[14.0] Missions To Saturn (2)

v1.2.3 / chapter 14 of 19 / 01 jun 09 / greg goebel / public domain

* The observations of Saturn performed by planetary probes have given us new insights into the ringed world, but much more remains to be learned, with the elaborate ring system still mysterious in many ways.

[14.1] SATURN IN THE SPACE AGE
[14.2] SATURN STATISTICS

[14.1] SATURN IN THE SPACE AGE

* The gross properties of the Saturn system had been apparent from Earth-based observations, and so the space probes that passed by revealed little dramatically new about the planet itself. The spacecraft did confirm the existence of a strong magnetic field; Saturn's magnetic field is 1,000 times stronger than the Earth's, but only about 1/20th as strong as Jupiter's. The magnetic poles are aligned to within 1 degree of the axis of rotation. Such a close alignment is unusual.

Images of the cloud features returned from the planet were interesting, but also revealed nothing that was particularly surprising. Saturn's cloud activity was known to be subdued compared to that of Jupiter, due to lower temperatures and gravity.

The sunlight falling on Saturn is relatively faint and weak, and so the average difference between polar and equatorial temperatures there is only about five degrees Kelvin. This means that most of the weather patterns are driven by the planet's internal energy and its fast rate of rotation. The end result is the planet's banded appearance, which is a consequence of zonal winds. These winds are very fast: the equatorial belt has a velocity of 500 meters per second. The zonal winds are symmetrical with respect to the hemispheres, demonstrating the relative unimportance of solar radiation on Saturn.

The probes did observe spots, the most spectacular being "Anne's Spot", a cyclonic storm observed by the Voyagers with a width of 5,000 by 3,000 kilometers, much smaller than Jupiter's Great Red Spot. In 2006, the Cassini probe observed a particularly interesting cyclonic storm at the planet's south pole, about 8,000 kilometers across. The storm was unusual in that it featured an "eye" like that of Earthly hurricanes, which hasn't been observed in other large cyclonic storms on the gas giant planets.

* Although observations of Saturn itself were more interesting than revolutionary, of course the unprecedented close-up images of the moons and particularly the ring system provided by spacecraft flybys provided plenty of surprises.

The probes, as well as exacting Earth-based observations performed during the missions and afterward, discovered some new small moons, and provided more detailed observations of the "classic" moons of Saturn. In order from outermost to innermost:

