Saturn is the sixth planet from the Sun and is the second largest in the solar system with an equatorial diameter of 119,300 kilometers (74,130 miles). Much of what is known about the planet is due to the Voyager explorations in 1980-81. Saturn is visibly flattened at the poles, a result of the very fast rotation of the planet on its axis. Its day is 10 hours, 39 minutes long, and it takes 29.5 Earth years to revolve about the Sun. The atmosphere is primarily composed of hydrogen with small amounts of helium and methane. Saturn is the only planet less dense than water (about 30 percent less). In the unlikely event that a large enough ocean could be found, Saturn would float in it. Saturn's hazy yellow hue is marked by broad atmospheric banding similar to, but fainter than, that found on Jupiter.
The wind blows at high speeds on Saturn. Near the equator, it reaches velocities of 500 meters a second (1,100 miles an hour). The wind blows mostly in an easterly direction. The strongest winds are found near the equator and velocity falls off uniformly at higher latitudes. At latitudes greater than 35 degrees, winds alternate east and west as latitude increases.
Saturn's ring system makes the planet one of the most beautiful objects in the solar system. The rings are split into a number of different parts, which include the bright A and B rings and a fainter C ring. The ring system has various gaps. The most notable gap is the Cassini [kah-SEE-nee] Division, which separates the A and B rings. Giovanni Cassini discovered this division in 1675. The Encke [EN-kee] Division, which splits the A Ring, is named after Johann Encke, who discovered it in 1837. Space probes have shown that the main rings are really made up of a large number of narrow ringlets. The origin of the rings is obscure. It is thought that the rings may have been formed from larger moons that were shattered by impacts of comets and meteoroids. The ring composition is not known for certain, but the rings do show a significant amount of water. They may be composed of icebergs and/or snowballs from a few centimeters to a few meters in size. Much of the elaborate structure of some of the rings is due to the gravitational effects of nearby satellites. This phenomenon is demonstrated by the relationship between the F-ring and two small moons that shepherd the ring material.
Radial, spoke-like features in the broad B-ring were also found by the Voyagers. The features are believed to be composed of fine, dust-size particles. The spokes were observed to form and dissipate in the time-lapse images taken by the Voyagers. While electrostatic charging may create spokes by levitating dust particles above the ring, the exact cause of the formation of the spokes is not well understood.
Saturn has 18 confirmed moons, the largest number of satellites of any planet in the solar system. In 1995, scientists using the Hubble Space Telescope sighted four objects which might be new moons.
Mass (kg) ......................................... 5.688e+26
Mass (Earth = 1) ................................. 9.5181e+01
Equatorial radius (km) ............................... 60,268
Equatorial radius (Earth = 1) .................... 9.4494e+00
Mean density (gm/cm^3) ................................. 0.69
Mean distance from the Sun (km) ............... 1,429,400,000
Mean distance from the Sun (Earth = 1) ............... 9.5388
Rotational period (hours) ............................ 10.233
Orbital period (years) ............................... 29.458
Mean orbital velocity (km/sec) ......................... 9.67
Orbital eccentricity ................................. 0.0560
Tilt of axis .......................................... 25.33°
Orbital inclination ................................... 2.488°
Equatorial surface gravity (m/sec^2) ................... 9.05
Equatorial escape velocity (km/sec) ................... 35.49
Visual geometric albedo ................................ 0.47
Magnitude (Vo) ......................................... 0.67
Mean cloud temperature ............................... -125°C
Atmospheric pressure (bars) ............................. 1.4
Atmospheric composition
Hydrogen ............................................ 97%
Helium ............................................... 3%
The following is a summary of the rings of Saturn.
Name Distance* Width Thickness Mass Albedo -------------------------------------------------------------------- D 67,000 km 7,500 km ? ? ? C 74,500 km 17,500 km ? 1.1x10^18 kg 0.25 Maxwell Gap 87,500 km 270 km B 92,000 km 25,500 km 0.1-1 km 2.8x10^19 kg 0.65 Cassini Div 117,500 km 4,700 km ? 5.7x10^17 kg 0.30 A 122,200 km 14,600 km 0.1-1 km 6.2x10^18 kg 0.60 Encke gap 133,570 km 325 km Keeler gap 136,530 km 35 km F 140,210 km 30-500 km ? ? ? G 165,800 km 8,000 km 100-1000 km 6-23x10^6 kg ? E 180,000 km 300,000 km 1,000 km ? ?
*The distance is measured from the planet center to the start of the ring.
