Other moons of Saturn

Excluding Titan, the six largest moons range in diameter from 397 km to 1530 km. In order of increasing distance from Saturn, these moons are Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus. These moons have average densities around 1.2 g/cm3, indicating they are made mostly of water ice with some rock. Having formed in a cold environment, these bodies retained water, methane, ammonia and nitrogen that condensed from the solar nebula. Some astronomers believe these mid-sized moons condensed from gas rings that surrounded Saturn about 4.5 billion years ago.

Mimas has a diameter of 397 km and is 185 520 km from Saturn. It takes only 23 hours to orbit Saturn. Mimas was discovered in 1789 by William Herschel. It is difficult to observe from Earth because it is so close to

Figure 10.9 Dione, one of Saturn's moons. (Photo: NASA)

Saturn. The Voyager probe showed that the surface of Mimas is dominated by a large impact crater, called Herschel, which is 130 km across, one-third the diameter of Mimas. The walls of this crater are about 5 km above the surface and parts of its floor are 10 km deep. It has a central peak that rises 6 km above the crater floor. Fractures can be seen on the opposite side of Mimas that may be due to the large impact. Other smaller impact craters also exist on Mimas.

Enceladus has a diameter of 500 km and is 238 000 km from Saturn. Herschel discovered it in 1789. The surface of this moon is covered with a smooth layer of water ice that makes it the most reflective of any known planetary body. There are many craters in one hemisphere and very few in the other hemisphere. The young surface of this moon was seen by Voyager 2 to contain a number of different types of formations, including ice flows, faults and striations. The crust is probably thin and lying on top of a molten interior. Ice volcanoes on Enceladus are a likely source for the particles that create Saturn's outermost E ring.

Fault Line Surface Moon
Figure 10.10 Surface of the moon Enceladus taken by Cassini. The surface is mostly cratered and crossed by numerous fault lines. (Photo: NASA)
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