Phoebe is a dark lump with a diameter of 120 kilometers. Its appearance differs from most of the other moons of Saturn, and its irregular orbit suggests that it was captured well after the formation of the Saturnian planetary system. Phoebe is also unusual in that it is one of the few moons in the solar system that does not have a synchronous orbit, or in other words it doesn't keep one face towards Saturn at all times.
Phoebe was imaged in considerable detail by the Cassini probe in a flyby on 17 June 2004. It is an indication of the distance of Phoebe to Saturn that Cassini didn't actually go into orbit around the planet until the end of the month! There were no great surprises, one mission scientist saying: "Phoebe looks icy and covered with dirt." -- but it was heavily battered with craters, with some of the more recent-appearing craters exposing bright patches in the dark surface, suggesting patches of water ice. Further analysis showed that Phoebe's composition was similar to that of Pluto and the other "Kuiper Belt Objects" (discussed later), hinting strongly that it was an interloper from the far regions of the Solar System that had been captured by Saturn.
Iapetus is a ball of water ice about 1,460 kilometers in diameter. Voyager images provided more detail of its odd appearance as seen from Earth observations, with an icy bright hemisphere in the "trailing" direction of its orbit, and a dark hemisphere in the "leading" direction of its orbit. The dividing line between the two hemispheres is surprisingly sharp. Obviously the dark hemisphere is the result of some deposition of material, but where the material came from is a mystery, since it's not as dark as Phoebe, and Phoebe is far away. Iapetus, although clearly a regular moon, also has an atypically high orbital inclination, over 14 degrees, suggesting the possibility of some major impact in the distant past.
Hyperion is a lumpy moon with dimensions of 410 by 260 by 220 kilometers. It is the largest known irregular body in the solar system. Like most of Saturn's other moons it appears to be made of water ice with some rock, but Voyager observations showed it to be unusually dark. Cassini took the first close-up pictures of Hyperion in June 2005, showing a body that was so pockmarked with craters that it looked like a sponge and had an unusual tan color. Since it is too small to collapse into a sphere under its own gravity, it is full of voids, giving a density only about 60% that of water ice.
Hyperion, like Phoebe, does not have a synchronous orbit. In fact, it doesn't even have a constant "day", since its rotation is "chaotic", changing on a continuous basis. The unusual appearance and rotation of Hyperion suggests that it is a fragment of a larger moon that was broken up in the distant past by an impact. The rotation should have straightened itself out, but Hyperion's 3:4 resonance with Titan may help keep the rotation in a chaotic state.
Titan, with a diameter of 5,150 kilometers, is the second biggest moon in the solar system, after Jupiter's moon Ganymede. Titan is bigger than Mercury and more massive than Pluto. Titan is the only moon in the solar system with a thick atmosphere. The thick atmosphere actually led astronomers to think for a long time that Titan was bigger than Ganymede.
Titan appears to have a rocky core covered by a thick layer of water ice. It is covered with an opaque atmosphere consisting mostly of nitrogen, with about 6% argon and a small percentage of methane. There are traces of hydrocarbons, such as ethane and acetylene, apparently created by the breakdown of methane in sunlight, creating the hazy "smog" that hides the surface of Titan. The atmospheric pressure at the surface appears to be about 1.5 times that of the Earth's sea-level air pressure. It has been described as an icy world with hydrocarbon streams, methane rivers, and ammonia volcanoes.
Titan features dark equatorial regions that were once thought to be seas of liquids, but Cassini observations show that they are huge fields of sand dunes. This was a bit surprising since it was also believed that the winds on Titan were weak, too weak to form dunes, but as it turns out, the moon's global weather system gets considerable drive from Saturn's tidal forces.
Rhea, Saturn's second biggest moon, is an iceball with a diameter of 1,530 kilometers and a surface covered with relatively subdued cratering. Rhea has a relatively high density compared to the other icy moons of Saturn, indicating that it probably contains a fair amount of rock along with ice. The leading and trailing hemispheres have different appearances. The leading hemisphere has a cratered surface, while the trailing hemisphere has relatively few craters and a pattern of bright streaks, apparently due to some kind of volcanism.
Dione is almost a twin to Rhea, with a diameter of 1,120 kilometers, similar subdued cratering, a composition of water with some rock, and leading and trailing hemispheres whose appearance differ in almost exactly the same way as they do on Rhea.
