|MANY MOONS, MANY WORLDS|

The two moons that Galileo thought he had seen, weren't there: instead, Saturn has many others. When the Voyager probes approached the planet, ten had been discovered: the first and largest one by the great Dutch astronomer Huygens, in 1655; the last, with a diameter of only 350 kilometers, was identified on December 17, 1966 by the French astronomer A. Dollfuss at the Pic du Midi observatory, in the Pyrenees. The Voyager probes added seven more satellites to this group: these are generally rather small objects, which are identified with a code. The largest, 1980S27, orbits on the inner edge of ring F and has a diameter of 220 kilometers; the smallest, 1980S28, with a diameter of only 30 kilometers, is the closest satellite to the surface of the planet discovered to this point. But the greatest interest was generated by the suface details that the two probes were able to obtain for most of the known satellites: Phoebe, Iapetus, Hyperion, Titan, Rhea, Dione, Tethys, Enceladus and Mimas. In general, the Voyager probes discovered that the period of rotation of all of Saturn's moons, except for Phoebe and perhaps Hyperion, is the same as that of the revolution - they always show the same side to the planet, just like the Moon. In addition, all of them, except Iapetus and Phoebe, have almost circular orbits in Saturn's equatorial plane. None, except Titan, the largest moon, has any appreciable atmosphere. But let's see how these worlds appeared to the eyes of the Voyager probes, starting with those in the outer part of the system. We don't have many details about the surface of Phoebe, Saturn's farthest moon, because it was photographed at a distance of no less than 2.2 million kilometers. We do know however that it is almost spherical, very dark and has a very inclined orbit that moves in the opposite direction to the other moons. Because of these characteristics, astronomers believe that this moon is an asteroid that was first captured by Jupiter and then stripped away by the gravitational field of Saturn. It has a diameter of 200 kilometers. Second in terms of size along with Rhea (with a diameter of about 1 500 kilometers) after Titan, Iapetus has a density of 1.1, which is very close to that of ice water. One of its main features is a bright hemisphere, the one facing the opposite direction of its motion in orbit, while the other hemisphere is much darker. One of the hypotheses developed by astronomers to explain this strange phenomenon states that the front hemisphere is covered by dark materials coming from Phoebe: in other words particles of matter were lifted and continue to be lifted from the surface of Phoebe, after each impact with micrometeorites which, attracted into Iapetus' orbit, are "swept" and collected by its front hemisphere. However, many researchers do not agree with this theory because the color of Phoebe seems to be different from that of the dark hemisphere of Iapetus. Another explanation, deduced by the presence of a ring of dark material, with a diameter of 100 kilometers, overlapping the two hemispheres, states that the flow of volcanic material has created the different colors of the two "faces". But this is a very special material: perhaps a type of fluid sludge consisting of ice, ammonia and some other still unknown dark substance. Hyperion is one of the moons which still baffles researchers. In fact, Voyager 2 showed that it is 400 kilometers long and slightly more than 200 wide - one of the largest bodies in the solar system with an irregular shape. Its axis is inclined by 45 degree with respect to Saturn. Instead, astronomical laws dictate that the longest axis of this satellite should be aligned with the center of Saturn. The strange behavior of this moon (that someone said resembles a hamburger) was probably caused by a collison with another celestial body, an impact that astronomers believe occurred in the last one hundred million years. Like Iapetus, Rhea has two hemispheres with different characteristics: the front hemisphere is brighter and uniform and there seems to be a rather large impact crater, of recent origin, with a width of one hundred kilometers. Instead, the rear hemisphere has a pattern of luminous strips, which stand out on a background that is darker than the front hemisphere, probably produced by the internal activity of the satellite. The entire surface is covered with craters of all sizes which make it resemble to the Moon and Mercury. According to astronomers, this would also mean that most of the formations would date back to the turbulent beginnings of the history of the solar system. Closer than Rhea to the planet, Dione has a diameter of 1 100 kilometers and a density of 1.5, the highest among all of Saturn's moons, except for Titan. This moon has a large number of craters and long criss-crossing streaks - evidence of its young age. Again in this case, the satellite underwent an intense period of cosmic bombardment followed by a period in which the surface was partially remodeled by the materials ejected from its interior, creating a vast plain of more recent craters. The main features of Tethys, whose diameter is almost identical to that of Dione but whose density is closer to ice water, include a shallow crater with an enormous diameter of 400 kilometers and a gigantic ramified canyon, with a width of one hundred kilometers and a depth of ten, which extends like a wound for at least one thousand kilometers in a north-south direction. The two structures provide clues to help reconstruct the history of this moon: in the past, Tethys was probably liquid and, when the meteorite collided with the surface, the crater produced did not maintain its shape for very long but instead "collapsed", smoothing out and adapting to the curvature of the satellite. Instead, the canyon probably was created in a later period and might be a type of "stretching" of the crust while the satellite cooled and solidified into ice. The last two moons, Mimas and Enceladus, with a diameter of about 500 kilometers, are the smallest among those known prior to our century. Mimas has a single, large impact crater with a diameter of 130 kilometers, almost a third of its own diameter, with very pronounced edges and a central peak, along with a myriad of smaller craters. On the other hand, Enceladus is much more uniform: it has almost no craters and is covered by plains containing a network of intersecting projections and wrinkles. Scientists are very interested in this unusual appearance. After analyzing the six types of ground that form the satellite, thanks to the pictures from the Voyager probes, they believe that it is one of the very few bodies in the solar system that still has a considerable amount of internal activity.
