SATURN - SATURN'S MOONS PAN: Pan is the innermost of Saturn's moons. It has an orbital of 133,583 km from Saturn and a diameter of 20 km. This moon was discovered by Mark R. Showalter in 1990 by pictures taken from the Voyager. Pan was discovered by reexamining the 10 year old Voyager photos at the predicted spot. Pan is located in the Encke division in Saturn's A ring. With careful analysis of the patterns in the edge of Saturn's ring, Showalter was able to predict the size and location of the small moon. ATLAS: Atlas is the second of Saturn's known moons. It has an orbit of 137,670 km from Saturn and a diameter of 30 km. Atlas was a Titan condemned by Zeus to support the heavens upon his shoulders. He was also the son of Iapetus and the Clymene and the brother of Prometheus and Epimetheus. It was discovered by R. Terrile in 1980 through various images taken by the Voyager. Atlas is a shepherd satellite of the A ring. PROMETHEUS: Prometheus is the third of Saturn's known satellites. It has an orbit of 139,350 km from Saturn and a diameter of 91 km. It also has a mass of 2.7e17 kg. Prometheus was the son of Iapetus and the brother of Atlas and Epimetheus. Prometheus is Greek for foresight. It was discovered by S. Collins and others in 1980 from Voyager photos. Prometheus is the inner shepherd satellite of the F ring. It has a number of ridges and valleys and several craters about 20 km in diameter but appears to be less cratered than its neighboring moons. From their very low densities and relatively high albedos, it seems likely that Prometheus, Pandora, Janus and Epimetheus are very porous icy bodies. PANDORA: Pandora is the fourth of Saturn's known satellites. It has an orbit of 141,700 km from Saturn and a diameter of 84 km. Its mass is 2.2e17 kg. Pandora, in Greek mythology, was the first woman, and wife of Epimetheus. The moon was discovered by Collins and others in 1980 by pictures taken by the Voyager. Pandora is the outer shepherd satellite of the F ring. Pandora has at least two large craters that are 30 km in diameter, and is even more cratered than Prometheus. It has no linear ridges or valleys. EPIMETHEUS: Epimetheus is the fifth of Saturn's known moons. It has an orbit of 151,422 km and a diameter of 115 km. The mass is also known, at 5.6e17 kg. Epimetheus was the son of Iapetus and brother of Prometheus and Atlas, as well as the husband of Pandora. Epimetheus is Greek for hindsight. Epimetheus was first observed by Walker in 1966. Epimetheus and Janus are coorbital. It has several craters larger than 30 km in diameter as well as both large and small ridges and grooves. The cratering indicates that Epimetheus must be fairly old. JANUS: Janus is the the sixth satellite of Saturn. It has an orbital of 151,472 km from Saturn and a diameter of 178 km. The mass of this moon is 2.01e18 kg. It was discovered by a French astronomer, Audouin Dollfus, in 1966. Janus and Epimetheus are known as co-orbital. The difference between the orbital radii of Janus and Epimetheus differ by only 50 km. The orbital velocities are almost the same and the lower, faster one overtakes the other. Upon approaching each other, they exchange momentum and the lower one is able to go into a higher orbit, and exchange places. The trading of places only occurs once every four years. Janus is highly cratered. It has many larger than 30 km. MIMAS: Mimas is the seventh moon of Saturn. It has an orbit of 185,520 km from Saturn and a diameter of 392 km. The mass of this moon is 3.80e19 kg. This satellite was discovered by Herschel in 1789. It has a very low density, 1.17, showing that it is mostly water ice and some amount of rock. Mimas' surface is dominated by an impact crater that is actually 130 km across. This crater is called Herschel, and is 1/3 of the diameter of the entire moon. It's walls are 5 km high, and its floor is about 10 km deep, and its central peak rises 6 km above the crater floor. The cause of this crater must have severely disrupted Mimas, there are even fractures on the opposite side of Mimas that may be due to the same reason. The surface of Mimas is covered with impact craters. The size of Herschel suggests that Mimas was impacted by even larger bodies than the one that created Herschel which completely disrupted the new moon and also in turn, wiping out the evidence of earlier large impacts. The impact debris fused together again to form what we know as Mimas. ENCELADUS: Enceladus is the eighth satellite of Saturn. It has an orbit of 238,020 km from Saturn and a diameter of 498 km. The mass is 7.30e19 kg. This moon was discovered in 1789 by Herschel. Enceladus has the highest albedo, greater than .9, than any body of matter in the solar system. The surface is made of clean ice, but its terrain has five different types (at least). Enceladus has, in addition to craters, smooth plains and extensive linear cracks and ridges. Some of the surface is relatively new, and probably formed less than 100 million years ago. This means that Enceladus must have been active until recently, maybe by water volcanism. Enceladus is very small and can not be heated by radioactive decay in the interior. Enceladus is locked in a 1:2 resonance with Dione. This may have provided a heat mechanism but not sufficient enough to melt the water ice. Therefore, Enceladus may be made up of a low-melting point material rather than just pure water. It may be the source of the material in Saturn's E ring. TETHYS: Tethys is the ninth satellite. It has an orbit of 294,660 km from Saturn and a diameter of 1060 km. The mass is 6.22e20 kg. It was discovered by Cassini in 1684. Tethys' low density indicates that it is almost completely composed of water ice. The western hemisphere has a large impact crater, called Odysseus. It is 400 km in diameter, and nearly 2/5 of Tethys. Since the impact didn't shatter Tethys, it indicates that it may have not very solid at the time. The crater is flat now, or conforms to Tethys' spherical shape. A second major feature on Tethys is a huge valley, called Ithaca Chasma. It is 100 km wide and 3 to 5 km deep. It runs about 2000 km or 3/4 of the way around Tethys' circumference. This shows that Tethys has not always been frozen solid. At some point, it was liquid. The impact craters have been smoothed out. As it froze and expanded, the surface may have cracked to accommodate for the extra volume producing Ithaca Chasma. The smaller impact craters we see today