Helene was discovered in 1980 by Earth-based observations. It is a lump with dimensions of 36 by 32 by 30 kilometers. It occupies the same orbit, or in other words is "co-orbital", with the much larger Dione. For this reason, it is sometimes referred to as "Dione B". Helene is trapped in one of the libration points, or stable gravitational positions, of the orbit, equidistant from Dione and Saturn. Helene leads Dione in their mutual orbit. Helene was finally imaged by Cassini in 2006, with the images showing an irregular dark lump.
Tethys is another iceball, 1,060 kilometers in diameter, apparently made mostly of water ice. It features light cratering, except for one crater about 400 kilometers in diameter that was named "Odysseus". The moon was probably liquid at the time of the impact that created Odysseus, since the event would have otherwise broken up the body. There is also a long valley named "Ithaca Chasma" a few kilometers deep and 100 kilometers wide that runs about three-quarters of the way around Tethys. The valley seems to be an artifact of the expansion of the watery moon as it solidified.
The very small moons "Telesto" and "Calypso" were discovered in 1980 by Earth-based telescopes. They are small lumps, with Telesto having dimensions of 34 by 28 by 36 kilometers and Calypso having dimensions of 34 by 22 by 22 kilometers. These two moons are co-orbital with the much larger Tethys. Telesto is in the "leading" Lagrangian point, preceding Tethys in their common orbit, while Calypso is in the "trailing" Lagrangian point.
Enceladus was found to be a very bright world, about 500 kilometers in diameter, covered with shiny ice with a spattering of relatively small craters and long cracks. It has the highest albedo, greater than 90%, of any object in the solar system. The relative smoothness of the surface suggested that Enceladus is geologically active; this notion was confirmed in 2005 when Cassini observed a number of geysers erupting to high altitudes from the south polar regions of Enceladus.
The observation suggested that there are liquid water oceans under the surface of Enceladus. As with Jupiter's Europa, that might provide an environment where traces of life could be found. The 1:2 resonance of Dione with Enceladus may induce tidal effects responsible for Enceladus's geological activity, though the heating wouldn't be very great.
Mimas proved to be about 400 kilometers in diameter, and had a crater about 130 kilometers across, roughly a quarter of the moon's diameter. The crater was named "Herschel" in honor of the moon's discoverer, and planetary astronomers believe the impact that created it came very close to shattering the world. Mimas is littered with smaller craters. Like Tethys, Mimas has a low density and appears to be mostly made up of water ice, and was probably still partly liquid when the Herschel impact occurred since it wouldn't have otherwise survived. The huge crater led some jokers to call Mimas the "Death Star", since it gave the moon something of the appearance of Darth Vader's doomsday weapon in the first STAR WARS movie.
Epimetheus and Janus, the last two moons discovered from Earth before spacecraft visited the planet, proved to have an intimate and startling relationship. Both are similar bodies, with Epimetheus having dimensions of 144 by 108 by 98 kilometers, and Janus with dimensions of 196 by 192 by 150 kilometers. They are cratered, relatively bright, and seem to be mostly made up of light ices with some rock.
As it turned out, the radii of their orbits differ by only 50 kilometers, less than their diameters, and to make things really confusing, about every four years they pass each other and undergo a transfer of momentum that drops one into the lower orbit and kicks the other into the higher orbit. It is suspected that this unusual orbit is due to the fact that the two moons are the product of a breakup of a single body in the distant past.
"Pandora" and "Prometheus" were discovered in 1980 from Voyager images. They are similar in many ways. Pandora has dimensions of 114 by 84 by 62 kilometers, while Prometheus has dimensions of about 145 by 85 by 62 kilometers. Both are cratered and have relatively bright surfaces. Highly uncertain estimates of their masses suggest they are made of ices and may be porous. Earth observations of Prometheus performed in 1995:1996 showed that it had shifted its orbital position, possibly by some interaction with an unknown satellite.
"Atlas" was discovered in 1980 from Voyager photos. It is an irregular object about 40 by 20 kilometers in size.
"Pan", the innermost known satellite, was discovered from Voyager photographs in 1990 orbiting in the Encke Division. While most of the new moons of Saturn had been discovered a decade earlier from Voyager imagery, Pan's existence was discovered indirectly from disturbances it set up in nearby ring components that called for a closer look at the old images. Pan is only about 20 kilometers in diameter and is little more than a speck on the Voyager photographs.