The two moons that Galileo thought he had seen, weren't there: instead, Saturn has many others. When the Voyager probes approached the planet, ten had been discovered: the first and largest one by the great Dutch astronomer Huygens, in 1655; the last, with a diameter of only 350 kilometers, was identified on December 17, 1966 by the French astronomer A. Dollfuss at the Pic du Midi observatory, in the Pyrenees. The Voyager probes added seven more satellites to this group: these are generally rather small objects, which are identified with a code. The largest, 1980S27, orbits on the inner edge of ring F and has a diameter of 220 kilometers; the smallest, 1980S28, with a diameter of only 30 kilometers, is the closest satellite to the surface of the planet discovered to this point. But the greatest interest was generated by the suface details that the two probes were able to obtain for most of the known satellites: Phoebe, Iapetus, Hyperion, Titan, Rhea, Dione, Tethys, Enceladus and Mimas. In general, the Voyager probes discovered that the period of rotation of all of Saturn's moons, except for Phoebe and perhaps Hyperion, is the same as that of the revolution - they always show the same side to the planet, just like the Moon. In addition, all of them, except Iapetus and Phoebe, have almost circular orbits in Saturn's equatorial plane. None, except Titan, the largest moon, has any appreciable atmosphere. But let's see how these worlds appeared to the eyes of the Voyager probes, starting with those in the outer part of the system. We don't have many details about the surface of Phoebe, Saturn's farthest moon, because it was photographed at a distance of no less than 2.2 million kilometers. We do know however that it is almost spherical, very dark and has a very inclined orbit that moves in the opposite direction to the other moons. Because of these characteristics, astronomers believe that this moon is an asteroid that was first captured by Jupiter and then stripped away by the gravitational field of Saturn. It has a diameter of 200 kilometers. Second in terms of size along with Rhea (with a diameter of about 1 500 kilometers) after Titan, Iapetus has a density of 1.1, which is very close to that of ice water. One of its main features is a bright hemisphere, the one facing the opposite direction of its motion in orbit, while the other hemisphere is much darker. One of the hypotheses developed by astronomers to explain this strange phenomenon states that the front hemisphere is covered by dark materials coming from Phoebe: in other words particles of matter were lifted and continue to be lifted from the surface of Phoebe, after each impact with micrometeorites which, attracted into Iapetus' orbit, are "swept" and collected by its front hemisphere. However, many researchers do not agree with this theory because the color of Phoebe seems to be different from that of the dark hemisphere of Iapetus. Another explanation, deduced by the presence of a ring of dark material, with a diameter of 100 kilometers, overlapping the two hemispheres, states that the flow of volcanic material has created the different colors of the two "faces". But this is a very special material: perhaps a type of fluid sludge consisting of ice, ammonia and some other still unknown dark substance. Hyperion is one of the moons which still baffles researchers. In fact, Voyager 2 showed that it is 400 kilometers long and slightly more than 200 wide - one of the largest bodies in the solar system with an irregular shape. Its axis is inclined by 45 degree with respect to Saturn. Instead, astronomical laws dictate that the longest axis of this satellite should be aligned with the center of Saturn. The strange behavior of this moon (that someone said resembles a hamburger) was probably caused by a collison with another celestial body, an impact that astronomers believe occurred in the last one hundred million years. Like Iapetus, Rhea has two hemispheres with different characteristics: the front hemisphere is brighter and uniform and there seems to be a rather large impact crater, of recent origin, with a width of one hundred kilometers. Instead, the rear hemisphere has a pattern of luminous strips, which stand out on a background that is darker than the front hemisphere, probably produced by the internal activity of the satellite. The entire surface is covered with craters of all sizes which make it resemble to the Moon and Mercury. According to astronomers, this would also mean that most of the formations would date back to the turbulent beginnings of the history of the solar system. Closer than Rhea to the planet, Dione has a diameter of 1 100 kilometers and a density of 1.5, the highest among all of Saturn's moons, except for Titan. This moon has a large number of craters and long criss-crossing streaks - evidence of its young age. Again in this case, the satellite underwent an intense period of cosmic bombardment followed by a period in which the surface was partially remodeled by the materials ejected from its interior, creating a vast plain of more recent craters. The main features of Tethys, whose diameter is almost identical to that of Dione but whose density is closer to ice water, include a shallow crater with an enormous diameter of 400 kilometers and a gigantic ramified canyon, with a width of one hundred kilometers and a depth of ten, which extends like a wound for at least one thousand kilometers in a north-south direction. The two structures provide clues to help reconstruct the history of this moon: in the past, Tethys was probably liquid and, when the meteorite collided with the surface, the crater produced did not maintain its shape for very long but instead "collapsed", smoothing out and adapting to the curvature of the satellite. Instead, the canyon probably was created in a later period and might be a type of "stretching" of the crust while the satellite cooled and solidified into ice. The last two moons, Mimas and Enceladus, with a diameter of about 500 kilometers, are the smallest among those known prior to our century. Mimas has a single, large impact crater with a diameter of 130 kilometers, almost a third of its own diameter, with very pronounced edges and a central peak, along with a myriad of smaller craters. On the other hand, Enceladus is much more uniform: it has almost no craters and is covered by plains containing a network of intersecting projections and wrinkles. Scientists are very interested in this unusual appearance. After analyzing the six types of ground that form the satellite, thanks to the pictures from the Voyager probes, they believe that it is one of the very few bodies in the solar system that still has a considerable amount of internal activity.