Moons of Saturn

Saturn

Of all the planetary bodies in the solar system, as of the time of this article being written, Saturn has the greatest number of moons. With 82 moons, Saturn has 3 more moons than Jupiter does. 53 of the 82 moons have formal names, with 29 not having official names yet. The largest moon of Saturn, Titan, comprises more than 90% of the all of mass in orbit around Saturn, including the rings. There is evidence that the second largest moon of Saturn, Rhea, may have its own rings; moreover, there are hundreds of moonlets in the rings of Saturn itself which have a size of 40 to 500 meters. These moonlets are not considered to real moons, so we will be spending some time to discuss some of the most familiar and well known of the 83 moons of Saturn.

Mimas

Mimas was discovered in 1789 by William Herschel and is named after Mimas, a son of Gaia in Greek mythology. With a diameter of 396 kilometers or 246 miles, it is the smallest astronomical body that is known to still be rounded in shape because of self-gravitation. To put the size of Mimas in perspective, Mimas has a surface area slightly less than the land area of the country of Spain. Although it is worth noting that Mimas is not actually in hydrostatic equilibrium for its current rotation. With a low density, Mimas is thought to have a composition largely of mostly of water ice with only a small amount of rock. Mimas’s most distinctive feature is a giant impact crater 81 miles across, named Herschel after the discoverer of Mimas, which means that Herschel’s diameter is almost a third of Mimas’s own diameter. Probes that have flown past Mimas include Pioneer 11, Voyager 1, Voyager 2, and the Cassini orbiter.

Fun Fact:  If there were a crater of an equivalent scale on Earth of Mimas’ largest crater, Herschel, such a crater would be over 2,500 miles in diameter, which would make it wider than Australia.

Enceladus

As the 6th largest moon of Saturn, Enceladus its about 310 miles in diameter. This size makes Enceladus 1/10th the size of Titan, Saturn’ss large moon for perspective. is the sixth-largest moon of Saturn. It is about 500 kilometers in diameter. William Herschel discovered Enceladus on August 28th , 1789, by William Herschel, but little was known about it until the Voyager 1 and Voyager 2 spacecrafts passed near to the moon. In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. Of particular note Cassini discovered water-rich plumes venting from the south polar region.[1] Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System. There are ice volcanos near the southern pole, which shoots shoot geyser-like jets of water vapor, molecular hydrogen, other volatile compounds, and solid material, including sodium chloride crystals and ice particles, into space.[2] Some of the water vapor falls back as “snow”; the rest escapes, and supplies most of the material making up Saturn’s E ring.[3]

Fun Fact: According to NASA scientists, the plumes are similar in composition to comets.[4] 

Tethys

Discovered by G. D. Cassini in 1684 and named after the titan Tethys of Greek mythology, Tethys has a density so low that the moon actually has the lowest of all the major moons in the Solar System. This has indicated to scientists Tethys is made of water ice with just a small fraction of rock, which was later confirmed by the spectroscopy of its surface. Tethys is heavily cratered and cut by a number of large faults. Like all other regular moons of Saturn, Tethys formed from the Saturnian subnebula, which is a disk of gas and dust that surrounded Saturn soon after its formation. Tethys has been approached by several space probes including Pioneer 11, Voyager 1, Voyager 2, and Cassini.

Dione

Discovered by by Italian astronomer Giovanni Domenico Cassini in 1684, the moon Dione was named after the Titaness Dione of Greek mythology. Dione is the 15th largest moon in the entire solar system at 697 miles in diameter. This makes Dione more massive than all of the all known Saturnian moons smaller than itself combined.

Fun Fact: About two thirds of Dione’s mass is water ice, and the remaining is a dense core, probably silicate rock.[5]

Rhea

The name Rhea comes from the Greek goddess, otherwise known as Titan, Rhea, who was the daughter of Uranus and Gaea. Her husband was Kronus, who is known in Roman culture, as Saturn. Rhea was also called the mother of the gods because she gave birth to several of the gods of Mount Olympus, including Zeus. Rhea is the second largest moon of Saturn, but with a mean radius of 475 miles, it is less than a third the radius of Saturn’s largest moon, Titan. Rhea is a small, cold, airless body that is very similar to sister moons Dione and Tethys.
Fun Fact: The density of Rhea is about 1.233 times that of liquid water, which suggests that Rhea is three quarters ice and one quarter rock.

Titan

Titan is the second largest moon in our solar system, with only Jupiter’s moon Ganymede being just 2% larger. It is also worth noting that Titan is larger in size than Earth’s moon, and larger than even the planet Mercury. Titan is the only moon in the solar system with a dense atmosphere, and it’s the only world besides Earth that has standing bodies of liquid, including rivers, lakes and seas, on its surface.[6] Titan is also tidally locked to Saturn. It is possible that Titan hosts life, but as the internal formation of the planet is unknown, more discoveries are yet to be made.

Iapetus

Discovered in October 1671 by French astronomer Giovanni Domenico Cassini, Iapetus is Saturn’s 3rd largest moon, and the 11th largest moon in the solar system. It is also the largest body in the Solar System known not to be in hydrostatic equilibrium.[7] After the discovery, Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere; moreover, Iapetus is tidally locked, always keeping the same face towards Saturn.  Iapetus is named after the Titan Iapetus from Greek mythology. The low density of Iapetus indicates that it is mostly composed of ice, with only a small amount of rocky materials.[8] Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles.[9]
Fun Fact: Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see.

Sources

Check out the Planetary Bodies Category for previous and upcoming articles on the solar system planets.

[1] = “Planetary Body Names and Discoverers”Gazetteer of Planetary Nomenclature. USGS Astrogeology Science Center. 
[2] =  Hansen, Candice J.; Esposito, L.; et al. (2006). “Enceladus’ Water Vapor Plume”. Science311 (5766): 1422–5. Bibcode:2006Sci…311.1422Hdoi:10.1126/science.1121254. PMID 16527971. S2CID 2954801.
[3] = “Icy Tendrils Reaching into Saturn Ring Traced to Their Source”NASA News. April 14, 2015. 
[4] = Battersby, Stephen (March 26, 2008). “Saturn’s moon Enceladus surprisingly comet-like”New Scientist.
[5] = Piazza, Enrico. “About Saturn & Its Moons: Dione”NASA – Jet Propulsion Laboratory.
[6] = https://solarsystem.nasa.gov/moons/saturn-moons/titan/in-depth/
[7] = Thomas, P. C. (July 2010). “Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission” (PDF). Icarus208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025.
[8] = Castillo-Rogez, J. C.; Matson, D. L.; Sotin, C.; Johnson, T. V.; Lunine, J. I.; Thomas, P. C. (2007). “Iapetus’ geophysics: Rotation rate, shape, and equatorial ridge”. Icarus190 (1): 179–202. Bibcode:2007Icar..190..179C. doi:10.1016/j.icarus.2007.02.018
[9] = Cowen, R. (2007). Idiosyncratic Iapetus, Science News vol. 172, pp. 104–106. references

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