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Useful Notes / The Moons of Saturn

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"You're not gonna believe this, I went to Phoebe instead of Titan. It's like, hello Saturn, after 10 moons it's time to stop naming and start numbering."
Rick Sanchez, Rick and Morty

You think Jupiter has a lot of moons? Well, Saturn has 146 of them! 53 of these have official names, and 7 of them have enough mass to become spherical under their own gravity. Technically the rings could be considered conglomerations of millions of moons, but there's only so much space on the Internet. They were originally all named after Titans of the Greek myths, (the Roman god Saturn being roughly equivalent to the Greek Kronos, king of the Titans)note  but so many have been discovered that they ran out of Titans, and thus the outer "irregular" moons have been divided into the "Gallic", "Norse" and "Inuit" groups after the sources of their names.

The low density of these moons, at least the largest ones, suggests that, like Saturn's rings, they are primarily composed of water ice with a more or less large proportion of rock. Most of the larger ones might even have formed from left-over debris created when two or more moons collided ~100 million years with the rings consisting of remnants from this event.

The Ringleaders: Pan, Daphnis, Atlas, Prometheus, etc.

Small moons that orbit near (or sometimes between) Saturn's various rings. They're also known as "shepherd moons" because they keep the particles that make up the rings in line. These moons are only a few kilometers in diameter, as this close to Saturn (i.e. within the Roche limit) anything larger would get slowly torn apart. Voyager and Cassini have taken pictures of the various kinks, twists and waves inflicted on the rings by their gravitational wakes.

Two of the largest of Saturn's inner moons, Janus and Epimetheus, are co-orbital. This means that when one of the moons catches up to the other, it gets sped up and the other slows down on their paths around Saturn. This is the only such configuration in the Solar System.

That's No Space Station, That's Mimas!
  • Diameter: 396 km
  • Mass: 0.0000063 of Earth
  • Density: 1.15 g/cm3
  • Surface Gravity: 0.006 g
  • Semi-major Axis: 185,539 km from Saturn
  • Orbital Period: 23 Hours
  • Rotational Period: 23 Hours (Tidally Locked)
  • Axial Tilt: 1.57° to Saturn's Equator (26.73°)
  • Average Surface Temperature: -209° C
  • Notable Features: Herschel Crater
  • Discovered: 1789 by Frederick William Herschel

A small moonnote  that got hit by a big rock at some point and just barely avoiding breaking up. This had led the moon to resemble the Death Star, with a 130-km crater named Herschel that almost exactly corresponds to the fictional space station's laser dish.note  The impact was probably also large enough to affect Mimas' orbit, as its eccentricity and inclination are both unusually high.note  It's also slightly egg-shaped, with the long end pointed toward Saturn and in an 2:1 orbital resonance with Tethys. Making two orbits around Saturn for every single done by Tethys but the small size of the two and distance between them means that they aren't affecting each other that much. It's also responsible for clearing the so called Cassini Division between Saturns A and B rings. Bizarrely, a temperature map produced by NASA showed that the warmer regions of the moon looks like Pac-Man.

Its surface features (apart from Herschel) are named after people and places from Arthurian Legend (Avalon Chasma, the crater Galahad, etc.).

Europa Two: Enceladus
  • Diameter: 504 km
  • Mass: 0.000018 of Earth
  • Density: 1.61 g/cm3
  • Surface Gravity: 0.01 g
  • Semi-major Axis: 237,948 km from Saturn
  • Orbital Period: 33 Hours
  • Rotational Period: 33 Hours (Tidally Locked)
  • Axial Tilt: 0.009° to Saturn's Equator (26.73°)
  • Average Surface Temperature: -198° C
  • Notable Features: The Tiger Stripes, Possible Sub-surface Ocean
  • Discovered: 1789 by Frederick William Herschel

Just like the Jovian moon, Enceladus is suspected to hide an ocean of liquid water underneath its surface, due to water vapour eruptions from Enceladus's many cryovolcanic vents. This was confirmed in September 2015 when it was determined that Enceladus's surface was shifting significantly relative to its interior, and have concluded its hypothetical subsurface ocean is global. Cassini flew through and found evidence of complex organic molecules. That, in addition to the discovery that the Enceladan water is salty, makes it a possible marker for life. By virtue of being covered with ice, it is the most reflective object in the Solar System. Enceladus is smaller than Europa and has almost no surface gravity, which is why streams of water and steam bursting from cracks in the ice become long, impressive plumes.

