Orbital parameters

Satellite	Density (kg/m^3)	Albedo	Period (days)	Inclination (degrees)	Eccentricity
Pan	N/A	0.5	0.5750	N/A	N/A
Atlas	N/A	0.9	0.6019	~0	~0
Prometheus	270	0.6	0.612986	0.0	0.0024
Pandora	420	0.9	0.628804	0.0	0.0042
Epimetheus	630	0.8	0.694590	0.34	0.009
Janus	650	0.8	0.694590	0.14	0.007
Mimas	1,140	0.5	0.9424218	1.53	0.0202
Enceladus	1,120	1.0	1.370218	0.02	0.0045
Tethys	1,000	0.9	1.887802	1.09	0.0000
Telesto	N/A	0.5	1.887802	~0	~0
Calypso	N/A	0.6	1.887802	~0	~0
Dione	1,440	0.7	2.736915	0.02	0.0022
Helene	N/A	0.7	2.736915	0.2	0.005
Rhea	1,240	0.7	4.517500	0.35	0.001
Titan	1,881	0.21	15.945421	0.33	0.0292
Hyperion	N/A	0.19 - 0.25	21.276609	0.43	0.1042
Iapetus	1,020	0.05 - 0.5	79.330183	7.52	0.0283
Phoebe	N/A	0.06	550.48R	175.3	0.163

*R indicates retrograde motion

Pan

Pan is the innermost of Saturn's known satellites:

orbit: 133,583 km from Saturn
diameter: 20 km
mass: N/A

Pan was the god of woods, fields, and flocks, having a human torso and head with a goat's legs, horns, and ears.

Discovered by Mark R. Showalter in 1990 from Voyager photos.

Pan is within the Encke Gap in Saturn's A ring.

Small moons near the rings produce wave patterns in the rings. Prior to the discovery of Pan, an analysis of the patterns in the edge of Saturn's A ring predicted the size and location of a small moon. Pan was discovered by reexamining the 10 year old Voyager photos at the predicted spot.

It is possible that there are more moons within Saturn's rings yet to be discovered.

Atlas is the second of Saturn's known satellites:

orbit: 137,670 km from Saturn
diameter: 30 km (40 x 20)
mass: N/A

Atlas was a Titan condemned by Zeus to support the heavens upon his shoulders; son of Iapetus and the nymph Clymene; brother of Prometheus and Epimetheus.

Discovered by R. Terrile in 1980 from Voyager photos.

Atlas seems to be a shepherd satellite of the A ring.

Prometheus

Prometheus ("pra MEE thee us") is the third of Saturn's known satellites:

orbit: 139,350 km from Saturn
diameter: 91 km (145 x 85 x 62)
mass: 2.7e17 kg

Prometheus was a Titan who stole fire from Olympus and gave it to humankind, for which Zeus punished him horribly; son of Iapetus; brother of Atlas and Epimetheus. "Prometheus" is Greek for "foresight".

Discovered by S. Collins and others in 1980 from Voyager photos.

Prometheus is the inner shepherd satellite of the F ring.

Prometheus has a number of ridges and valleys and several craters about 20 km in diameter but appears to be less cratered than the neighboring moons Pandora, Janus and Epimetheus.

From their very low densities and relatively high albedos, it seems likely that Prometheus, Pandora, Janus and Epimetheus are very porous icy bodies. (Note, however, that there is a lot of uncertainty in these values.)

The 1995/6 Saturn Ring Plane Crossing observations found that Prometheus was lagging by 20 degrees from where it should have been based on Voyager 1981 data. This is much more than can be explained by observational error. It is possible that Prometheus's orbit was changed by a recent encounter with the F ring, or it may have a small companion moon sharing its orbit.

Pandora

Pandora ("pan DOR uh") is the fourth of Saturn's known satellites:

orbit: 141,700 km from Saturn
diameter: 84 km (114 x 84 x 62)
mass: 2.2e17 kg

In Greek mythology Pandora was the first woman, bestowed upon humankind by Zeus as a punishment for Prometheus' theft of fire. Entrusted with a box containing all the ills that could plague people, she opened it out of curiosity and thereby released all the evils of human life; wife of Epimetheus.

