It may seem that we have already seen the most interesting satellites in the Solar System in the previous article, devoted to the satellites of Jupiter. However, it is far from that. If anyone can compete with Jupiter in the number of satellites — it is Saturn, the sixth planet from the Sun, a huge and bright gas giant, which is surrounded by thousands of sparkling rings.

The planet is home to a large number of unique worlds. Saturn has 62 known natural satellites with a confirmed orbit — just five satellites less than Jupiter. In addition, Saturn’s moons are not less interesting for the researchers of the Solar System: here we have Titan with a cloud-shrouded surface, riddled with craters Phoebe, and Mimas which looks like the Death Star.

Titan

Dutch mathematician and astronomer Christiaan Huygens discovered the first moon of Saturn in 1655, which was named as “Titan”. It is the largest satellite of Saturn and the second in the Solar System after Ganymede.

Titan’s diameter is equal to 5152 kilometers — larger than the Moon (3474 km) and even Mercury (4865 km is the diameter of this planet)! It is a very large satellite; it is slightly less than Mars (6670 km)! For a long time it was considered that the diameter of Titan was 5550 kilometers and it was ranked first among all satellites of the Solar System. But now, thanks to the Voyager 1 spacecraft we were able to find out its real size. Titan is so massive that it affects the orbits of other moons that are close to it!

Before 2004 it was unknown how the surface of Titan looks like, as the satellite was enveloped by an incredibly thick atmosphere which makes the research difficult. But due to the Cassini-Huygens spacecraft, we know much more about this mysterious moon. The Huygens probe was released on 25 December 2004 and landed on Titan with the help of a parachute on 14 January 2005. After the landing the first data on the moon’s surface became known.

Huygens is the first device created by man that is located on the surface of a planet’s moon (except for the vehicles on the Moon). The descent by parachute through the atmosphere took two hours and 27 minutes during which the probe took samples of the atmosphere. Huygens landed on solid ground (even though the probe was also designed for a sea landing). Due to the external microphone, it was even possible to record the sound of the wind on Titan!

The Huygens probe did send the data back to Cassini and from there they came to the Earth. The probe sent 500 megabytes of data including 350 images. It was planned to send 700 images, but due to a failure in the program half of the photos was lost.

With the help of the obtained data it turned out that Titan’s surface is relatively young in geological terms and is covered with sedimentary organic compounds and water ice. The entire surface is almost flat, only a few mountains and craters. Surface temperature is 170–180 degrees below zero Celsius.

Photos taken with Cassini-Huygens in 2005 proved the presence of methane-ethane lakes and rivers. Their presence makes Titan the second object (the first is Earth) in our System, where the presence of liquid on the surface is proven. Since the moon has fluid and an atmosphere, a hypothesis was presented that there may be a primitive form of life. The atmosphere has mainly nitrogen (approximately 95 %) as well as methane and ethane. The limit of Titan’s atmosphere is about ten times higher than on Earth! For comparison, Earth’s atmosphere has 77 % nitrogen.

Titan has signs of volcanic activity. But the volcanoes there are not silicate like on Earth, Venus or Mars. They are called cryovolcanoes — a volcano that erupts a water-ammonia mixture with an admixture of hydrocarbons in the atmosphere.

Titan is located outside the Saturn rings. Just like the Earth’s Moon it performs synchronous rotation around its planet. Scientists presume the existence of underground water reservoirs on the satellite and this, combined with the low gravity, makes Titan a great candidate for colonization.

Around 2020s a new mission is planned to study Titan, Saturn and Enceladus. The spacecraft will consist of a space station and two probes that will study Titan. One probe will float among the clouds in the atmosphere and it will have to make at least one revelation around the planet, the second probe should splash down in the polar sea of hydrocarbons. We are only left to wait for the launch and then to follow the research of the satellite.

Iapetus

The second discovered moon of Saturn was Iapetus. Giovanni Cassini saw the satellite in his telescope in 1671. It is the third-largest Saturn’s satellite (its diameter is 1494 km). It is a very interesting satellite not only because of its unusual color (one half is white and bright as snow and the other is dark as the black velvet), but also because it has a ridge encircling the entire satellite on the equator.

The contrasting sides of the satellite are divided with a quite visible boundary, only partly they come to each other’s territory — the dark at the equator and the light at the poles. The main color of the satellite is white, since it is made up of water ice. Scientists suggest that Phoebe affects the dark side of the satellite by strewing dust to Iapetus. Phoebe constantly collides with other cosmic bodies and therefore it has a black dust tail. And since Phoebe flies towards Iapetus, the latter gets all the dust that Phoebe leaves in space.