Saturn has 18 officially recognized and named satellites. In addition, there are other unconfirmed satellites. One circles in the orbit of Dione, a second is located between the orbits of Tethys and Dione, and a third is located between Dione and Rhea. The unconfirmed satellites were found in Voyager photographs, but were not confirmed by more than one sighting. Recently, the Hubble Space Telescope imaged four objects that might be new moons.
Several generalizations can be made about the satellites of Saturn. Only Titan has an appreciable atmosphere. Most of the satellites have a synchronous rotation. The exceptions are Hyperion, which has a chaotic orbit, and Phoebe. Saturn has a regular system of satellites. That is, the satellites have nearly circular orbits and lie in the equatorial plane. The two exceptions are Iapetus and Phoebe. All of the satellites have a density of < 2 gm/cm3. This indicates they are composed of 30 to 40% rock and 60 to 70% water ice. Most of the satellites reflect 60 to 90% of the light that strikes them. The outer four satellites reflect less than this and Phoebe reflects only 2% of the light that strikes it.
The following table summarizes the radius, mass, distance from the planet center, discoverer and the date of discovery of each of the confirmed satellites of Saturn:
Radius Mass Distance
Moon # (km) (kg) (km) Discoverer Date
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Pan XVIII 9.655 ? 133,583 M. Showalter 1990
Atlas XV 20x15 ? 137,640 R. Terrile 1980
Prometheus XVI 72.5x42.5x32.5 2.7e+17 139,350 S. Collins 1980
Pandora XVII 57x42x31 2.2e+17 141,700 S. Collins 1980
Epimetheus XI 72x54x49 5.6e+17 151,422 R. Walker 1966
Janus X 98x96x75 2.01e+18 151,472 A. Dollfus 1966
Mimas I 196 3.80e+19 185,520 W. Herschel 1789
Enceladus II 250 8.40e+19 238,020 W. Herschel 1789
Tethys III 530 7.55e+20 294,660 G. Cassini 1684
Telesto XIII 17x14x13 ? 294,660 B. Smith 1980
Calypso XIV 17x11x11 ? 294,660 B. Smith 1980
Dione IV 560 1.05e+21 377,400 G. Cassini 1684
Helene XII 18x16x15 ? 377,400 Laques-Lecacheux 1980
Rhea V 765 2.49e+21 527,040 G. Cassini 1672
Titan VI 2,575 1.35e+23 1,221,850 C. Huygens 1655
Hyperion VII 205x130x110 1.77e+19 1,481,000 W. Bond 1848
Iapetus VIII 730 1.88e+21 3,561,300 G. Cassini 1671
Phoebe IX 110 4.0e+18 12,952,000 W. Pickering 1898
Saturn XVIII - 1990S18, 1981S13
Pan, the innermost known satellite, was found from photographs taken by Voyager during its encounter with Saturn. It was discovered by Mark R. Showalter in 1990, 9 years after the Voyager encounter. Pan is located 133,583 kilometers from the center of Saturn and is within the Encke Gap of Saturn's A-ring. It acts as a shepherd and is responsible for keeping the Encke gap open. It has a diameter of 20 kilometers.
Discovered by ............................. Mark R. Showalter Date of discovery ...................................... 1990 Mass (kg) ................................................. ? Equatorial radius (km) ................................ 9.655 Equatorial radius (Earth = 1) .................... 1.5138e-03 Mean density (gm/cm^3) .................................... ? Mean distance from Saturn (km) ...................... 133,583 Rotational period (days) .................................. ? Orbital period (days) ................................ 0.5750 Mean orbital velocity (km/sec) ........................ 16.90 Orbital eccentricity ................................... 0.00 Orbital inclination ..................................... 0.0° Visual geometric albedo ................................. 0.5 Magnitude (Vo) ............................................ ?
Saturn XV - 1980S28
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.