Discovered by Collins and others in 1980 from Voyager photos.

Pandora is the outer shepherd satellite of the F ring.

More heavily cratered than nearby Prometheus, Pandora has at least two large craters 30 km in diameter.
But it shows no linear ridges or valleys.

Epimetheus

Epimetheus ( "ep eh MEE thee us" ) is the fifth of Saturn's known satellites:

orbit: 151,422 km from Saturn
diameter: 115 km (144 x 108 x 98)
mass: 5.6e17 kg

Epimetheus was the son of Iapetus and brother of Prometheus and Atlas; husband of Pandora. "Epimetheus" is Greek for "hindsight".

Discovered by Walker, Larson, and Fountain in 1978 and confirmed in 1980 by Voyager 1. It was probably also observed by both Walker and Dollfus in 1966 when they discovered Janus.

Epimetheus and Janus are "co-orbital".

There are several craters larger than 30 km in diameter as well as both large and small ridges and grooves.

The dark line across the surface in the image above is actually the shadow of the Saturn's F-ring.

Janus

Janus ( "JAY nus" ) is the sixth of Saturn's known satellites:

orbit: 151,472 km from Saturn
diameter: 178 km (196 x 192 x 150)
mass: 2.01e18 kg

Janus was the god of gates and doorways, depicted with two faces looking in opposite directions; also the root of the English word "January".

Discovered by the French astronomer Audouin Dollfus in 1966.

Dollfus is credited with the discovery of Janus but it's not really certain whether the object he saw was Janus or Epimetheus. (Walker discovered it independently but his telegram arrived a few hours after Dollfus'.) Larson and Fountain determined in 1978 that there are in fact two moons at about 151000 km from Saturn. This was confirmed in 1980 by the Voyager spacecraft.

Janus and Epimetheus are "co-orbital". The orbital radii of Janus and Epimetheus differ by only 50 km, less than the diameter of either. Their orbital velocities are thus very nearly equal and the lower, faster one slowly overtakes the other. As they approach each other they exchange a bit of momentum the end result of which is to boost the lower one into a higher orbit and to drop the higher one to a lower orbit. They thus exchange places. The exchange takes place about once every four years. The orbital data given here is as of the time of the Voyager encounters.

Janus is extensively cratered with several craters larger than 30 km but few linear features. Its surface appears to be older than Prometheus' but younger than Pandora's.

Mimas

Mimas ("MY mas") is the seventh of Saturn's known satellites:

orbit: 185,520 km from Saturn
diameter: 392 km
mass: 3.80e19 kg

The pronounciation "MEE mas" is also acceptable.

Mimas was one of the Titans slain by Hercules.

Discovered in 1789 by Herschel.

Mimas' low density (1.17) indicates that it is composed mostly of water ice with only a small amount of rock.

The surface of Mimas is dominated by an impact crater 130 km across, known as Herschel; it's almost 1/3 of the diameter of the entire moon. Herschel's walls are approximately 5 km high, parts of its floor measure 10 km deep, and its central peak rises 6 km above the crater floor. The impact that made this crater must have nearly disrupted Mimas. Fractures can be seen on the opposite side of Mimas that may be due to the same impact.

The surface is saturated with impact craters. But no others are nearly as large as Herschel. This suggests that early in its history, Mimas was probably impacted by even larger bodies than the one that created Herschel which completely disrupted the new moon (wiping out the evidence of earlier large impacts) but that the impact debris then coalesced again to form present-day Mimas.

Enceladus

Enceladus ("en SEL a dus") is the eighth of Saturn's known satellites:

orbit: 238,020 km from Saturn
diameter: 498 km
mass: 7.30e19 kg

In Greek mythology Enceladus was a Titan who was defeated in battle and buried under Mount Etna by Athena.

Discovered in 1789 by Herschel.

Enceladus has the highest albedo (>0.9) of any body in the solar system. Its surface is dominated by fresh, clean ice.

