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The cratered surface of Saturn's moon Mimas
The cratered surface of Saturn's moon Mimas

Mimas, Enceladus, Tethys, Dione, and Rhea are approximately spherical in shape and appear to be composed mostly of water ice. Enceladus reflects almost 100 percent of the sunlight that strikes it. All five satellites represent a size range that had not been explored before.

Mimas, Tethys, Dione, and Rhea are all cratered; Enceladus appears to have by far the most active surface of any satellite in the system (with the possible exception of Titan, whose surface was not photographed). At least five types of terrain have been identified on Enceladus. Although craters can be seen across portions of its surface, the lack of craters in other areas implies an age less than a few hundred million years for the youngest regions. It seems likely that parts of the surface are still undergoing change, since some areas are covered by ridged plains with no evidence of cratering down to the limit of resolution of Voyager 2's cameras (2 kilometers or 1.2 miles). A pattern of linear faults crisscrosses other areas. It is not likely that a satellite as small as Enceladus could have enough radioactive material to produce the modification. A more likely source of heating appears to be tidal interaction with Saturn, caused by perturbations in Enceladus' orbit by Dione (like Jupiter's satellite Io). Theories of tidal heating do not predict generation of enough energy to explain all the heating that must have occurred. Because it reflects so much sunlight, Enceladus' current surface temperature is only 72 Kelvins (-330 degrees Fahrenheit).

Photos of Mimas show a huge impact crater. The crater, named Herschel, 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.

Photos of Tethys taken by Voyager 2 show an even larger impact crater, named Odysseus, nearly one-third the diameter of Tethys and larger than Mimas. In contrast to Mimas' Herschel, the floor of Odysseus returned to about the original shape of the surface, most likely a result of Tethys' larger gravity and the relative fluidity of water ice. A gigantic fracture 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, although the expansion of theinterior due to freezing would not be expected to cause only one large crack. The canyon has been named Ithaca Chasma. Tethys' surface temperature is 86 Kelvins (-305 degrees Fahrenheit).

Hyperion shows no evidence of internal activity. Its irregular shape causes an unusual phenomenon: Each time Hyperion passes Titan, the larger satellite's gravity gives Hyperion a tug and it tumbles erratically, changing orientation. The irregular shape of Hyperion and evidence of bombardment by meteors make it appear to be the oldest surface in the Saturn system.

Iapetus has long been known to have large differences in surface brightness. Brightness of the surface material on the trailing side has been measured at 50 percent, while material on the leading side reflects only 5 percent of the sunlight. Most dark material is distributed in a pattern directly centered on the leading surface, causing conjecture that dark material in orbit around Saturn was swept up by Iapetus. The trailing face of Iapetus, however, has craters with dark floors. That implies that the dark material originated in the satellite's interior. It is possible that the dark material on the leading hemisphere was exposed by ablation (erosion) of a thin, overlying, bright surface covering.

Voyager 2 photographed Phoebe after passing Saturn. 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 in nine hours. Thus, unlike the other Saturnian satellites (except Hyperion), it does not always show the same face to the planet. If, as scientists believe, Phoebe is a captured asteroid with its composition unmodified since its formation in the outer solar system, it is the first such object that has been photographed at close enough range to show shape and surface brightness.

Both Dione and Rhea have bright, wispy streaks that stand out against an already-bright surface. The streaks are probably the results of ice that evolved from the interior along fractures in the crust.

 

 
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