The Moons of Saturn

Saturn has 62 confirmed natural satellites, but only the seven largest, Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Iapetus [1] have sufficient mass/self gravity to have attained a hydrostatic equilibrium (spherical shape):

Mimas, sometimes referred to as the "Death Star" because of its resemblance to the fictional moon-sized space station in the movie Star Wars, is the innermost of Saturn's major moons and with a diameter of 397 kilometers the seventh largest. Apparently, due to its proximity, it's also responsible for clearing the material from the Cassini Division, a 4,800 km (2,980 mile) gap between Saturn's A and B rings. [2]

Mimas has a surface temperature of -200 Celsius and appears to be comprised of water ice surrounding a small rocky core. Its most prominent surface feature is Herschel a huge impact crater 130 km in diameter with walls 5 km high and a floor 10 km deep (a crater to scale on Earth would be wider than Canada).

Enceladus, with a diameter of 494 kilometers, is the sixth largest of Saturn’s moons.

Initial data from the Voyager spacecraft revealed an exterior that is old and cratered in some areas, younger and smooth in others. Fissures, crevices and other deformations are the major features. The surface temperature due to its high reflectivity is a frosty -200 Celsius.

In 2005 the Cassini probe discovered a plume of water vapor spewing from cracks in an area dominated by water ice near the satellite's south pole, the data confirmed by low level flybys in 2008. (Cassini's first direct flyby through the Enceladus plume, at a hight of approximately 62.5 miles or 100 kilometers, took place November 2, 2009.)

An analysis of this outgassing would seem to suggest sub-surface liquid water. [3] If so this tiny moon offers yet another potential habitat for extraterrestrial life. It is also probable, considering that Enceladus orbits in the densest part of the E ring, that it is the originator of the rings material.

On March 27, 2012, Cassini returned to Enceladus, its flyby at a hight of 46 miles (74 kilometers) plunging it deeper into the plume than ever before the spacecraft "tasting" the jets as they erupted from the cracks now bearing the name "tiger stripes."

Carolyn Porco, planetary scientist and leader of the probes imaging team stated "More than 90 jets of all sizes near Enceladus's south pole are spraying water vapor, icy particles and organic compounds all over the place. Cassini has flown several times now through this spray and has tasted it. And we have found that aside from water and organic material, there is salt in the icy particles. The salinity is the same as that of Earth's oceans."

Porco's exuberance notwithstanding it does simplify matters that Enceladus is venting its (still hypothetical) habitable zone into space when compared to say Ganymede or Europa, moons of Jupiter, whose (also still hypothetical) habitable zones are buried beneath miles of ice.

"It's erupting out into space where we can sample it. It sounds crazy but it could be snowing microbes on the surface of this little world. In the end, it's the most promising place I know of for an astrobiology search. We don't even need to go scratching around on the surface. We can fly through the plume and sample it. Or we can land on the surface, look up and stick our tongues out.  And voilą . . . we have what we came for."

Cassini returned for an encore visit to Enceladus April 14, 2012, before moving on to Tethys (closest approach 5,700 miles [9,100 kilometers]) its best imaging encounter with the mid-sized satellite since September 2005. 

On May 2, 2012, another visit to Enceladus the probe approaching within 46 miles (74 km) before exiting past Dione at a distance of 5,000 miles (8,000 km) on its way to a reunion with Titan.

Begining in mid-October, 2015, three final flybys - first a close-up look at Enceladus' north polar region, a region masked by darkness during previous visits; second, closer views of the south polar region and further sampling of the moon's icy spray (earlier data collected at higher altitudes was inconclusive); and lastly, completing the encounter, a high altitude flyby, the spaceprobe measuring heat flow from the moon's interior. [4]

Tethys, with a diameter of 1,060 km, is the fifth largest of Saturn’s moons.