Like Mimas and Tethys. Enceladus is in an 2:1 orbital resonance with another mid-size moon. Namely Dione and their gravitational interactions are believed to fuel Enceladus' cryovolcanism.

Its surface features are named after people and places from the stories of the Arabian Nights (Baghdad Sulcus, the crater Sindbad, etc.).

Ithaca is Gorges: Tethys
  • Diameter: 1,062 km
  • Mass: 0.000103 of Earth
  • Density: 0.984 g/cm3
  • Surface Gravity: 0.01 g
  • Semi-major Axis: 294,619 km from Saturn
  • Orbital Period: 45 Hours
  • Rotational Period: 45 Hours (Tidally Locked)
  • Axial Tilt: 1.12° to Saturn's Equator (26.73°)
  • Average Surface Temperature: -187° C
  • Notable Features: Ithaca Chasma, Odysseus Crater
  • Discovered: 1684 by Giovanni Cassini

A mid-size moon marked by a very long (2000km), very wide (100km) and very deep (3-5km) canyon system, Ithaca Chasma. Some scientists theorized that Tethys used to have subsurface liquid water before it all froze solid, causing the surface to "crack". In fact, this moon has the lowest density of all major ones of the Solar System which indicates it's mostly composed of water ice with just a bit of "contamination" by rocky materials.

Tethys is an 2:1 orbital resonance with Mimas, and like its partner also has a very big crater called Odysseus, which is 400 km in diameter (or approximately 2/5 of Tethys'). It is much shallower than Mimas's Herschel though. Two smaller, irregular, "Trojan" moons, Telesto and Calypso, also share the same orbit around Saturn as Tethys. Also like Mimas, a thermal map of Tethys shows Pac-Man.

Its surface features are named after people and places from The Odyssey (the aforementioned Ithaca Chasma, the crater Telemachus, etc.).

Some... thing is wrong on Saturn 3note .

A Cracking Good Time at Dione
  • Diameter: 1,122 km
  • Mass: 0.000328 of Earth
  • Density: 1.48 g/cm3
  • Surface Gravity: 0.02 g
  • Semi-major Axis: 377,396 km from Saturn
  • Orbital Period: 2.7 Days
  • Rotational Period: 2.7 Days (Tidally Locked)
  • Axial Tilt: 0.02° to Saturn's Equator (26.73°)
  • Average Surface Temperature: -186° C
  • Notable Features: Eurotas and Palatine Chasmata
  • Discovered: 1684 by Giovanni Cassini

Another mid-size moon with surface features (cracks, troughs, smooth plains, rises and depressions) which seems to indicate past or present tectonic or volcanic activities. Pictures from the Voyager probes showed wispy white lines that were thought to be frost; Cassini proved them to be cliffs hundreds of meters high. Like Tethys, it also has orbit-mates, or Trojan moons — Helene and Polydeuces are irregular lumps of rock. It's also believed to be responsible for the cryovolcanism seen on Enceladus due to their 2:1 orbital resonance. But whether Enceladus affects it the same way is unknown.

Its surface features are named after people and places from The Aeneid (Carthage Fossae, the crater Aeneas, etc.).

