We have explored the five largest moons of Saturn in the first part. This part will deal with the remaining 57.
The new moons of Saturn have been found only a hundred years after the discovery of Tethys and Dione. Astronomer William Herschel made a telescope in 1789 with mirrors of 1.2 meters in diameter with the help of which he could see Saturn’s new satellites — Enceladus and Mimas.
Enceladus, despite its modest size (diameter of 500 kilometers), turned out to be a very bright moon and that made it possible to discover it in the XVIII century. Since the satellite reflects 90 % of sunlight, therefore, the temperature drops there to −241 degrees Celsius.
Enceladus moves in the midst of one of the rings of the planet — the dim and wide outer E Ring. The surface of this moon reminds us of another satellite — Europa: same color, same cracks on the surface and same large and smooth areas covered with water ice.
The surface of the moon has cracks, known as “tiger stripes”; material is periodically ejected into space from them. Gravitational forces open and also close up fractures on the satellite. This happens when the moon is approaching or moving away from Saturn. These icy emissions facilitate the fulfillment of the E Ring of Saturn, the largest ring in the Solar System. This ring covers more than a million kilometers. The substance source for the geysers is beneath the ice, the ocean is most likely there. It is presumably located just a few hundred meters from the surface. In 2014 it was announced that an analysis of data collected by Cassini gives reason to suppose the existence of an ocean under the moon’s surface comparable in size to Lake Superior (Earth’s largest freshwater lake by surface area).
The small satellite has gained much attention from many scientists since it has: carbon, liquid water, nitrogen and organic matter. This rather rare combination for the Solar System makes one think of the possibility of the emergence of life forms under the icy layer of the satellite.
Herschel’s son gave name to the satellite in honor of the titan of the Greek mythology. The size of the moon is 400 kilometers, but, in spite of such a small size, it has its own gravity. That makes it the smallest cosmic body that has a rounded shape.
Mimas does not have high density, which means that it is largely composed of water ice and a minor amount of stones. The temperature on the moon does not fall below −209 degrees Celsius.
The satellite has a one distinguishing feature, thanks to which it cannot be confused with other satellites. There is a huge crater on the surface of the moon with a diameter of 139 kilometers. It is called the “Herschel” crater and it is an indication of a collision of Mimas with a giant asteroid. The collision could have resulted in a split of the satellite, what the numerous cracks located on the opposite side from the impact say. The moon also has many smaller craters, but none of them can compete with the Herschel crater.
A gap formed between the widest Saturn’s rings due to the influence of gravitational forces of Mimas. The gap is called the “Cassini Division”.
Another interesting feature of the satellite is its similarity with the Death Star from the Star Wars universe. Some even believe that the satellite was an inspiration to the creators of the movie. However, it is not possible, since the shots on which the crater was discovered were made only three years after the film’s release in 1977.
Mimas and Enceladus almost entirely consist of ice. But the orbit of Mimas is closer to Saturn than Enceladus’ orbit. And despite the fact that Mimas is closer to Saturn, the moon does not experience much heat, meanwhile Enceladus becomes so heated that the ice melts and geysers are formed. This mystery remains unsolved as scientists cannot yet explain this phenomenon.
Hyperion was discovered by William Bond, director of the Harvard Observatory, in 1848. Its average diameter is about 266 kilometers. It is the largest known non-spherical satellite of the Solar System. It never orbits the same because, in addition to Saturn, it is affected by Titan’s gravitational forces.
Satellite’s density is very small; most likely it consists of water ice (60 %) with a small amount of metals and stones and emptiness make up the remaining 40 % of its internal volume. The satellite looks like a sponge — everything that Hyperion faces deeply penetrates. Since its density is slightly more than that of water, therefore, in case the moon was put in an ocean of an appropriate size, it would remain on the surface.
The satellite was found by William Pickering in 1898. Since Phoebe rotates around Saturn in a direction opposite to that of large satellites, it is believed that Phoebe was formed in the Kuiper belt and was later captured by Saturn’s gravity.
Janus and Epimetheus
These two satellites were discovered by a French astronomer Audouin Dollfus in 1966. The moons represent porous bodies that are mainly composed of ice. The satellites density is below the water’s density. They actually move along the same orbit, the distance between their orbits is only fifty kilometers, which is significantly less than their size. Meanwhile they periodically change places.
How does this happen? Janus and Epimetheus move in their orbits independently of each other until the inner moon begins to catch up the outer. At the same time, under the influence of gravitational forces Epimetheus is pushed into a higher orbit and Janus moves to a closer to Saturn orbit, that is, they change places. This occurs once in four years. Probably in the past these two satellites composed a single cosmic body that later divided into two satellites.
Prometheus and Pandora
The moons were discovered in October 1980 by astronomer Stewart Collins. Prometheus and Pandora are “shepherd moons” — with their gravity they exert influence on Saturn’s F Ring. The orbits of the satellites are unstable and are in a resonance with each other. The most noticeable changes in their orbits are visible about every 6.2 years.
The gravitational field of Prometheus creates loops and fractures in the F Ring, while at the same time taking material from it.
Atlas and Pan
Atlas and Pan are also shepherd moons. They are distinguished by their unusual form: they have a prominent equatorial ridge that gives the satellites a special form of a flying saucer. The projections on the equator were formed from the particles of the A Ring.
Pan is within the Encke Gap of the A Ring. The Gap is a 325 km passage in the A Ring for the clearance of which from particles Pan is responsible.
The remaining 47 moons are smaller than forty kilometers. Perhaps, these are not Saturn’s all satellites and there are more of them, but most likely the new satellites will have a small size.
When somebody talks about Saturn, it is most likely that the rings of the planet will come up. Undoubtedly, it is Saturn’s most striking feature.
The rings of the planet mostly consist of particles of ice. The size of the particles varies from a dust particle to large ice floes the size of a car. The thickness of the rings is very small in comparison with their width (from one kilometer to ten meters). Therefore, if you observe the planet from the side, the rings are simply not visible. The main rings are composed of water ice with impurities of silicate dust.
The first who saw the rings was Galileo Galilei — it happened in 1610. The first data showed that the planet has only two rings. But later, thanks to flights of spacecrafts, it turned out that there are many more.
So how many rings does Saturn have? After Cassini’s and Voyagers’ flights it became clear that Saturn has not two or three rings: during approach the rings break into smaller ones and the smaller ones break into even smaller and so on to infinity. Therefore, at this moment it is not possible to answer this question.
A new ring was discovered in 2009 — the Phoebe Ring, the diameter of which turned out to be more than 13 million kilometers. Presumably, the ring consists of particles of the Phoebe satellite.
This concludes the story of Saturn’s moons and we will continue our journey through the Solar System. Moons of Nepture and Uranus are still waiting for us.