Discovered by .................................... R. Terrile Date of discovery ...................................... 1980 Mass (kg) ................................................. ? Radius (km) ........................................... 20x15 Radius (Earth = 1) ............................... 3.1358e-03 Mean density (gm/cm^3) .................................... ? Mean distance from Saturn (km) ...................... 137,640 Rotational period (days) .................................. ? Orbital period (days) ................................ 0.6019 Mean orbital velocity (km/sec) ........................ 16.63 Orbital eccentricity .................................. 0.000 Orbital inclination ..................................... 0.0° Visual geometric albedo ................................. 0.9 Magnitude (Vo) ......................................... 18.0
Saturn XVI - 1980S27
Prometheus [pra-MEE-thee-us] is the third of Saturn's known satellites. It was discovered from photographs taken by Voyager during its encounter with Saturn by S. Collins and others. Prometheus acts as a shepherd satellite for the inner edge of Saturn's F Ring. The moon is extremely elongated about 145 by 85 by 62 kilometers (90 by 53 by 39 miles) in diameter. It has a number of ridges and valleys on its northern side. Several craters about 20 kilometers (12 miles) in diameter are visible, but it appears to be less cratered than its nearby neighbors Pandora, Janus and Epimetheus. The density of Prometheus has been estimated to be quite low, indicating that is a porous, icy body; however, there is a lot of uncertainty in these values.
Discovered by ........................... S. Collins & others Date of discovery ...................................... 1980 Mass (kg) .......................................... 2.7e+17* Mass (Earth = 1) ................................. 4.5181e-08 Radius (km) .................................. 72.5x42.5x32.5 Radius (Earth = 1) ............................... 1.1367e-02 Mean density (gm/cm^3) ................................. 0.7* Mean distance from Saturn (km) ...................... 139,350 Rotational period (days) .................................. ? Orbital period (days) ................................ 0.6130 Mean orbital velocity (km/sec) ........................ 16.54 Orbital eccentricity .................................. 0.003 Orbital inclination ..................................... 0.0° Escape velocity (km/sec) ............................. 0.0223 Visual geometric albedo ................................. 0.6 Magnitude (Vo) ......................................... 15.8
Saturn XVII - 1980S26
Pandora [pan-DOR-uh] is the fourth of Saturn's known satellites. It was discovered from photographs taken by Voyager during its encounter with Saturn by S. Collins and others. Pandora is the outer shepherd satellite for Saturn's F-ring. It has a diameter of about 114 by 84 by 62 kilometers (71 by 52 by 38 miles) and appears to be very heavily cratered. The two largest craters are about 30 kilometers (19 miles) in diameter. No linear valleys or ridges are visible. Very limited data is available to analyze Pandora.
Discovered by ........................... S. Collins & others Date of discovery ...................................... 1980 Mass (kg) .......................................... 2.2e+17* Mass (Earth = 1) ................................. 3.6814e-08 Radius (km) ........................................ 57x42x31 Radius (Earth = 1) ............................... 8.9370e-03 Mean density (gm/cm^3) ................................. 0.7* Mean distance from Saturn (km) ...................... 141,700 Rotational period (days) .................................. ? Orbital period (days) ................................ 0.6285 Mean orbital velocity (km/sec) ........................ 16.40 Orbital eccentricity .................................. 0.004 Orbital inclination ..................................... 0.0° Escape velocity (km/sec) ............................. 0.0227 Visual geometric albedo ................................. 0.9 Magnitude (Vo) ......................................... 16.5
Saturn XI - 1980S3
Epimetheus [ep-eh-MEE-thee-us] is the fifth satellite of Saturn. It was discovered by R. Walker in 1980. Epimetheus was the son of Iapetus and brother to Prometheus and Atlas. Epimetheus means hindsight in Greek. It has an irregular shape with a size of 144x108x98 kilometers (89x67x61 miles) in diameter. It is traversed by several large and small grooves, valleys and ridges. Several craters larger than 30 km can be seen on its surface. The pervasive cratering indicates that its surface must be several billion years old. The above image shows the shadow of Saturn's F Ring crossing its surface.
Epimetheus and Janus share the same orbit of 151,472 kilometers from Saturn's center or 91,000 kilometers above the cloud tops. They are only separated by about 50 kilometers. As these two satellites approach each other they exchange a little momentum and trade orbits; the inner satellite becomes the outer and the outer moves to the inner position. This exchange happens about once every four years. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites. If this is the case, the disruption must have happened early in the history of the satellite system.