Like Saturn, Like Rhea
  • Diameter: 1,527 km
  • Mass: 0.00039 of Earth
  • Density: 1.24 g/cm3
  • Surface Gravity: 0.02 g
  • Semi-major Axis: 527,108 km from Saturn
  • Orbital Period: 4 Days
  • Rotational Period: 4 Days (Tidally Locked)
  • Axial Tilt: 0.34° to Saturn's Equator (26.73°)
  • Average Surface Temperature: -197° C
  • Notable Features: Yamsi Chasmata, Tirawa Basin, Inktomi Crater, possible ring system
  • Discovered: 1672 by Giovanni Cassini

The second-largest Saturnian moonnote  and the only spherical one to not be in an orbital resonance or show any evidence to ever have been in one. It's named after the Wife of Saturn in mythology. The most interesting feature of this otherwise heavily-cratered moon is... a large crater, called Tirawa, with bright rays radiating outwards. Perhaps to compensate for general boringness Rhea, like Saturn, may have a ring system (the only moon in the Solar System which would). The rings, however, would be very dark and very tenuous and Cassini ended up finding no trace of them.

Its surface features are named after elements from non-European creation myths (the craters Izanagi and Izanami, Kunlun Linea, etc.).

Rhea has a thin atmosphere. (And by "thin", we mean "more rarefied than a laboratory vacuum".)

Orange Haze All Up on Titan
  • Diameter: 5,149 km
  • Mass: 0.0225 of Earth
  • Density: 1.88 g/cm3
  • Surface Gravity: 0.14 g
  • Semi-major Axis: 1,221,870 km from Saturn
  • Orbital Period: 15 Days
  • Rotational Period: 15 Days (Tidally Locked)
  • Axial Tilt: 0.34° to Saturn's Equator (26.73°)
  • Average Surface Temperature: -179° C
  • Atmospheric Pressure: 1.45 atm
  • Notable Features: Shangri-La, Kraken Mare, Ontario Lacus, Selk Crater
  • Discovered: 1655 by Christiaan Huygens
  • Number of Total Missions: 1

The second biggest moon in the Solar System (after Jupiter's Ganymede) note  and comprising 96% of all mass in orbit around Saturn. Despite its size (larger than Mercury), it only has one-seventh the Earth's gravity and its bulk density is less than twice that of water — there's a lot of ice and rock mixed in. Nevertheless it is the only Saturnian moon with a gravitational pull comparable to our own, as well as to the Galilean moons; on all the others the gravity is so low that even basic locomotion would be challenging.

It's covered in a thick hydrocarbon hazenote , making it the only moon to have a full-blown atmosphere. It's also the only known celestial body in the Solar System, apart from Earth, to have both a solid surface and basins of liquid on it. Unlike Earth's oceans, the liquids in question are light hydrocarbons, mostly methane and ethane — basically the same stuff that the atmosphere is made of. note  As a bonus, Titan's equatorial regions are abundant in massive dunefields, the largest of the Solar System, whose grains are thought to be made of organic compounds too, maybe with a core of water ice.

Its orbit is also somewhat unusual due having an high degree of eccentricity yet no resonance partner that would explain it (it's in an 4:3 resonance with Hyperion. But the sponge is way to small to affect Titan to any significant degree). Its possibly it used to be in one with Iapetus prior to whatever caused the later to get thrown out to its current position. As it would explain booth their eccentricities. Its significant size also causes the so called Colombo gap in the rings due being in resonance with a small ringlet embedded inside of it.

Due to its uniqueness, Titan occasionally appears in fiction.