* The probes provided very significant revelations about Saturn's ring system. They discovered three two more main rings, including the broad and faint outermost "E Ring" (which had been tentatively spotted by Feibelman in the 1960s), the faint but smaller "G Ring" inside the E Ring, and the strandlike "F Ring", which circled the A Ring. The rings were determined to have a typical depth of about 200 meters.

The discoveries led to a modern breakdown of the ring system as follows, arranged from outermost to innermost:

The outermost ring, the E Ring, is broad and tenuous, which is why it hadn't been clearly observed from Earth. It has an outer radius of 480,000 kilometers, as measured from the center of Saturn, and an inner radius of 180,000 kilometers, or from eight Saturn radii (RS) to three RS.
The moons Phoebe, Iapetus, Hyperion, Titan, and Rhea orbit outside of the ring system. The moons Dione, Helene, Tethys, Calypso, Telesto, Enceladus, and Mimas all pass through the E Ring. It appears that the E Ring is created by venting of ice crystals from Enceladus, which give the ring a blue color.
The G Ring is faint and relatively narrow, with an outer radius of about 173,800 kilometers (2.9 RS) and an inner radius of about 165,800 kilometers (2.76 RS).
The F ring is bright but very narrow, with an outer radius of roughly 140,350 kilometers and an inner radius of about 140,200 kilometers (2.34 RS). It is flanked by Pandora in an orbit above the ring, and by Prometheus in an orbit below the ring.
The F Ring should have spread out, but the two moons seem to keep it confined to its narrow width. They are known as "ring shepherd" or just "shepherd" moons for this reason, and this sort of arrangement is regarded as one of the major influences on structure in planetary ring systems.
The bright A Ring has an outer radius of 136,000 kilometers (2.27 RS) and an inner radius of 122,200 kilometers (2.04 RS). The Encke division cuts a 325-kilometer-wide swath through it, from 133,825 kilometers to 133,500 kilometers (2.23 RS).
The tiny moon Atlas orbits just outside the A Ring, probably acting as a shepherd to define the ring's boundary, while the tiny moon Pan orbits within the Encke division and is almost certainly the cause of it. There is a narrow division near the outer edge of the ring, known as the "Keeler Gap"; in 2005 the Cassini probe photographed a moonlet about six kilometers wide in the middle of the gap. The moonlet was provisionally eventually named "Daphnis" and the image revealed an interesting pattern of waves on the edge of the gap, caused by the moonlet.
The bright B Ring is separated from the A Ring by the Cassini Division. The B Ring has an outer radius of 117,500 kilometers (1.96 RS) and an inner radius of 92,000 kilometers (1.53 RS).
The bright C Ring is separated from the B ring by the Maxwell Division. The C Ring has an outer radius of 92,000 kilometers (1.53 RS) and an inner radius of 74,500 kilometers (1.24 RS).
The faint D Ring is separated from the C ring by the Guerin Division. The D Ring has an outer radius of 74,500 kilometers (1.24 RS) and an inner radius of 67,000 kilometers (1.12 RS).

One of the big surprises was that each of the "main" rings consists of hundreds of very narrow rings. One of the outer rings has an oddly twisted or braided appearance, the reason for which is still not fully understood. The spokelike features that O'Meara had vaguely seen were observed in the B Ring by Voyager 1, and still remain mysterious. They may be an interaction with Saturn's magnetic field.

Cassini returned an even bigger surprise by providing evidence that the rings are much more stable than previously thought, and in fact may have been around for billions of years. The conventional wisdom was that impacts of ring particles would gradually pulverize them and the rings would eventually dissipate, but it appears instead that the particles will either clump together or break apart, and so the rings are in a good approximation of a steady state.

* Nine new moons of Saturn were thought to have been discovered in 1981 from analysis of Voyager 2 data; seven more were thought to have been discovered in 1995 from Hubble Space Telescope imagery. All of these were in the vicinity of or associated with the ring system, but they were not confirmed and are now generally suspected to have been at best just temporary clumpings of ring material. They have now been generally discounted, though Cassini observations should provide conclusive proof one way or another. However, over 30 irregular moonlets were discovered by Earth-based telescopes in the 21st century:

13 were discovered in 2000, eventually being named "Albiorix", "Erriapus", "Iijiraq", "Kiviuq", "Mundilfari", "Paaliaq", "Siarnaq", "Skathi", "Suttungr", "Tarvos", "Thrymr", and "Ymir",
One moonlet, "Narvi", was discovered in 2003, with 13 more discovered in 2004; nine of these moonlets were eventually given the names of "Aegir", "Bebhionn", "Bergelmir", "Bestla", "Farbauti", "Fenrir", "Fornjot", "Hati", and "Hyrrokkin", while four remain unnamed or uncomfirmed.
Eight moonlets were discovered in 2006, with six eventually given the names of "Kari", "Loge", "Skoll", "Surtur", "Jarnsaxa", and "Greip", the other two remaining unnamed or unconfirmed.
Three were discovered in 2007, one being named "Tarqeq", the other two remaining unnamed or uncomfirmed.