Discovered by ..................................... R. Walker Date of discovery ...................................... 1980 Mass (kg) .......................................... 5.6e+17* Mass (Earth = 1) ................................. 9.3708e-08 Radius (km) ........................................ 72x54x49 Radius (Earth = 1) ............................... 1.1289e-02 Mean density (gm/cm^3) ................................. 0.7* Mean distance from Saturn (km) ...................... 151,422 Rotational period (days) ............................. 0.6942 Orbital period (days) ................................ 0.6942 Mean orbital velocity (km/sec) ........................ 15.87 Orbital eccentricity .................................. 0.009 Orbital inclination .................................... 0.34° Escape velocity (km/sec) ............................. 0.0322 Visual geometric albedo ................................. 0.8 Magnitude (Vo) ......................................... 15.7
Saturn X - 1980S1
Janus [JAY-nus] is the sixth satellite of Saturn. It was discovered by Audouin Dollfus in 1966 and was named after the god of gates and doorways. It is depicted with two faces looking in opposite directions. Janus has an irregular shape with a size of 196x192x150 kilometers (122x119x93 miles) in diameter. It is heavily cratered with several craters 30 kilometers (19 miles) in diameter. The pervasive cratering indicates that its surface must be several billion years old. Prometheus appears to have fewer craters indicating a younger surface while Pandora appears to have an older surface. Janus has few linear features.
Janus and Epimetheus share the same orbit of 151,472 kilometers (94,125 miles) from Saturn's center or 91,000 kilometers (56,547 miles) above the cloud tops. They are only separated by about 50 kilometers (31 miles). As these two satellites approach each other they exchange a little momentum and trade orbits; the inner satellite becomes the outer and the outer moves to the inner position. This exchange happens about once every four years. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites. If this is the case, the disruption must have happened early in the history of the satellite system.
Discovered by ............................... Audouin Dollfus Date of discovery ...................................... 1966 Mass (kg) ......................................... 2.01e+18* Mass (Earth = 1) ................................. 3.3635e-07 Radius (km) ........................................ 98x96x75 Radius (Earth = 1) ............................... 1.5365e-02 Mean density (gm/cm^3) ................................ 0.67* Mean distance from Saturn (km) ...................... 151,472 Rotational period (days) ............................. 0.6945 Orbital period (days) ................................ 0.6945 Mean orbital velocity (km/sec) ........................ 15.87 Orbital eccentricity .................................. 0.007 Orbital inclination .................................... 0.14° Escape velocity (km/sec) ............................. 0.0523 Visual geometric albedo ................................. 0.8 Magnitude (Vo) ......................................... 14.5
Saturn I
Mimas [MY-mass] is one of the innermost moons of Saturn. Mimas was a Titan who was slain by Hercules. William Herschel discovered the moon in 1789. The surface is icy and heavily cratered. Mimas has a low density, meaning it probably consists mostly of ice. Because Mimas has such a low temperature of about -200° C (-328°F), the impact features may date back to the time of the moon's creation.
One of the craters, named Herschel, is surprisingly large in comparison to the size of the moon. The crater is 130 kilometers (80 miles) wide, one-third the diameter of Mimas. Herschel is 10 kilometers (6 miles) deep, with a central mountain almost as high as Mount Everest on Earth. This central peak rises 6 kilometers (4 miles) above the crater floor. This impact probably came close to disintegrating the moon. Traces of fracture marks can be seen on the opposite side of Mimas.
Although Mimas is heavily cratered, the cratering is not uniform. Most of the surface is covered with craters greater than 40 kilometers (25 miles) in diameter but in the south polar region, craters greater than 20 kilometers (12 miles) are generally lacking. This suggests that some process removed the larger craters from these areas.
Discovered by .............................. William Herschel Date of discovery ...................................... 1789 Mass (kg) .......................................... 3.80e+19 Mass (Earth = 1) ................................. 6.3588e-06 Equatorial radius (km) .................................. 196 Equatorial radius (Earth = 1) .................... 3.0731e-02 Mean density (gm/cm^3) ................................. 1.17 Mean distance from Saturn (km) ...................... 185,520 Rotational period (days) ........................... 0.942422 Orbital period (days) .............................. 0.942422 Mean orbital velocity (km/sec) ........................ 14.32 Orbital eccentricity ................................. 0.0202 Orbital inclination .................................... 1.53° Escape velocity (km/sec) .............................. 0.161 Visual geometric albedo ................................. 0.5 Mean surface temperature ............................. -200°C Magnitude (Vo) ......................................... 12.9
Saturn II
Enceladus [en-SELL-ah-dus] is one of the innermost moons of Saturn. It is quite similar in size to Mimas but has a smoother, brighter surface. Enceladus reflects almost 100 percent of the sunlight that strikes it. Unlike Mimas, Enceladus displays at least five different types of terrain. Parts of Enceladus shows craters no larger than 35 km in diameter. Other areas show regions with no craters indicating major resurfacing events in the geologically recent past. There are fissures, plains, corrugated terrain and other crustal deformations. All of this indicates that that interior of the moon may be liquid today, even though it should have frozen aeons ago. It is postulated that Enceladus is heated by a tidal mechanism similar to Jupiter's moon Io. It is perturbed in its orbit by Saturn's gravitational field and by the large neighboring satellites Tethys and Dione. Because Enceladus reflects so much sunlight, the surface temperature is only -201° C (-330° F).