At least five different types of terrain have been identified on Enceladus. In addition to craters there are smooth plains and extensive linear cracks and ridges.

This means that Enceladus must have been active until very recently (and perhaps is still active today). Perhaps some sort of "water volcanism" is at work.

Enceladus is much too small to still be heated by the decay of radioactive material in its interior (the heat would have all dissipated long ago).

Enceladus is locked in a 1:2 resonance with Dione (similar to the situation between Io and Europa). This may provide a heating mechanism but it is probably insufficient to melt water ice. Enceladus may therefore be composed of some low-melting point material rather than pure water.

Enceladus may be the source of the material in Saturn's tenuous E ring. And since the material cannot persist in the ring for more than a few thousand years, it may be due to very recent activity on Enceladus. Another possibility, though, is that the rings are maintained by high-velocity collisions between dust particles and the various moons.

Tethys

Tethys ( "TEE this" ) is the ninth of Saturn's known satellites:

orbit: 294,660 km from Saturn
diameter: 1060 km
mass: 6.22e20 kg

In Greek mythology Tethys was a Titaness and sea goddess who was both sister and wife of Oceanus.

Discovered by Cassini in 1684.

Tethys' low density indicates that it is almost completely composed of water ice, similar to Dione and Rhea.

The western hemisphere is dominated by a huge impact crater, called Odysseus, whose 400 km diameter is nearly 2/5 of that of Tethys itself. That such an impact didn't shatter Tethys completely indicates that it may have been liquid or at least not very solid at the time. The crater is now quite flat (or more precisely, it conforms to Tethys' spherical shape), like the craters on Callisto, without the high ring mountains and central peaks commonly seen on the Moon and Mercury.

The second major feature seen on Tethys is a huge valley (called Ithaca Chasma) 100 km wide and 3 to 5 km deep which runs 2000 km or 3/4 of the way around Tethys' circumference.

Clearly then, Tethys has not always been frozen solid. At some point in its past it was probably liquid. The impact craters from that era have been smoothed out. As it froze and expanded, the surface must have cracked to accommodate the extra volume producing Ithaca Chasma. The smaller impact craters we see today are more recent.

There are no albedo features on Tethys as there are on Rhea and Dione.

Telesto and Calypso orbit in Tethys' Lagrange points (60 degrees ahead and behind Tethys in the same orbit).

Telesto

Telesto ("tah LESS toh") is the tenth of Saturn's known satellites:

orbit: 294,660 km from Saturn
diameter: 29 km (34 x 28 x 36)
mass: N/A

In Greek mythology Telesto was a daughter of Oceanus and Tethys.

Discovered by Smith, Reitsema, Larson and Fountain in 1980 from ground-based observations.

Telesto is in Tethys' leading Lagrange point.

Calypso

Calypso ("ka LIP so") is the eleventh of Saturn's known satellites:

orbit: 294,660 km from Saturn
diameter: 26 km (34 x 22 x 22)
mass: N/A

In Greek mythology Calypso was a sea nymph who delayed Odysseus on her island for seven years.

Discovered by Pascu, Smith and others in 1980 from ground-based observations with prototype cameras destined for the HST.

Calypso is in Tethys' trailing Lagrange point.

Calypso and Telesto are among the smallest moons in the solar system.

Dione

Dione ( "dy OH nee" ) is the twelfth of Saturn's known satellites:

orbit: 377,400 km from Saturn
diameter: 1120 km
mass: 1.05e21 kg

In Greek mythology Dione was the mother of Aphrodite (Venus) by Zeus (Jupiter).

Discovered by Cassini in 1684.

Dione is the densest of Saturn's moons (aside from Titan, whose density is increased by gravitational compression). It is composed primarily of water ice but must have a considerable fraction of denser material like silicate rock.

Though somewhat smaller, Dione is otherwise very similar to Rhea. They both have similar compositions, albedo features and varied terrain. Both rotate synchronously and have dissimilar leading and trailing hemispheres.

