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Saturn's moons , while very cold, would be a little more hospitable. Though none has a breatheable atmosphere, they are solid, like Earth. From Saturn's moons, you could see the Sun as a tiny, bright disk. And depending on where you stood, Saturn itself might loom overhead -- a giant, bejeweled neighbor in the sky.
After Jupiter, Saturn has more moons (22) than any other planet in the solar system.
Pan is the innermost of Saturn's known satellites. 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.
Atlas, the second of Saturn's known satellites, orbits near the outer edge of the A-ring and is about 40 by 20 kilometers (25 by 15 miles) in size. It is probably a shepherd satellite for Saturn's A-ring. Atlas was discovered by R. Terrile in 1980 from photographs taken by Voyager during its encounter with Saturn.
Prometheus is the third of Saturn's known satellites. 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.
Pandora is the fifth of Saturn's known satellites. Discovered by Collins and others in 1980 from Voyager photos. Pandora is the outer shepherd satellite of the F ring. The image at right shows both Pandora, the faint F ring, Prometheus and part of the A 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 is the fifth of Saturn's known satellites. Epimetheus was first observed in 1966. But the situation was confused since Janus is in a very similar orbit. So officially the discovery of Epimetheus was confirmed in 1980 by Voyager 1. Epimetheus has several craters larger than 30 km in diameter as well as both large and small ridges and grooves. The extensive cratering indicates that Epimetheus must be quite old.
Janus is the sixth of Saturn's known satellites. 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 is the seventh of Saturn's known satellites: 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 is the eighth of Saturn's known satellites it 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. At least some of the surface is relatively young, probably less than 100 million years. 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 be heated by the decay of radioactive material in its interior at present (the heat would have all dissipated long ago) and 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 is the ninth of Saturn's known satellites, 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 (right). 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.
Telesto and Calypso are the tenth and eleventh of Saturn's known satellites Discovered by Pascu, Seidelmann, Baum and Currie in 1980 from ground-based observations with prototype cameras destined for the HST. elesto is 34 by 28 by 26 kilometers (21 by 17 by 16 miles) and Calypso is 34 by 22 by 22 kilometers (21 by 14 by 14 miles).
Telesto and Calypso are called the Tethys Trojans because they circle Saturn in the same orbit as Tethys, about 60 degrees ahead of and behind that body. Telesto is the leading Trojan and Calypso is the trailing Trojan.
Dione is the twelfth of Saturn's known satellites and 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 of Dion there is a network of bright streaks on a dark background and few visible craters (left). The streaks overlay the craters, indicating that they are newer. The leading hemisphere is heavily cratered and uniformly bright (right). Like Callisto, the craters lack the high relief features seen on the Moon and Mercury.
Helene is the thirteenth of Saturn's known satellites Discovered by Laques and Lecacheux in 1980 from ground-based observations. Helene is in Dione's leading Lagrange point and hence was sometimes referred to as "Dione B".
Rhea is the fourteenth of Saturn's known satellites and the second largest. Though Rhea is somewhat larger than 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 of Rhea is heavily cratered and uniformly bright. 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.
Titan is the fifteenth of Saturn's known satellites and the largest. 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.
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.
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| This view of Saturn's largest satellite was taken from a position looking almost directly back toward the sun at the Mercury-sized body. Titan's hydrocarbon-rich nitrogen atmosphere scatters sunlight in a forward direction over the limb around the entire disk.
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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) and water. 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. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started.
There are scattered variable clouds in Titan's atmosphere in addition to the overall deep haze. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space.
Hyperion is the sixteenth of Saturn's known satellites. Hyperion is the largest highly irregular (non-spherical) body in the solar system. It seems likely that Hyperion is a fragment of a larger body that was broken by a large impact in the distant past. 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.
Iapetus is the seventeenth of Saturn's known satellites and the third largest. 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. Iapetus is inclined almost 15 degrees.
Phoebe is the outermost of Saturn's known satellites. Phoebe is almost 4 times more distant from Saturn than its nearest neighbor (Iapetus). 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.
4 new satellites of the planet Saturn. On October 26 2000 the most recent discovery was announced. Four new irregular moons of that planet. The newly discovered satellites are small (about 50 km across) and likely icy moons; the remnants of a long-ago capture event. The satellites orbit Saturn at distances of around 15 million kilometers from planet.
Observed from Earth, these moons appear as faint dots of light moving around the planet. The discovery's were made on September 23 2000. Two of the satellites (S/2000 S 1 and S/2000 S 2) where discovered using the European Southern Observatory's 2.2 meter telescope and the other two (S/2000 S 3 and S/2000 S 4) were discovered using the Canada France Hawaii telescope.
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