Discovered by .............................. William Herschel Date of discovery ...................................... 1789 Mass (kg) .......................................... 8.40e+19 Mass (Earth = 1) ................................. 1.4056e-05 Equatorial radius (km) .................................. 250 Equatorial radius (Earth = 1) .................... 3.9197e-02 Mean density (gm/cm^3) ................................. 1.24 Mean distance from Saturn (km) ...................... 238,020 Rotational period (days) ........................... 1.370218 Orbital period (days) .............................. 1.370218 Mean orbital velocity (km/sec) ........................ 12.64 Orbital eccentricity ................................. 0.0045 Orbital inclination .................................... 0.02° Escape velocity (km/sec) .............................. 0.212 Visual geometric albedo ................................ 0.99 Mean surface temperature ............................. -201°C Magnitude (Vo) ......................................... 11.7
Saturn III
Tethys [TEE-this] was discovered by Giovanni Cassini in 1684. It is an icy body similar in nature to Dione and Rhea. The density of Tethys is 1.21 gm/cm3, indicating that it is composed almost entirely of water-ice. Tethys's icy surface is heavily cratered and contains cracks caused by faults in the ice. There is one enormous trench on Tethys about 65 kilometers (40 miles) wide and extending from above the center to the extreme left. It covers three-fourths of Tethys' circumference. The fissure is about the size scientists would predict if Tethys were once fluid and its crust hardened before the interior. The canyon has been named Ithaca Chasma. A vast expanse of relative young plains also exists on Tethys. Tethys' surface temperature is -187° C (-305° F).
Discovered by ..................... Giovanni Domenico Cassini Date of discovery ...................................... 1684 Mass (kg) .......................................... 7.55e+20 Mass (Earth = 1) ................................. 1.2634e-04 Equatorial radius (km) .................................. 530 Equatorial radius (Earth = 1) .................... 8.3098e-02 Mean density (gm/cm^3) ................................. 1.21 Mean distance from Saturn (km) ...................... 294,660 Rotational period (days) ........................... 1.887802 Orbital period (days) .............................. 1.887802 Mean orbital velocity (km/sec) ........................ 11.36 Orbital eccentricity ................................. 0.0000 Orbital inclination .................................... 1.09° Escape velocity (km/sec) .............................. 0.436 Visual geometric albedo ................................. 0.9 Magnitude (Vo) ......................................... 10.2 Mean surface temperature ............................. -187°C
Telesto [tah-LESS-toh] and Calypso [ka-LIP-so] 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. Both were found in 1981 by B. Smith through ground-based observations made in 1980. Telesto 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).
Discovered by ............................. B. Smith & others Date of discovery ...................................... 1980 Mass (kg) ................................................. ? Radius (km) ........................................ 17x14x13 Radius (Earth = 1) ............................... 2.6654e-03 Mean density (gm/cm^3) .................................... ? Mean distance from Saturn (km) ...................... 294,660 Rotational period (days) .................................. ? Orbital period (days) ................................ 1.8878 Mean orbital velocity (km/sec) ........................ 11.36 Orbital eccentricity ................................... 0.00 Orbital inclination ..................................... 0.0° Visual geometric albedo ................................. 0.5 Magnitude (Vo) ......................................... 18.7
Discovered by ............................. B. Smith & others Date of discovery ...................................... 1980 Mass (kg) ................................................. ? Radius (km) ........................................ 17x11x11 Radius (Earth = 1) ............................... 2.6654e-03 Mean density (gm/cm^3) .................................... ? Mean distance from Saturn (km) ...................... 294,660 Rotational period (days) .................................. ? Orbital period (days) ................................ 1.8878 Mean orbital velocity (km/sec) ........................ 11.36 Orbital eccentricity ................................... 0.00 Orbital inclination ..................................... 0.0° Visual geometric albedo ................................. 0.6 Magnitude (Vo) ......................................... 18.0
Saturn IV
Dione [dy-OH-nee] was discovered in 1684 by Giovanni Cassini. It is an icy body similar to Tethys and Rhea. Its density is 1.43 gm/cm3, which makes it the densest moon of Saturn other than Titan. Dione is probably composed of a rocky core making up one-third of the moon's mass, with the rest water-ice. Its ice coverage is less than that of Tethys and Rhea.