On the trailing hemisphere there is a network of bright streaks on a dark background and few visible craters. The streaks overlay the craters, indicating that they are newer.

The leading hemisphere is heavily cratered and uniformly bright. Like Callisto, the craters lack the high relief features seen on the Moon and Mercury.

This is interpreted as follows: shortly after its formation Dione was active. Some processes (ice volcanism?) resurfaced much of Dione leaving the the pattern of streaks, probably on the whole surface. Later, after the internal activity and resurfacing ceased, a much less intense series of impacts (which left craters too small to be seen in Voyager's images) occurred. This was concentrated on the leading hemisphere and wiped out the streak patterns but left them intact on the trailing hemisphere.

Helene orbits in Dione's leading Lagrange point.

Helene

Helene is the thirteenth of Saturn's known satellites:

distance from Saturn: 377,400 km
diameter: 33 km (36 x 32 x 30)

a.k.a. 1980 S 6, Dione B

Discovered by Laques and Lecacheux in 1980 from ground-based observations.

Helene is in Dione's leading Lagrange point.

Rhea

Rhea ("REE a") is the fourteenth of Saturn's known satellites and the second largest:

orbit: 527,040 km from Saturn
diameter: 1530 km
mass: 2.49e21 kg

In Greek mythology Rhea was the sister and wife of Cronus (Saturn) and the mother of Demeter, Hades (Pluto), Hera, Hestia, Poseidon (Neptune), and Zeus (Jupiter).

Discovered by Cassini in 1672.

Though somewhat larger, Rhea is otherwise very similar to Dione. They both have similar compositions, albedo features and varied terrain. Both rotate synchronously and have dissimilar leading and trailing hemispheres.

Rhea is composed primarily of water ice with rock making up less than 1/3 of its mass.

The leading hemisphere is heavily cratered and uniformly bright (left). Like Callisto, the craters lack the high relief features seen on the Moon and Mercury.

On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters.

Rhea's history is probably very similar to Dione's.

Titan

Titan is the fifteenth of Saturn's known satellites and the largest:

orbit: 1,221,830 km from Saturn
diameter: 5150 km
mass: 1.35e23 kg

In Greek mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus.

Discovered by Huygens in 1655.

It was long thought that Titan was the largest satellite in the solar system but recent observations have shown that Titan's atmosphere is so thick that its solid surface is slightly smaller than Ganymede's. Titan is nevertheless larger in diameter than Mercury and larger and more massive than Pluto.

One of the principal objectives of the Voyager 1 mission was the study of Titan. Voyager 1 came within 4000 km of the surface. We learned more in the few minutes of that encounter than in the previous 300 years.

And yet our knowledge is frustratingly incomplete. Titan is surrounded by a thick, opaque atmosphere; the surface cannot be seen at all in visible light. (The Cassini mission will map Titan's surface with radar as Magellan did at Venus.) All that the Voyager images show is a slight variation in color between the northern and southern hemispheres. Some surface detail is visible in the infrared with HST.

Titan is similar in bulk properties to Ganymede, Callisto, Triton and (probably) Pluto. It is not known whether it has any internal structure like Ganymede or is uniform like Callisto.

Titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser because it is so large that its gravity compresses its interior.

Alone of all the satellites in the solar system, Titan has a significant atmosphere. At the surface, its pressure is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide). The organics are formed as methane, which dominates in Titan's upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker.

Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere. It is therefore directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.

At the surface, Titan's temperature is about 94 K (-290 F). At this temperature water ice does not sublimate and the water at the surface cannot participate in the chemistry of the atmosphere. Nevertheless, there appears to be a lot of chemistry going on; the end result seems to be a lot like a very thick smog.

There are probably two layers of clouds at about 200 and 300 km above the surface. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space.

It seems likely that the ethane clouds would produce a rain of liquid ethane onto the surface perhaps producing an "ocean" of ethane (or an ethane/methane mixture) up to 1000 meters deep. Recent ground-based radar observations have cast this into doubt, however.