Tethys is composed almost entirely of water ice. Its surface is dominated by two main features, a huge impact crater called Odysseus, almost 450 km in diameter, and a massive 2,000 km long valley called Ithaca Chasma. The surface temperature is -187 Celsius.

Dione, with a diameter of 1,120 kilometers is the fourth largest of Saturn’s moons.

Dione is composed of water ice with a rocky core. Its surface is heavily cratered the major features a series of towering ice cliffs a result of past tectonic activity and a number of giant fractures known as chasma (Latium Chasma recently imaged by Cassini is approximately 300 kilometers long, 1 kilometer deep and 8 to 12 kilometers wide). The surface temperature is -186 degrees Celsius.

Rhea, with a diameter of 1,528 kilometers, is the second largest of Saturn’s moons.

Rhea’s surface is heavily cratered in some areas, in other areas lightly cratered and marbled with bright streaks which appear to be ice cliffs similar to those found on Dione. The surface temperature is -200 degrees Celsius.

Titan, with a diameter of 5,150 kilometers, is Saturn’s largest moon (larger by volume than the planet Mercury).

Titan is unique in the Solar System in that it is the only satellite with a substantial atmosphere, an atmosphere composed primarily of nitrogen and small amounts of methane.

Titan’s internal structure is a rocky core surrounded by differing layers of ice, a layer of liquid water and ammonia possibly sandwiched between the ice and core.

A number of spacecraft have visited Titan, with Cassini sending back the latest and most up to date information. On December 25, 2004, Cassini launched a probe named Huygens into the moon's murky atmosphere, soon parachutes opened and Huygens began recording the sights and sounds of this distant world. The images stunned observers, parallel rows of hydrocarbon sand dunes hundreds of meters high and hundreds of kilometers long, drainage channels, dried lake beds and then finally a bumpy touchdown on a surface strewn with rocks and ice.

Titan is a veritable organic factory, its nitrogen atmosphere laced with carbon compounds, a world similar to Earth 3.8 billion years ago before the advent of life. Indeed life may already exist in some ammonia-water ocean far below the surface something Cassini Huygens was not equipped to search for.

Iapetus, with a diameter of 1,436 kilometers, is the third largest moon of Saturn and has an average surface temperature of approximately -150 degrees Celsius.
Iapetus’ main surface features are several large impact craters and a 20 km wide and in some places 20 km high equatorial ridge discovered by the Cassini probe when it imaged the moon in 2004. The origin of the ridge is at present undetermined, but American professors William McKinnon and Andrew Dombard have proposed an interesting hypothesis, that the ridge was caused by the long-ago breakup of an orbiting body which as its orbit decayed was torn apart by tidal forces the debris eventually crashing into the moon's equator. The theory has yet to gain general acceptance.

The 55 minor moons lack the mass/self-gravity necessary to assume a spherical shape:

Pan has a diameter of 20 km and acts as a shepherd moon. [5] 
Daphnis has a diameter of 7 km and acts as a shepherd moon.
Atlas has a diameter of 32 km and acts as a shepherd moon.
Prometheus has a diameter of 100 km and acts as a shepherd moon.
Pandora has a diameter of 84 km and acts as a shepherd moon.
Epimetheus has a diameter of 119 km and along with Janus is a truly co-orbital natural satellite (not a trojan). [6]
Janus has a diameter of 178 km and along with Epimetheus is a truly co-orbital natural satellite (not a trojan).
Methone has a diameter of 3 km.
Pallene has a diameter of 4 km.
Telesto has a diameter of 24 kilometers and is one of two (co-orbital) trojan moons (leading) that bracket Tethys. [7]
Calypso has a diameter of 19 kilometers and is one of two (co-orbital) trojan moons (trailing) that bracket Tethys.
Helene has a diameter of 32 kilometers and is one of two (co-orbital) trojan moons (leading) that bracket Dione.
Polydeuces has a diameter of 13 kilometers and is one of two (co-orbital) trojan moons (trailing) that bracket Dione.
Hyperion with a diameter of 266 kilometers is Saturn’s eighth largest moon and the largest non spherical body in the Solar System.
Kiviuq has a diameter of 16 km.
Ijiraq has a diameter of 12 km.
Phoebe has a diameter of 220 kilometers, travels in the opposite direction to its companion moons (retrograde) and is probably a captured comet or Kuiper Belt object.
Paaliaq has a diameter of 22 km.
Skathi has a diameter of 8 km.
Albiorix has a diameter of 32 km.
Erriapus has a diameter of 10 km.
Siarnaq has a diameter of 40 km.
Tarvos has a diameter of 15 km.
Mundilfari has a diameter of 7 km.
Narvi has a diameter of 7 km.
Suttungr has a diameter of 7 km.
Thrymr has a diameter of 7 km.
Ymir has a diameter of 18 km.
Aegir has a diameter of 6 km.
Bebhionn has a diameter of 6 km.
Bergelmir has a diameter of 6 km.
Bestla has a diameter of 7 km.
Farbauti has a diameter of 5 km.
Fenrir has a diameter of 4 kilometers and is one of the faintest known moons in the Solar System.
Fornjot has a diameter of 6 km.
Hati has a diameter of 6 km.
Hyrrokkin has a diameter of 8 km.
Kari has a diameter of 7 km.
Loge has a diameter of 6 km.
Skoll has a diameter of 6 km.
Surtur has a diameter of 6 km.
Greip has a diameter of 6 km.
Jarnsaxa has a diameter of 6 km.
Tarqeq has a diameter of 7 km.
Anthe has a diameter of 2 km.
Aegaeon has a diameter of a half kilometer and is likely a contributor to Saturn's G ring.
The other nine satellites S/2004 S 7, S/2004 S 13, S/2006 S 1, S/2004 S 17, S/2004 S 12, S/2006 S 3, S/2007 S 2, S/2007 S 3 and S/2009 S 1, were discovered in 2004, 2006, 2007 and 2009 and are as yet unnamed. They range from 300 meters to 7 kilometers in size.

[1] The names of Saturn's seven major moons (all derived from Greek mythology) were suggested by John Herschel the son of William Herschel discoverer of Uranus.

[2] A planetary ring is a flat disk shaped band composed of rock or ice dust, larger rocks, boulders and ice chunks which circle in a planet's equatorial plane.

[3] Once just conjecture, NASA has now confirmed the existence of a global subsurface ocean on Enceladus.

[4] This last visit by Cassini to Enceladus wrapped on December 19, 2015. The mission as a whole ended on September 15, 2017, with a fiery sterilizing plunge into the atmosphere of Saturn. A final signal was received at 7.55 a.m. EDT (11:55 GMT).

[5] “Shepherd” moons are small satellites orbiting within gaps or at the periphery of planetary rings their gravitational presence producing a distinct edge to the matter comprising the ring.

* The term "co-orbital moon" refers to natural satellites that share the same average orbit. Only Saturn is known to have co-orbital moons and it has 3 sets.

[6] Moons that are truly co-orbital are natural satellites that share an orbit one chasing the other, and because the difference in the mean orbital radius from Saturn to Epimetheus or Janus is 50 km (smaller than either moon's diameter), and because closer orbits have higher velocities the two moons inevitably close any gap a collision appearing unavoidable. In the case of Epimetheus and Janus, however, physics (orbital dynamics) comes into play, the moons as they close oscillating about their mass-weighted mean orbit (they "trade" orbits and speed, then, seeming to repel each other, move apart again). The nearest they ever approach is actually about 10,000 kilometers, the exchange taking place approximately once every four years.

[7] A trojan moon is a natural satellite that shares an orbit (is co-orbital) with a larger satellite, avoiding collision because it orbits around one of the two Lagrangian points of stability, L4 and L5, which lie 60 degrees ahead of and behind the larger body. There are four known trojan moons (2 sets) both in the Saturnian system.

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