The irregular moonlets occur in three groups:

The "Gallic" group, all with French-Celtic names, in medium-inclination prograde orbits.
The "Inuit" group, all with names from Inuit tribal mythology, in medium-inclination prograde orbits.
The "Norse" group, all with names from Norse mythology, all in medium-inclination retrograde orbits. Phoebe turned out to be part of this group. Unlike the other two groups, the Norse group is not very homogeneous, and includes at least two subgroups, one clustered around Narvi, the other clustered around Skathi; some of the Norse moons don't seem to be closely associated with either group.

Cassini observations have also suggested the existence of very small moonlets, a few hundred meters across, associated with the ring system. Given that the observations only covered a small sector of the rings, there are likely millions more moonlets of similar size. Naming them all might be a challenge. In fact, sooner or later the search for moonlets around the gas giant planets is certain to run into diminishing returns. Astronomers believe that many more moons will be found as telescope technology improves, until eventually nobody finds it interesting any more. As one astronomer put it: "At some point, you have to decide when to quit."

BACK_TO_TOP

[14.2] SATURN STATISTICS

* Statistics for Saturn:

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  mean distance from Sun              9.54 AU (1427.0 x 10^6 kilometers)
  orbital period (sidereal)           29.46 years
  orbital eccentricity                0.056
  orbital inclination                 2.5 degrees
  equatorial diameter                 120,000 km (9.41 Earth)
  mass (relative to Earth)            95.159
  mean density (relative to water)    0.70
  gravity (relative to Earth)         1.08
  escape speed                        35.6 kilometers per second
  rotation period                     0.44 days
  oblateness                          1/9
  inclination of equator              26.7 degrees
  albedo                              0.47
  max surface temperature             -180 degrees Celsius (cloud tops)
  atmosphere (major constituents )    H, He, CH4
                                      clouds of NH3, NH4SH, H2O
  atmospheric pressure at surface     not applicable
  number of known moons               18
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* Regular moons of Saturn, from outermost to innermost, with pronunciations in parenthesis.

Radii are given from the center of Saturn. The abbreviation "RS" stands for the radius of Saturn, while "RM" stands for the radius of the orbit of the Earth's Moon, and "M" stands for the diameter or mass of the Earth's Moon. Densities are given relative to water, which is equivalent to grams per cubic centimeter. Orbital and rotation periods are given in days and fractions of days, with days-hours-minutes added for periods under two days.

  __________________________________________________________________________

  PHOEBE ("FEE-bee") / Saturn IX / irregular (Norse group):

  mean distance from planet           12,952,000 km / 216 RS / 33.71 RM
  orbital period (sidereal)           550.46 days
  orbital eccentricity                0.163
  orbital inclination                 175.3 degrees (retrograde)
  equatorial diameter                 220 km / 0.06 M
  mass                                0.000054 M
  mean density                        0.7
  albedo                              0.06
  year of discovery                   1898 (Pickering)
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  IAPETUS ("eye-AP-i-tus") / Saturn VIII:

  mean distance from planet           3,561,300 km / 59.4 RS / 9.26 RM
  orbital period (sidereal)           79.331 days
  orbital eccentricity                0.028
  orbital inclination                 14.7 degrees
  equatorial diameter                 1,460 km  / 0.42 M
  mass                                0.026 M
  mean density                        1.2
  albedo                              0.2
  year of discovery                   1671 (Cassini)
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  HYPERION ("hy-PEER-ee-en") / Saturn VII:

  mean distance from planet           1,481,000 km / 24.7 RS / 3.85 RM
  orbital period (sidereal)           21.276 days
  orbital eccentricity                0.104
  orbital inclination                 0.4 degrees
  dimensions                          410 x 260 x 220 km
  mass                                0.00024 M
  mean density                        1.4
  albedo                              0.3
  year of discovery                   1848 (Bond, Bond, Lassell)
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  TITAN ("TY-tun") / Saturn VI:

  mean distance from planet           1,221,850 km / 20.4 RS / 3.18 RM
  orbital period (sidereal)           15.945 days
  orbital eccentricity                0.029
  orbital inclination                 0.33 degrees
  equatorial diameter                 5,150 km / 1.48 M
  mass                                1.83 M
  mean density                        1.88
  albedo                              0.21
  year of discovery                   1655 (Huygens)
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  RHEA ("REE-a") / Saturn V:

  mean distance from planet           527,040 km / 8.77 RS / 1.37 RM
  orbital period (sidereal)           4.517 days / 4 days 12 hours 10 minutes
  orbital eccentricity                0.001
  orbital inclination                 0.35 degrees
  equatorial diameter                 1,530 km / 0.44 M
  mass                                0.034 M
  mean density                        1.3
  albedo                              0.7
  year of discovery                   1672 (Cassini)
  __________________________________________________________________________

  DIONE ("dy-OH-nee") / Saturn IV:

  mean distance from planet           377,400 km / 6.3 RS / 0.98 RM
  orbital period (sidereal)           2.737 days
  orbital eccentricity                0.002
  orbital inclination                 0.02 degrees
  equatorial diameter                 1,120 km / 0.32 M
  mass                                0.014 M
  mean density                        1.4
  albedo                              0.7
  year of discovery                   1684 (Cassini)

  HELENE / Saturn XII:

     CO-ORBITAL with Dione, at leading libration point.

  dimensions                          36 x 32 x 30 km
  mass                                UNKNOWN
  albedo                              0.7
  year of discovery                   1980 (Laques, Lecacheux)

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  POLYDEUCES ("polly-deuces") / Saturn XXXIV (S/2004 S5)

  mean distance from planet           377,396 km / 6.29 RS / 0.99 RM
  orbital period (sidereal)           2.737 days
  orbital eccentricity                0
  orbital inclination                 0
  equatorial diameter                 3.5 km
  mass / density:                     UNKNOWN
  albedo                              UNKNOWN
  year of discovery                   2004 (Cassini)
  __________________________________________________________________________

  TETHYS ("TEE-this") / Saturn III:

  mean distance from planet           294,660 km / 4.92 RS / 0.77 RM
  orbital period (sidereal)           1.888 days / 1 day 21 hours 19 minutes
  orbital eccentricity                0
  orbital inclination                 1.1 degrees
  equatorial diameter                 1,060 km / 0.30 M
  mass                                0.010 M
  mean density                        1.2
  albedo                              0.9
  year of discovery                   1684 (Cassini)

  CALYPSO ("ka-LIP-so") / Saturn XIV:
  
    CO-ORBITAL with Tethys, at leading libration point

  dimensions                          34 x 22 x 22 km
  mass / density                      UNKNOWN
  albedo                              0.6
  year of discovery                   1980 (Pascu, Seidelmann, Baum, Currie)

  TELESTO ("ta-LESS-toh") / Saturn XIII:
  
    CO-ORBITAL with Tethys, at trailing libration point

  dimensions                          34 x 28 x 26 km
  mass / density                      UNKNOWN
  albedo                              0.5
  year of discovery                   1980 (Smith, Larson, Reitsma)