Cassini carried an atmospheric entry probe, Huygens (built and managed by the European Space Agency and named after the discoverer of this moon), along on its trip to the system. On its way there the mission control team realized the Doppler shift caused by the differing velocities of Cassini and Huygens would cause its radio signals to become unreadable. They changed Huygens' approach vector to reduce the shift, and the data and pictures were successfully transmitted as the probe parachuted east of the Adiri regionnote . Unfortunately, half of them were never picked up because a programming error prevented Cassini from listening to one of the two data channels; nevertheless the pictures revealed a strangely Earth-like landscape of what looked like hills and sand dunes, drainage channels, islands and seas (all produced by hydrocarbons). After landing, pictures taken by Huygens from the surface showed a gloomy, orange-tinted plain covered with "ice pebbles" and a thick hydrocarbon fog. To date, these images are the most distant ones captured from a celestial body other than Earth and the Moon. The Huygens probe also carried a microphone, becoming the second probe after Venera 14 did so on Venus back in the 80s to record audio from a world other than Earth note . This mission will be followed by NASA's Dragonfly rotorcraft drone, expected to be launched if everything goes as planned next decade.

Similar to Venus, the thick haze layer on Titan renders any optical observation of its surface from orbit impossible, so outside of close-up images taken from the Huygens lander, the map of Titan was constructed using Cassini's radar imaging instrument. The actual surface wouldn't appear reddish-orange (that's due to its atmosphere), but rather a combination of whites, grays and the occasional black patches due to its mostly icy composition, similar to the other icy moons in the outer Solar System.

The lakes, craters and drainage channels on Titan are named after lakes and rivers from Earth (e.g. Ontario Lacus), while other features are named after sacred and enchanted places on Earth and deities of peace, happiness, beauty and wisdom (Shangri-La, Ganesa Macula, etc.). Titanian planitia (low plains) are named after planets from Frank Herbert's Dune (e.g. Arrakis Planitia, Caladan Planitia, etc.), and its mountains' names come from Tolkien's Legendarium (e.g. Erebor Montes, Doom Mons, etc.).

  • Gattaca: Vincent is faking Jerome's identity to get into the space program—specifically an expedition to Titan. When Jerome wants to know what Titan is like, Vincent blows cigarette smoke into a wine glass to simulate the smoggy world, puzzling Jerome as to why anyone would want to go there.
  • The Puppetmasters: The invading mind-controlling slugs come from that world, and the novel ends with a human expedition being sent to attack it.
  • The Titan involves a project to genetically transform humanity so Titan can be colonised.
  • James P. Hogan's duology Code of the Lifemaker and The Immortality Option is about a civilisation descended from alien mining robots that got slightly fried on an interstellar voyage to Titan, and as a result ended up as a part of a diverse Mechanical Ecosystem rather than just being an automated mining operation.
  • Dead Space 2 takes place on a colony, referred to as the Sprawl, set up on Titan, and Issac Clark has to once again contain the Necromoprph outbreak that was unleashed by the Marker stashed in the Sprawl.
  • Teen Titans (1996): The alien race that the protagonists are hybrids of, the H'san Natall, are situated on Titan. This makes the meaning of the team a little different since the team are literally "Teen(agers from) Titan".

SpongeBob Not-So-Square-Moon: Hyperion
  • Diameter: 270 km
  • Mass: 5.6199×10^18kg
  • Density: 0.544 g/cm3
  • Surface Gravity: 0.017–0.021 m/s2 (depending on location)
  • Semi-major Axis: 1,481,009 km km from Saturn
  • Orbital Period: 21 Days
  • Rotational Period: Various (averages around 13 days)
  • Axial Tilt: Unmeasurable
  • Average Surface Temperature: −180 C° C
  • Notable Features: Chaotic rotation, Very low density with a high degree of porosity
  • Discovered: 1848 by William Bond, George Bond, and William Lassell

A small irregular moon (smaller than Mimas) shaped and colored like a sponge. Hyperion is notable for being the only moon in the solar system to have a "chaotic rotation"note , meaning it lacks poles (no, not the Central European kind, though it lacks those, toonote ) or a stable axis of rotation. This essentially makes Hyperion's orientation in space unpredictable. A very hyperactive moon, indeed.

The moon is also notable for having one of the highest degrees of porosity in the solar system. With 46% of its volume being made up of empty space. It's also in an 4:3 Orbital resonance with titan and located very close to its partner. Causing it to be stuck in a tug-of-war between it and Saturn itself and which would probably also cause its current rotation.