Dione's icy surface includes heavily cratered terrain, moderately cratered plains, lightly cratered plains, and whispy material. The heavily cratered terrain has numerous craters greater than 100 kilometers in diameter. The plains area tends to have craters less than 30 kilometers in diameter. Some of the plains are heavily cratered while others are not. Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains area existing on the leading hemisphere. This is opposite from what some scientists expected. Shoemaker and Wolfe proposed a cratering model for a tidally locked satellite with the highest cratering rates on the leading hemisphere and the lowest on the trailing hemisphere. This suggests that during the period of heavy bombardment, Dione was tidally locked to Saturn in the opposite orientation. Because Dione is relatively small, an impact causing a 35 kilometer (21 mile) crater could have spun the satellite. Since there are many craters larger than 35 kilometers (21 miles), Dione could have been repeatedly spun.
Dione has probably been tidally locked in its current position for the past several billion years. This is reflected in the average surface albedo of the leading and trailing hemispheres. The surface albedo decreases from the leading to the trailing hemispheres due to a higher micrometeor dusting on the leading hemisphere.
The origin of the bright whispy material is somewhat obscure. Apparently, it is material with a high albedo and is thin enough that it doesn't obscure the surface feature underneath. It might have formed from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash.
Discovered by ..................... Giovanni Domenico Cassini Date of discovery ...................................... 1684 Mass (kg) .......................................... 1.05e+21 Mass (Earth = 1) ................................. 1.7570e-04 Equatorial radius (km) .................................. 560 Equatorial radius (Earth = 1) .................... 8.7802e-02 Mean density (gm/cm^3) ................................. 1.43 Mean distance from Saturn (km) ...................... 377,400 Rotational period (days) ........................... 2.736915 Orbital period (days) .............................. 2.736915 Mean orbital velocity (km/sec) ........................ 10.03 Orbital eccentricity ................................. 0.0022 Orbital inclination .................................... 0.02° Escape Velocity (km/sec) .............................. 0.500 Surface gravity (m/sec^2) ............................. 0.223 Visual geometric albedo ................................. 0.7 Magnitude (Vo) ......................................... 10.4 Mean surface temperature ............................ -186.56°C
Saturn XII - 1980S6
Helene was discovered by P. Laques and J. Lecacheus in 1980 through ground based observations. Helene is called a Dione Trojan satellite because it shares the same orbit as Dione at about 60 degrees ahead of its larger companion. Its diameter is about 36 x 32 x 30 kilometers (22 x 20 x 19 miles).
Discovered by ...................... P. Laques & J. Lecacheus Date of discovery ...................................... 1980 Mass (kg) ................................................. ? Radius (km) ........................................ 18x16x15 Radius (Earth = 1) ............................... 2.8222e-03 Mean density (gm/cm^3) .................................... ? Mean distance from Saturn (km) ...................... 377,400 Rotational period (days) .................................. ? Orbital period (days) ................................ 2.7369 Mean orbital velocity (km/sec) ........................ 10.03 Orbital eccentricity .................................. 0.005 Orbital inclination ..................................... 0.2° Visual geometric albedo ................................. 0.7 Magnitude (Vo) ......................................... 18.5
Rhea [REE-a] is the largest airless satellite of Saturn. It was discovered in 1672 by Giovanni Cassini. Rhea is an icy body with a density of 1.33 gm/cm3. The low density indicates that it is composed of a rocky core taking up less than one-third of the moon's mass, with the rest composed of water-ice. Rhea is somewhat similar to Dione. They both have similar composition, albedo features, varied terrain and synchronous rotations. The temperature on Rhea is -174°C with illumation from the Sun and between -200°C and -220°C (-328°F and -364°F) in the shade.
Rhea is heavily cratered with bright whispy markings. Its surface can be divided into two geologically different areas based on crater density. The first area contains craters which are larger than 40 kilometers (25 miles) in diameter. The second area, in parts of the polar and equatorial regions, has craters under 40 kilometers (25 miles) in diameter. This suggests that a major resurfacing event occurred some time during its formation.