Recent observations with the Hubble Space Telescope show remarkable near infrared views of Titan's surface. Voyager's camera couldn't see through Titan's atmosphere but in the near infrared the haze becomes more transparent, and HST's pictures suggest that a huge bright "continent" exists on the hemisphere of Titan that faces forward in its orbit. These Hubble results don't prove that liquid "seas" exist, however, only that Titan has large bright and dark regions on its surface. The landing site for the Huygens probe has been chosen in part by examining these images. It will be just "offshore" of the largest "continent" at 18.1 degrees North, 208.7 degrees longitude.

The observations by HST also indicate that Titan's rotation is in fact synchronous like most of Saturn's other
moons.

Hyperion

Hyperion ("hi PEER ee en") is the sixteenth of Saturn's known satellites:

orbit: 1,481,100 km from Saturn
diameter: 286 km (410 x 260 x 220)
mass: 1.77e19 kg

In Greek mythology Hyperion was a Titan, the son of Gaea and Uranus and the father of Helios.

Discovered by Bond and Lassell in 1848.

Hyperion is the largest highly irregular (non-spherical) body in the solar system. Proteus is quite a bit larger but is almost spherical.

Like most of Saturn's moons, Hyperion's low density indicates that it is composed of water ice with only a small amount of rock.

But unlike most of Saturn's moons, Hyperion has a low albedo (.2 - .3) indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe (which is much darker) that got past Iapetus.

The Voyager images indicate that Hyperion's rotation is chaotic, i.e. its spin rate and orientation is completely unpredictable. More recent analysis, however, suggests that there may be a more regular 13 day rotation period.

Hyperion's odd rotation may account for the fact that Hyperion's surface is more or less uniform, in contrast to many of Saturn's other moons which have distinctly different leading and trailing hemispheres.

Iapetus

Iapetus ("eye AP i tus" ) is the seventeenth of Saturn's known satellites and the third largest:

orbit: 3,561,300 km from Saturn
diameter: 1460 km
mass: 1.88e21 kg

In Greek mythology Iapetus was a Titan, the son of Uranus, the father of Prometheus and Atlas and an ancestor of the human race.

Discovered by Cassini in 1671.

With a density of only 1.1, Iapetus must be composed almost entirely of water ice.

The leading and trailing hemispheres of Iapetus are radically different. The albedo of the leading hemisphere is between .03 and .05, as dark as lampblack, whereas the trailing hemisphere's albedo is .5, almost as bright as Europa. This difference is so striking that Cassini noted that he could see Iapetus only on one side of Saturn and not on the other.

One explanation of this is that the leading hemisphere is dusted with a coating of material knocked off of Phoebe. However, the color of the leading half of Iapetus and that of Phoebe don't quite match. Another possibility is that some active process within Iapetus is responsible. The puzzle is compounded by the fact that the dividing line between the two sides is inexplicably sharp.

All of Saturn's moons except for Iapetus and Phoebe are very nearly in the plane of Saturn's equator. lapetus is inclined almost 15 degrees.

Phoebe

Phoebe ("FEE bee") is the outermost of Saturn's known satellites. Phoebe is almost 4 times more distant from Saturn than its nearest neighbor (Iapetus).

orbit: 12,952,000 km from Saturn
diameter: 220 km
mass: 4.0e18 kg

Phoebe (Artemis) was the goddess of the hunt and the Moon; twin sister of Apollo.

Discovered by Pickering in 1898.

Most of Saturn's moons are bright but Phoebe's albedo is very low (.05), as dark as lampblack.

All of Saturn's moons except for Phoebe and Iapetus orbit very nearly in the plane of Saturn's equator. Phoebe's orbit is inclined almost 175° (its north pole is in the opposite direction to Saturn's).

Phoebe's eccentric, retrograde orbit and unusual albedo indicates that it may be a captured asteroid or Kuiper Belt object.

Phoebe is also unusual in that it does not rotate synchronously as do all the other moons of Saturn except Hyperion.

Material knocked off of Phoebe's surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion and the leading hemisphere of Iapetus.