Its surface features are named after solar and lunar deities (e.g. the crater Helios).

Yin and Yang OR the Frosted Walnut: Iapetus
  • Diameter: 1,469 km
  • Mass: 0.00030 of Earth
  • Density: 1.09 g/cm3
  • Surface Gravity: 0.02 g
  • Semi-major Axis: 3,560,820 km from Saturn
  • Orbital Period: 79 Days
  • Rotational Period: 79 Days (Tidally Locked)
  • Axial Tilt: 17.28° to Ecliptic Plane, 15.47° to Saturn's Equator (26.73°), 8.13° to Laplace Plane
  • Average Surface Temperature: -163° C
  • Notable Features: Equatorial Ridge, Cassini Regio, Engelier Crater
  • Discovered: 1671 by Giovanni Cassini

Famously very white on its trailing side and very black (i.e. as dark as coal) on its leading side, Iapetus was discovered by Giovanni Cassini in 1671. The moon is tidally locked. Its two-tone nature was a mystery until 2009, when the Spitzer Space Telescope discovered a very dim ring of dust generated by the outer moon Phoebe. Since Phoebe orbits Saturn in the opposite direction, as the "Phoebe ring" spreads out it smacks into Iapetus head-on and darkens that side. Cassini also discovered a gigantic equatorial ridge that some people have compared to plastic sprue or the surface of a walnut. Also, because it orbit at an incline relative to the plane of the rings (unlike all the other major moons), Iapetus is the only major moon of Saturn with a full view of the spectacular ring system. On the other moons, the rings appear at best as a thin line in the sky (at worst, you're on Titan and while you have some marvelous mountains and methane seas to look at, the sky is just a red-orange haze). But on Iapetus, the sky looks like this. One imagines that if interplanetary space tourism ever becomes a thing, Iapetus, with its pretty mountains and view of Saturn, would become quite a popular destination.

Like Rhea it lacks an orbital resonance partner, but it's theorised that it once had one in Titan as it would explain booth their current orbital eccentricities. The cause of its huge inclination is also something of a mystery as its unlikely to have formed with it.

Its surface features are named after people and places from The Song of Roland (Roncevaux Terra, the crater Ganelon, Toledo and Tortelosa Montes, etc.)

The setting of the book 2001: A Space Odyssey.

In the Empire from the Ashes series, the Achuultani attempt to hurl Iapetus at Earth as a weapon.

Retro Girl Phoebe
  • Diameter: 213 km
  • Mass: 8.292×10^18 kg
  • Density: 1.638 g/cm3
  • Surface Gravity: 0.038 g
  • Semi-major Axis: 12,960,000 km from Saturn
  • Orbital Period: 550 Days
  • Rotational Period: 9 h and 16 min
  • Axial Tilt: 152.14° (to orbit)
  • Average Surface Temperature: ~—200° C
  • Notable Features: Leto Regio, Retrograde orbit, Phoebe Ring
  • Discovered: 1899 by W.H. Pickering

Phoebe was the first moon to be discovered photographically (in 1898) and was the outermost of Saturn's known moons until more were discovered in the 2000s (it's now considered a member of the "Norse group" of irregular satellites and by far their largest member. The rest having a diameter smaller than 10km). Its main claim to fame is its retrograde orbitnote  and very low albedo, and it was long thought to be a captured asteroid. Recent evidence points to it being a captured "centaur" (a Kuiper belt object pulled in from its normal stomping grounds out by Pluto by the permutations of the gas giants). Its also the progenitor of Saturns largest and outermost ring (the so called Phoebe ring). Consisting of material expulsed from the moon by countless meteorites.

Its surface features are named after people and places from the Classical myth of Jason and the Argonauts (e.g. the crater Hylas).

Home of the protomolecule in The Expanse and its TV adaptation.