Discovered by ..................... Giovanni Domenico Cassini Date of discovery ...................................... 1672 Mass (kg) .......................................... 2.49e+21 Mass (Earth = 1) ................................. 4.1667e-04 Equatorial radius (km) .................................. 765 Equatorial radius (Earth = 1) .................... 1.1994e-01 Mean density (gm/cm^3) ................................. 1.33 Mean distance from Saturn (km) ...................... 527,040 Rotational period (days) ........................... 4.517500 Orbital period (days) .............................. 4.517500 Mean orbital velocity (km/sec) ......................... 8.49 Orbital eccentricity ................................. 0.0010 Orbital inclination .................................... 0.35° Escape velocity (km/sec) .............................. 0.659 Visual geometric albedo ................................. 0.7 Magnitude (Vo) .......................................... 9.7 Maximum surface temperature .......................... -174°C Minimum surface temperature .......................... -220°C
Saturn VI
Titan [TY-tun] is the largest moon of Saturn and the second largest moon in the solar system, rivaled only by Jupiter's moon Ganymede. Before the Voyager encounters, astronomers suspected that Titan might have an atmosphere. Scientists also believed they might find liquid seas or pools of methane or ethane; water would be frozen due to Titan's low surface temperature. Expecting an unusual world, Voyager 1 was programmed to take numerous close up views of Titan as it flew past in November of 1980. Unfortunately, all that was revealed was an impenetrable layer of atmosphere and clouds. Only slight color and brightness variations were observed.
Although Titan is classified as a moon, it is larger than the planets Mercury and Pluto. It has a planet-like atmosphere which is more dense than those of Mercury, Earth, Mars and Pluto. The atmospheric pressure near the surface is about 1.6 bars, 60 percent greater than Earth's. Titan's air is predominantly made up of nitrogen with other hydrocarbon elements which give Titan its orange hue. These hydrocarbon rich elements are the building blocks for amino acids necessary for the formation of life. Scientists believe that Titan's environment may be similar to that of the Earth's before life began putting oxygen into the atmosphere.
Titan's surface temperature appears to be about -178°C (-289°F). Methane appears to be below its saturation pressure near Titan's surface; rivers and lakes of methane probably don't exist, in spite of the tantalizing analogy to water on Earth. On the other hand, scientists believe lakes of ethane exist that contain dissolved methane. Titan's methane, through continuing photochemistry, is converted to ethane, acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide. The last is an especially important molecule; it is a building block of amino acids.
The Voyager spacecraft were not able to penetrate the thick layers of clouds but they did reveal that Titan is one of the more interesting places in the solar system. What kind of landscape lies below the layers of clouds? What mysteries are held beneath these orange curtains? These questions will have to wait until future spacecraft are launched to visit this unusual moon. In October 1997, the Cassini spacecraft is scheduled to be launched for a rendezvous with Saturn in June 2004. Later that year, it will release the European-built Huygens probe for a descent through Titan's atmosphere. Cassini will have more than 30 encounters with Titan, mapping the moon's surface with a synthetic aperature radar similar to the one Magellan used to map Venus.
Discovered by ............................ Christiaan Huygens Date of discovery ...................................... 1655 Mass (kg) .......................................... 1.35e+23 Mass (Earth = 1) ................................. 2.2590e-02 Equatorial radius (km) ................................ 2,575 Equatorial radius (Earth = 1) .................... 4.0373e-01 Mean density (gm/cm^3) ................................. 1.88 Mean distance from Saturn (km) .................... 1,221,850 Rotational period (days) ........................... 15.94542 Orbital period (days) .............................. 15.94542 Mean orbital velocity (km/sec) ......................... 5.58 Orbital eccentricity ................................. 0.0292 Orbital inclination .................................... 0.33° Escape velocity (km/sec) ............................... 2.65 Visual geometric albedo ................................ 0.21 Magnitude (Vo) ......................................... 8.28 Mean surface temperature ............................. -178°C Atmospheric pressure (bars) ............................. 1.5
Saturn IV
Hyperion [hi-PEER-ee-en] is one of the smaller moons of Saturn. It has a pock-marked body and is the largest irregularly shaped satellite ever observed. Hyperion might have had a major collision which blew part of the moon away. Its eccentric orbit makes it subject to gravitational forces from Saturn which have set it tumbling out of control. The moon's rotational period is not constant and varies from one orbit to the next. Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus. The largest crater on its surface is approximately 120 kilometers (75 miles) in diameter and 10 kilometers (6.2 miles) deep. The irregular shape of Hyperion and evidence of bombardment by meteors makes it appear to be the oldest surface in the Saturn system.