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  ENCELADUS ("en-SELL-a-dus") / Saturn II:

  mean distance from planet           238,000 km / 3.97 RS / 0.62 RM
  orbital period (sidereal)           1.370 days / 1 day 8 hours 53 minutes
  orbital eccentricity                0.004
  orbital inclination                 0.02 degrees
  equatorial diameter                 500 km / 0.14 M
  mass                                0.0011 M
  mean density                        1.24
  albedo                              1.0
  year of discovery                   1789 (Herschel)
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  PALLENE ("pah-LEE-nee") / Saturn XXXIII (S/2004 S2)

  mean distance from planet           212,280 km / 3.54 RS / 0.56 RM
  orbital period (sidereal)           1.154 days / 27 hours 42 minutes
  orbital eccentricity                0
  orbital inclination                 0.2
  equatorial diameter                 4 km
  mass / density:                     UNKNOWN
  albedo                              UNKNOWN
  year of discovery                   2004 (Cassini)
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  METHONE ("me-THO-nee") / Saturn XXXII (S/2004 S1)

  mean distance from planet           194,440 km / 3.24 RS / 0.51 RM
  orbital period (sidereal)           1.010 days / 24 hours 14 minutes
  orbital eccentricity                0
  orbital inclination                 0
  equatorial diameter                 ~3 km
  mass / density:                     UNKNOWN
  albedo                              UNKNOWN
  year of discovery                   2004 (Cassini)
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  MIMAS ("MY-mass") / Saturn I:

  mean distance from planet           185,520 km / 3.12 RS / 0.49 RM
  orbital period (sidereal)           0.942 days / 22 hours 36 minutes
  orbital eccentricity                0.020
  orbital inclination                 1.5 degrees
  equatorial diameter                 392 km / 0.11 M
  mass                                0.00052 M
  mean density                        1.17
  albedo                              0.5
  year of discovery                   1789 (Herschel)
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  JANUS ("JAY-nus") / Saturn X:
  
     CO-ORBITAL with Epimetheus, in unusual "reciprocal" orbit.

  mean distance from planet           151,472 km / 2.52 RS / 0.39 RM 
  orbital period (sidereal)           0.694 days / 16 hours 39 minutes
  orbital eccentricity                0.007
  orbital inclination                 0.14 degrees
  dimensions                          196 x 192 x 150 km
  mass                                0.000003 M?
  mean density                        0.67?
  albedo                              0.8
  year of discovery                   1966 (Dollfus)

  EPIMETHEUS ("ep-eh-MEE-thee-us") / Saturn XI
  
     CO-ORBITAL with Janus, in unusual "reciprocal" orbit.

  dimensions                          144 x 108 x 98 km
  mass                                0.000008 M?
  mean density                        0.7?
  albedo                              0.8
  year of discovery                   1966 (Fountain, Larson)
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  PANDORA ("pan-DOH-uh") / Saturn XVII:

  mean distance from planet           141,700 km / 2.37 RS / 0.37 RM
  orbital period (sidereal)           0.628 days / 15 hours 4 minutes
  orbital eccentricity                0.004
  orbital inclination                 0
  dimensions                          114 x 84 x 62 km
  mass                                0.000003 M?
  mean density                        0.7?
  albedo                              0.9
  year of discovery                   1980 (Collins, Carlson)
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  PROMETHEUS ("pra-MEE-the-us") / Saturn XVI:

  mean distance from planet           139,350 km / 2.32 RS / 0.36 RM 
  orbital period (sidereal)           0.613 days / 14 hours 17 minutes
  orbital eccentricity                0.003
  orbital inclination                 0
  dimensions                          145 x 85 x 65 km
  mass                                0.000004 M?
  mean density                        0.7?
  albedo                              0.6
  year of discovery                   1980 (Collins, Carlson)
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  ATLAS / Saturn XV:

  mean distance from planet           137,640 km / 2.28 RS / 0.36 RM
  orbital period (sidereal)           0.602 days / 14 hours 27 minutes
  orbital eccentricity                0
  orbital inclination                 0
  dimensions                          40 x 30 km
  mass / density:                     UNKNOWN
  albedo                              0.9
  year of discovery                   1980 (Terrile)
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  DAPHNIS ("dafniss") / Saturn XXXV (S/2005 S1)

  mean distance from planet           136,504 km / 2.28 RS / 0.36 RM
  orbital period (sidereal)           0.594 days / 14 hours 15 minutes
  orbital eccentricity                0
  orbital inclination                 0
  equatorial diameter                 ~7 km
  mass / density:                     UNKNOWN
  albedo                              UNKNOWN
  year of discovery                   2005 (Cassini)
  __________________________________________________________________________

