogen, 1.4% methane, and 0.1% hydrogen with other elements in traces. Before Voyager 1 passed by Saturn in 1980, it was thought that Titan was larger than Ganymede, due to Titan’s atmosphere making it apparently larger when viewed from Earth.

Titan’s high orbital eccentricity is explained by theorised impacts that destroyed moons that had been created from material around Saturn soon after the planet’s creation. From the debris of that first generation of Saturn’s moons, Titan, Iapetus and Rhea are believed to have formed.

Titan’s oceans are could have complex organic molecules and is, beside Europa, perhaps the best place to look for alien life in the Solar System, because it is thought to be similar to Earth long time ago.

It was discovered in 1655 by Christiaan Huygens, the first of many Saturn’s moons, which is no wonder considering its size.

Other Large Saturn’s Moons

Beside Titan, there are six other Saturn’s moons large enough to have ellipsoidal shape - Mimas, Enceladus, Tethys, Dione and aforementioned Rhea and Iapetus. Titan has 96% of the mass around Saturn, the other six about 4% and all other moons and rings only 0.04%.

Mimas, Enceladus, Tethys and Dione are inner large moons with diameters 396 km, 504 km, 1,062 km and 1,123 km respectively. All of them have very short orbital period of less than three Earth days, which is understandable considering their proximity to Saturn. All of them are closer to Saturn than the Moon is to Earth, but Saturn’s gravity is stronger than Earth’s so these moons need higher velocity to stay in orbit. Enceladus is the smallest known geologically active moon in the Solar System. Tethys and Dione have two trojan moons each (see Trojan Moons).

Outer large moons are Rhea, Titan, Hyperion and Iapetus. Rhea is the second largest Satrun’s moon with the diameter of 1,527 km. Hyperion is actually not large enough to become ellipsoid, but with 270 km diameter, it is still considered large moon compared to other, tiny moons. Hyperion is locked with Titan in 4:3 resonance i.e. for every four orbits Titan makes around Saturn, Hyperion makes exactly three. It has highly irregular and unpredictable rotation, making it the only moon in the Solar System without clearly defined equator or poles.

Irregular Saturn’s Moons and Alkyonides

Saturn’s irregular moons were most likely created elsewhere and then captured by the planet. They all have large radii, high inclination and out of 38 of them, 29 have retrograde orbit. They are divided into three groups - Inuit, Gallic and Norse group. Most of them are under 10 km in diameter, with Phoebe in Norse group standing out with 213 km in diameter. Phoebe, being discovered in the late 19th century, kept its Greek name, but all other moons received names from Norse mythology.

Alkyonides are three small, regular moons that orbit between Mimas and Enceladus. They are Methone, Anthe and Pallene.

Trojan Moons

Saturn is the only planet to have so-called trojan moons. They are moons at two Lagrangian points, L4 and L5, before and after another moon on the same orbit, leading and trailing the said moon. Tethys has two such moons, Telesto leading it and Calypso trailing it. Dione also has two, Helene as the leading trojan moon and Polydeuces trailing Dione. For example, Tethys, Telesto and Calypso all share the same orbit, but can never collide because it takes all of them the same time to complete their orbit.

Ring Moonlets, Ring Shepherds and Co-orbitals

Ring moonlets are small moons inside Saturn’s rings and there may be thousands of them. Shepherds on the other hand are moons that shape the planet’s rings by orbiting just beyond rings or between them, accreting material and giving rings their sharp edges and forming gaps between rings. They are Pan, Daphnis. Atlas, Prometheus and Pandora.

Co-orbitals are two moons, Janus and Epimetheus, which are similar in size and have orbits around Saturn so close to each other, they would collide if there wasn’t for a unique mechanism that makes them swap their orbits every four years. Their respective orbits are only 60 km apart, less than their diameters, but as the inner moon, which is slower in speed but has shorter orbit and thus can complete it in less time, reaches the outer moon, gravitation interplay between them causes the inner moon to speed up and increase its orbital semi-major axis i.e. moves farther away from the planet. At the same time, the outer moon slows down and essentially becomes inner moon via the same process. This confused discoverers of Janus and Epimetheus in 1966, who of course thought they were observing a single moon, but couldn’t determine its orbital period.

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