Discovered by ........................... William Cranch Bond Date of discovery ...................................... 1848 Mass (kg) .......................................... 1.77e+19 Mass (Earth = 1) ................................. 2.9618e-06 Radius (km) ..................................... 205x130x110 Radius (Earth = 1) ............................... 3.2142e-02 Mean density (gm/cm^3) .................................. 1.4 Mean distance from Saturn (km) .................... 1,481,000 Rotational period (days) ............................ chaotic Orbital period (days) .............................. 21.27661 Mean orbital velocity (km/sec) ......................... 5.07 Orbital eccentricity ................................. 0.1042 Orbital inclination .................................... 0.43° Escape velocity (km/sec) .............................. 0.107 Visual geometric albedo ................................. 0.3 Magnitude (Vo) ........................................ 14.19
Saturn VIII
Iapetus [eye-AP-i-tus] is one of the stranger moons of Saturn. Its density is similar to that of Rhea, indicating that it has a small allotment of rocky materials. Its leading side is dark with a slight reddish color while its trailing side is bright. The dark surface might be composed of matter that was either swept up from space or oozed from the moon's interior. The real reason is still unknown. The dark material might be a thin layer of organic material perhaps similar to the complex substances found in the most primitive meteorites. However, there are no bright rimed craters present on the dark hemisphere. If the dark material is thin, it must be constantly renewed since a meteor impact would punch through the layer to reveal brighter surface material.
The dark material may have originated from Phoebe, which has a very low albedo. Micrometeor impacts could kick dark matter off Phoebe which is then swept up by Iapetus. (Phoebe, however, has a slightly different color from that of the dark surface of Iapetus.) The fact that the material is on the leading hemisphere seems to support this theory. On the other hand, the dark material seems to be concentrated in crater floors. This would indicate an internal origin. Since Iapetus is so far from Saturn, it may have formed with methane or ammonia ice in its interior. The dark material could be explained by eruptions of methane from its interior. This theory is supported by a dark ring of material about 100 kilometers (62 miles) in diameter that straddles the border between the leading and trailing hemispheres of Iapetus. Such rings formed on the Moon and on Mars when dark volcanic material flowed into impact craters and filled around the central peak.
Discovered by ..................... Giovanni Domenico Cassini Date of discovery ...................................... 1671 Mass (kg) .......................................... 1.88e+21 Mass (Earth = 1) ................................. 3.1459e-04 Equatorial radius (km) .................................. 730 Equatorial radius (Earth = 1) .................... 1.1446e-01 Mean density (gm/cm^3) ................................. 1.21 Mean distance from Saturn (km) .................... 3,561,300 Rotational period (days) ........................... 79.33018 Orbital period (days) .............................. 79.33018 Mean orbital velocity (km/sec) ......................... 3.27 Orbital eccentricity ................................. 0.0283 Orbital inclination ................................... 14.72° Escape velocity (km/sec) .............................. 0.586 Visual geometric albedo ................................. 0.2 Magnitude (Vo) .................................... 10.2-11.9
Saturn IX
Phoebe [FEE-bee] is the last of the known satellites of Saturn, at a distance of 12.952 million kilometers (8 million miles). Phoebe orbits Saturn in a retrograde direction (opposite to the direction of the other satellites' orbits) in a plane much closer to the ecliptic than to Saturn's equatorial plane. Voyager 2 found that Phoebe has a roughly circular shape, and reflects about 6 percent of the sunlight. It also is quite red. Phoebe rotates on its axis about once every nine hours. Thus, unlike the other Saturnian satellites (except Hyperion), it does not always show the same face to the planet.
Scientists believe that Phoebe may be a captured asteroid with a composition unmodified since the time it was formed in the outer Solar System. It resembles the common class of dark carbonaceous asteroids. These asteroids are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula. Since they are so small, they never heated up sufficiently to change chemical composition. If this is the case, Phoebe is the first such object that has been photographed at close enough range to show shape and surface brightness. Phoebe also might be the source of dark material on Iapetus.
Discovered by ....................... William Henry Pickering Date of discovery ...................................... 1898 Mass (kg) ........................................... 4.0e+18 Mass (Earth = 1) ................................. 6.6934e-07 Equatorial radius (km) .................................. 110 Equatorial radius (Earth = 1) .................... 1.7247e-02 Mean density (gm/cm^3) .................................. 0.7 Mean distance from Saturn (km) ................... 12,952,000 Rotational period (days) ................................ 0.4 Orbital period (days) ............................... -550.48 Mean orbital velocity (km/sec) ........................ -1.71 Orbital eccentricity ................................. 0.1633 Orbital inclination ................................... 175.3° Escape velocity (km/sec) ............................. 0.0697 Visual geometric albedo ................................ 0.06 Magnitude (Vo) ........................................ 16.45