We have already seen the Galilean moons of Jupiter (i.e., Io, Europa, Ganymede, and Callisto), and see why they have attracted our interest. Now will do the same for the moons of Saturn. Saturn has 82 confirmed moons that are not embedded in its ring system, but only 13 of them have a diameter greater than 50 km. Through space exploration programs, we were able to extract crucial information for all these moons that have left us stunned. So, let’s see below what may classify fun facts satellites of Saturn.
Saturn’s moons in numbers
The moons of Saturn that have attracted our interest are: Titan, Rhea, Enceladus, and Mimas.
Titan is the largest satellite of Saturn and the second-largest in the Solar system, and it was discovered in 1655 by Christiaan Huygens. Titan is named after the ancient Greek mythology of pre-Olympian Gods, the Titans.
It has a radius of 2,574 km. Titan is larger than Mercury, but due to its low density (i.e., 1.88 g/cm3), its mass is only 40% of that of Mercury (1.35×1023 kg). All these correspond to the surface gravity of 0.138g (i.e., 1.352 m/s2, lower than the Moon).
Titan orbits around Saturn at a mean distance of 1,221,870 km, and it completes an orbit around the planet in 15 days and 22 hours. Just like our Moon, Titan is tidally locked, meaning it is in a synchronous orbit, showing the same side on Saturn. Finally, the mean temperature in Titan is -179.5°C (-291°F).
Rhea is the second-largest moon of Saturn and the ninth-largest in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini. It is named after Rhea, the wife of Kronos, and the mother of the Olympian gods. It has a radius of 764 km, a mass of 2.3×1021 kg, while its mean density is 1.23 g/cm3. These correspond to a gravity acceleration of 0.265 m/s2.
Rhea orbits around Saturn at a mean distance of 527,100 km. The satellite is tidally locked, so it always shows the same face on Saturn, and it needs around 4 days and 12 hours to complete an orbit. Finally, the temperature in Rhea varies between -174°C (-281°F) in direct sunlight and -200°C (-328°F), and -220°C (-364°F) in the shade.
Enceladus is the sixth-largest satellite of Saturn, and it was discovered in 1789 by William Herschel. It is named after Enceladus, a giant from the Greek mythology. The mean radius of Enceladus is 252 km, and its mass is 1.08×1020 kg. The mean density of Enceladus is 1.61 g/cm3, and all these correspond to a surface gravity acceleration of 0.113 m/s2.
Enceladus orbits Saturn at a mean distance of 237,950 km, and it is tidally locked, white the satellite needs 1 day and 9 hours approximately for an orbit around Saturn. The temperature on Enceladus varies between -128°C (-198°F), and -240°C (-400°F).
Mimas is the seventh-largest satellite of Saturn, and it was discovered in 1789 by William Herschel. Its name comes from Mimas, a giant and son of Gaia in Greek mythology. The mean radius of Mimas is just shy of 200 km (i.e., ~ 198 km), and its mass is 3.75×1019 kg. The mean density of Mimas is 1.15 g/cm3, and all these correspond to a surface gravity acceleration of 0.064 m/s2.
Mimas orbits Saturn at a mean distance of 181,900 km, it is tidally locked, and Mimas needs 22 hours and 35 minutes approximately, for an orbit around Saturn. The mean temperature on Mimas is around -344.5°C (-588°F).
Structure of the moons
As a moon, Titan is undoubtedly one of a kind. Titan is the only satellite that has a dense atmosphere, and it’s also the only other place apart from Earth that has rivers and lakes. Titan’s atmosphere is thicker than Earth’s, and it has a surface pressure of 1.45 atm, and it is also ~1.2 times more massive. Its composition is 97% nitrogen, 2.7% methane, 0.1-0.2% hydrogen, with traces of other gases, such as ethane, propane, acetylene, carbon dioxide, hydrogen cyanide, argon, and helium.
Moving to Titan itself, its density indicates that it is composed of 50% of water, and 50% rocky-material. Most likely, Titan has a rocky core that extends out to 2,000 km from its center. The core is surrounded by a shell of water ice known as ice-VI. This ice type is only found at extremely high-pressure. The ice-shell is surrounded by a layer of salty liquid water, and on top of it sits an outer crust of water ice. The surface is coated with organic molecules. These have settled from the atmosphere, in the form of sands and liquids, or as rain.