are more recent. Telesto and Calypso in Tethys' Lagrange points, meaning that they are 60 degrees ahead and behind Tethys in the same orbit. TELESTO: Telesto is the tenth satellite of Saturn. Its orbit is 294,660 km from Saturn and it has a diameter of 29 km. The mass is unknown. This moon was discovered in 1980 by Smith, Reitsema, Larson and Fountain from ground-based observations. Telesto is in Tethys' leading Laggrange point. CALYPSO: Calypso is the eleventh of Saturn's known satellites. It has an orbit of 294,660 km from Saturn and a diameter of 26 km. The mass is unknown. This satellite was discovered by Pascu, Seidelmann, Baum and Currie 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 entire solar system. DIONE: Dione is the twelfth satellite of Saturn. It has an orbital of 377,400 km from Saturn and a diameter of 1120 km. The mass is 1.05e21 kg. This moon was discovered by Cassini in 1684. Dione is the densest of Saturn's moons. It is made mostly of water ice and also must have a fraction of denser material such as silicate rock. Dione is otherwise similar to Rhea, but smaller. They both have similar compositions, albedos and terrain. They 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 a few visible craters. These streaks overlay the craters, indicating that they are new. The leading hemisphere is heavily cratered and uniformly bright. It is believed that shortly after its formation Dione was active. Some processes resurfaced Dione, leaving the pattern of streaks, on the whole surface. After the internal activity and resurfacing ended, a less intense series of impacts 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. It has an orbit of 377,400 km and a diameter of 33 km. Helene was discovered by Laques and Lecacheux in 1980 from ground-based observations. Helene is the moon in Dione's leading Lagrange point and therefore is sometimes referred to as "Dione B". RHEA: Rhea is the fourteenth moon as well as the second largest. It has an orbit of 527,040 km from Saturn and a diameter of 1530 km. The mass of this moon is 2.49e21 kg. In Greek myths, Rhea is the sister and wife of Cronus (Saturn) and the mother of Demeter, Hades (Pluto), Hera, Hestia, Poseidon (Neptune), and Zeus (Jupiter). Rhea was discovered by Cassini in 1672. Rhea is very similar to Dione, just larger. They have similar compositions, albedos, and terrain. They 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. The trailing hemisphere has a network of bright swaths on a dark background and few visible craters. TITAN: Titan is the fifteenth of Saturn's satellites and the largest. It has an orbit of 1,221,830 km from Saturn and a diameter of 5150 km. The mass is 1.35e23 kg. It was discovered by Huygens in 1655. It was long believed that 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 actually slightly smaller than Ganymede's. Titan is larger in diameter than Mercury, and even more massive than Pluto. Titan is surrounded by a thick, opaque atmosphere. The surface cannot be seen at all in visible light. Titan is similar in bulk properties to Ganymede, Callisto, Triton and maybe even Pluto. We do not know if it has an internal structure like Ganymede or if it is uniform like Callisto. Titan is probably half water ice and half rocky material, differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. The interior may still be hot. It is denser because it is so large that its gravity compresses its interior. Titan has a significant atmosphere. At the surface, the pressure is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen with no more than 6% argon and a few percent methane. There are also trace amounts of organic compounds such as ethane, hydrogen cyanide, carbon dioxide and water. The organics are formed as methane, that dominates in Titan's upper atmosphere, and is destroyed by sunlight. This produces smog found over large cities, but much thicker. This is similar to the conditions on Earth early in its history. Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere. Therefore it is directly exposed to 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, meaning that there is little water vapor in the atmosphere. There are two layers of clouds at about 200 and 300 km above the surface. Other complex chemicals must be responsible for the orange color as seen from space. Ethane clouds would produce a rain of liquid ethane onto the surface, and may produce an "ocean" of ethane mixture up to 1000 meters deep. HYPERION: Hyperion is the sixteenth moon. It has an orbit of 1,481,100 km from Saturn and a diameter of 286 km. The mass is 1.77e19 kg. It was discovered by Bond and Lassell in 1848. Hyperion is the largest highly irregular or non-spherical body in the solar system. Hyperion seems to be a fragment of a larger body that was broken by a large impact in the past. Hyperion's low density indicates that it is composed of water ice with only a small amount of rock. Unlike most of Saturn's moons, Hyperion has a low albedo (.2 - .3). This means that it is covered by a thin layer of dark material. Hyperion's rotation is chaotic, meaning that its its axis of rotation wobbles so much that its orientation in space is unpredictable. Hyperion is the only known body in the solar system that rotates chaoticly but simulations seem to indicate that other irregular satellites may have done so in the past. Hyperion is unique in that it is very irregularly shaped, has a highly eccentric orbit, and is near another large moon. The 3:4 orbital resonance between Titan and Hyperion may also make chaotic rotation more likely. Hyperion's odd rotation probably accounts 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 is the seventeenth of Saturn's known satellites and the third largest. It has an orbit of 3,561,300 km from Saturn and a diameter of 1460 km. The mass is 1.88e21 kg. It was discovered by Cassini in 1671. Iapetus only has a density of 1.1, meaning that it is 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. 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. The color of the leading half of Iapetus and that of Phoebe do not 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: Phoebe is the outermost satellite. Phoebe is almost 4 times more distant from Saturn than its nearest neighbor, Iapetus. It has an orbit of 12,952,000 km from Saturn and a diameter of 220 km. The mass is 4.0e18 kg. Phoebe was 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.