The surface of Iapetus is covered with numerous craters of various depth and diameter. Thanks to the Cassini spacecraft an interesting peculiarity of the relief was discovered. Right on the equator the satellite is encircled by the mountain range that stretches for 1300 kilometers with a height of less than 13 km and a width of about twenty km. These characteristics make the “Wall of Iapetus” one of the highest massifs in the Solar System. In the leading (black) hemisphere the ridge is continuous and in the bright part of Iapetus — these are separate mountains.

There are several opinions about its appearance. Some scientists believe that this mountain range was formed due to tectonic shifts in the formation of Iapetus. No tectonic plates and volcanic activity were now seen on Iapetus. Meanwhile astronomer Andrew Dombard thinks that rings could be the cause of the ridge. The rings were formed from an icy space object that was orbiting for a long time on Iapetus orbit. The convergence of bodies continued as long as the tidal forces did not rip the supposed Iapetus’ satellite on a large number of pieces from which the rings emerged. Then, under the influence of gravity, these pieces fell on the surface of Iapetus and that led to the appearance of a ring of the mountain range.

Rhea

A year after the discovery of Iapetus, Giovanni Cassini discovered another Saturn’s moon — Rhea. It is the second largest moon of Saturn. Mostly, the satellite consists of water ice; stone rocks occupy less than one third of the total weight. The ice on the moon is solid as a rock — this is due to very low temperatures, in dark areas it drops to −220 degrees Celsius. Rhea is two times less than the Moon — its diameter is 1528 km.

During formation the satellite underwent strong meteorite bombardment and, therefore, it has a much cratered surface. Rhea is considered a satellite with the largest number of craters in the Solar System. The hemisphere that is turned to the planet is mostly of a bright color while the other hemisphere has dark areas with bright dashes. Scientists have determined that it is a series of cliffs and ridges of ice.

Thanks to Cassini we know that Rhea has a thin layer of rarefied atmosphere containing oxygen (70 %) and carbon dioxide (30 %). The density of the atmosphere is twelve times less than that of the Earth. But how does Rhea have an atmosphere? It turns out that water ice splits with a flow of ions from the radiation belts of Saturn. And carbon dioxide occurs, perhaps, from the subsoil of the satellite or during oxidation of organic compounds on the surface.

Dione

In 1684 Giovanni Cassini saw two more satellites — Dione and Tethys. Among Saturn’s satellites Dione is the fourth-largest, its diameter is 1132 kilometers. By density it is second only to Titan, which may indicate the presence of solid core under the layer of ice.

Dione is very similar in composition and the appearance of the surface to Rhea. This satellite also underwent meteorite bombardment during the formation and, therefore, its surface is covered with craters. Some craters are up to a hundred kilometers in diameter. As on the Rhea satellite, Dione’s surface has white stripes which are fractures and ridges of ice.

Strange is the fact that most of the craters are on the trailing hemisphere, although usually the leading hemisphere takes all the hits. There is a hypothesis that the moon turned 180 degrees due to a collision with a cosmic body. The position in which Dione is now has been around for billions of years.

A thin layer of rarefied atmosphere was discovered on the satellite. Usually the bodies of such small sizes have no atmosphere because they have not enough gravity force to hold it. But the atmosphere is formed here, as on Rhea, due to the bombing of the moon’s surface by ions from the radiation belts of Saturn.

Two more satellites move on the same Dione’s orbit. One — Polydeuces, it lags behind Dione 60 degrees, and Helene (second), on the contrary, outruns Dione. Such celestial bodies are called trojan moons.

Tethys

As said, Tethys was discovered by Giovanni Cassini in 1684. Its diameter is about 1060 kilometers. Tethys has low density because it consists mainly of water ice with a small amount of dark material. The temperature on the surface of the satellite is −187 degrees and the surface has also a high reflectivity.

The satellite has a several topographic features — the Ithaca Chasma and a huge crater Odysseus. The Ithaca Chasma valley covers almost three-quarters of its diameter. Its emergence is associated with an ancient catastrophe that occurred on the satellite when it was still in the molten state. The large canyon emerged at the time when the liquid layers solidified. Other astronomers attribute the appearance of the canyon with the fall of the cosmic body which left the Odysseus crater. Such a hard blow could destroy the satellite if it consisted of solid material. But only Odysseus with a diameter of 450 kilometers and a depth of five remained after the impact

Same as Dione, Tethys has its own trojan moons. These are two small moons — Calypso and Telesto; they are located at the Lagrange points in front and in the back at a distance of 60 degrees.

These were the five largest Saturn’s satellites, and now let’s compare their size with the Earth and the Moon.

In the next part we will conclude our study of Saturn’s satellites and will also learn about its rings.