If we exclude the low temperature and the different chemical composition, Titan’s surface is one of the most Earth-like in the Solar system. Its surface is carved by flowing methane and ethane, which creates rivers and forms lakes of liquid hydrocarbons. The other similarity with Earth’s surface is the low number of visible impact craters. This means that the surface of Titan is young, and also there is a process that “erases” the craters. On Earth, this happens due to wind, water flow, and from the activity of plate tectonics. On Titan, the process is similar (i.e., wind, liquid flow), but scientists have not found evidence of seismic activity.
The density of Rhea suggests that the satellite is made ~75% of water ice and ~25% of rock. It is not clear if Rhea has a rocky core or not, thus it could be the largest object in the Solar system that doesn’t have distinct layers (i.e., crust, mantle, and core). Images from the Voyager probes have shown that the surface of Rhea is heavily cratered. Additionally, Voyager images have shown wispy structures with lengths that reach 100s of kilometers, cutting through plains and craters. Higher resolution images from Cassini in 2006, have shown that these wispy areas are subsidence fractures that make canyons. Such features could result from tectonic activity.
Cassini also detected a thin atmosphere composed of oxygen and carbon dioxide. This makes Rhea the only celestial object (if we exclude Earth) in the Solar system that has oxygen in its atmosphere. The abundance is five trillion times less than the amount of oxygen in our atmosphere. The source of oxygen most likely comes from the surface of Rhea. As the magnetic field of Saturn rotates over Rhea, high-energy particles, trapped in the magnetic field, decompose the surface and release oxygen.
Little was known about Enceladus until August 1981, when Voyager 2 showed the first detailed observations. Images revealed that Enceladus has at least five distinct types of terrain. Thus, there are several cratered regions and regions where the surface is smooth, and without craters (i.e., young areas). The latter often borders areas of ridged terrain. Additionally, extensive linear cracks and scarps were observed. Cassini provided further information on Enceladus, when rifts were found on its surface. These surface structures can be up to 200 km long, 5-10 km wide, and 1 km deep.
The fact that Enceladus has a young surface means that geological activity is needed for renewing it with “fresh” material. Thus, Enceladus most likely has active water volcanism. Further evidence for the volcanism activity comes from the fact that Enceladus is near the E ring. So volcanism in Enceladus, not only renews its surface, but it also feeds material to the E ring, in the form of water and volatiles (i.e., nitrogen, ammonia, etc.).
The density of Enceladus indicates that it has a higher abundance of silicates and iron than the other similar size satellites of Saturn. Thus, Enceladus could have a rocky core, followed by a layer of liquid water, surrounded by an icy crust.
The low density of Mimas (1.15 g/cm3), indicates that it is composed mostly of water ice with a small amount of rock. The most distinctive feature of Mimas’ surface is a giant impact crater that is 130 km across. The crater is named Herschel in honor of the discoverer of the satellite. The crater’s width is equal to one-third of Mimas’ diameter, its walls are 5 km high, and its depth can reach down to 10 km deep. The central peak of the crater reaches an altitude of 6 km. An equivalent impact on Earth would result in a crater 4,000 km across.
Another eye-catchy feature of Mimas is that it is a solid-frozen world, unlike Enceladus. What makes it more interesting is that Mimas is closer to Saturn, meaning that the tidal heating should be higher. This paradox has led to the creation of what is known as the “Mimas test”. This means if there is a theory to explain the frozen nature of Mimas, then the same theory should explain the water geysers of Enceladus.
Interesting facts of the moons of Saturn
- Before the arrival of Voyager 1, we thought Titan was the largest satellite in the Solar System. The overestimation was caused by the thick atmosphere that Titan has.
- If you have an oxygen mask and the analogous clothing for the temperature, you will be able to walk on Titan’s surface without the need of a spacesuit.
- If Titan has volcanic activity, the role of the molten rock lava is replaced by liquid water
- Titan’s atmosphere rotates faster than Titan itself.
- In 2008, NASA announced the presence of orbiting material around Rhea. This would make Rhea the first satellite with a ring system. This still needs confirmation.
- Over 100 geysers have been identified on Enceladus.
- Due to its volcanoes that spread fresh-ice, Enceladus has the most reflective surface in the Solar system.
- The temperature map of Mimas has revealed a temperature distribution that resembles Pac-Man.
- Due to the extent of the Herschel crater and appearance, Mimas is usually compared with the Imperial Death-star.