  PAN / Saturn XVIII:

  mean distance from planet           133,583 km / 2.23 RS / 0.35 RM
  orbital period (sidereal)           0.575 days / 13 hours 48 minutes
  orbital eccentricity                0
  orbital inclination                 0
  equatorial diameter                 20 km
  mass / density:                     UNKNOWN
  albedo                              0.5
  year of discovery                   1990 (Showalter)
  __________________________________________________________________________

* Irregular moons of Saturn: _____________________________________________________________ preliminary moon designation group _____________________________________________________________ YMIR Saturn XIX S/2000 S1 Norse group PAALIAQ Saturn XX S/2000 S2 Inuit group TARVOS Saturn XXI S/2000 S4 Gallic group IIJIRAQ Saturn XXII S/2000 S6 Inuit group SUTTUNGR Saturn XXIII S/2000 S12 Norse group KIVIUQ Saturn XXIV S/2000 S5 Inuit group MUNDILFARI Saturn XXV S/2000 S9 Norse group ALBIORIX Saturn XXVI S/2000 S11 Gallic group SKATHI Saturn XXVII S/2000 S8 Norse group ERRIAPUS Saturn XXVIII S/2000 S10 Gallic group SIARNAQ Saturn XXIX S/2000 S3 Inuit group THRYMR Saturn XXX S/2000 S7 Norse group NARVI Saturn XXXI S/2003 S1 Norse group AEGIR Saturn XXXVI S/2004 S10 Norse group BEBHIONN Saturn XXXVII S/2004 S11 Gallic group BERGELMIR Saturn XXXVIII S/2004 S15 Norse group BESTLA Saturn XXXIX S/2004 S18 Norse group FARBAUTI Saturn XL S/2004 S9 Norse group FENRIR Saturn XLI S/2004 S16 Norse group FORNJOT Saturn XLII S/2004 S8 Norse group HATI Saturn XLIII S/2004 S14 Norse group HYRROKKIN Saturn XLIV S/2004 S19 Norse group KARI Saturn XLV S/2006 S2 Norse group LOGE Saturn XLVI S/2006 S5 Norse group SKOLL Saturn XLVII S/2006 S6 Norse group SURTUR Saturn XLVIII S/2006 S7 Norse group JARNSAXA Saturn L S/2006 S6 Norse group GREIP Saturn LI S/2006 S4 Norse group TARQEQ Saturn S/2007 S1 Inuit group _____________________________________________________________ unnamed moons, all of Norse group: _____________________________________________________________ S/2004 S7 S/2004 S12 S/2004 S13 S/2004 S17 S/2006 S1 S/2006 S3 S/2007 S2 S/2007 S3 _____________________________________________________________
* Rings of Saturn: __________________________________________________________________________ E RING 480,000 - 180,000 km / 8.0 - 3.0 RS / 1.25 - 0.47 RM G RING 173,800 - 165,800 km / 2.9 - 2.76 RS / 0.45 - 0.43 RM F RING 140,350 - 140,200 km / 2.34 RS / 0.37 RM A RING 136,000 - 122,200 km / 2.27 - 2.04 RS / 0.35 - 0.32 RM cassini division B RING 117,500 - 92,000 km / 1.96 - 1.53 RS / 0.31 - 0.24 RM maxwell division C RING 92,000 - 74,500 km / 1.53 - 1.24 RS / 0.24 - 0.19 RM guerin division D RING 74,500 - 67,000 km / 1.24 - 1.12 RS / 0.19 - 0.17 RM __________________________________________________________________________ The Encke division bisects the A Ring at 2.23 RS. __________________________________________________________________________
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