Saturn's Major Moons Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 16, 2011


Iapetus orbits at a great distance from Saturn: 3.6 million km (2.2 million mi). Its diameter is about 1,450 km (900 mi). Like Rhea, Iapetus is only a little more dense than water, and analysis of light reflected from its surface indicates that this moon is made up mostly of water ice.

The leading side of Iapetus is much darker than the trailing side. This is the opposite of the situation with Rhea. The contrast is great; the leading side is nearly as white as snow, whereas the trailing side is nearly as dark as tar. Also in contrast with Rhea, most of the craters on Iapetus are on the trailing side. This has caused some befuddlement among astronomers. Did something stain the leading side of Iapetus and cover up the craters there? Did this “dye” come from inside Iapetus, or did it come from space? Or is it the result of some reaction of material on the surface with ultraviolet light or high-speed particles from the Sun?

Iapetus is the only major moon of Saturn that does not orbit almost exactly in the plane of Saturn’s equator. Instead, Iapetus is inclined by 15 degrees. One theory concerning this inclination is that Iapetus did not form along with the Saturnian system but instead was once a huge wandering protocomet or planetoid that was captured by Saturn’s gravitation. Another theory holds that Iapetus originally orbited in the plane of Saturn’s equator but was knocked out of kilter by a large asteroid.


Dione has a diameter of 1,120 km (690 mi) and orbits Saturn in an almost perfect circle at a distance of 377,000 km (234,000 mi). Dione’s density is about 1.4 times that of water. This fact and the analysis of the light reflected from its surface indicate that Dione, like most of the other moons of Saturn, is made up largely of water ice. It is thought that the proportion of ice to rock is higher near the surface and lower near the core.

There are variations in the reflectivity of the surface of this moon, but the demarcation is not as great as is the case with Iapetus. The leading side is generally brighter than the darker side. The trailing side has wispy markings that suggest that volatile material, perhaps water vapor, has escaped from the interior and fallen back on the surface to freeze. Some areas of Dione are heavily cratered, whereas other regions contain virtually no craters.

Dione exhibits a property that is sometimes found in the satellite systems of large planets: orbital resonance with one of the other moons. In this case the other moon is Enceladus , one of the minor satellites of Saturn. While Dione takes 66 Earth hours to orbit once around Saturn, Enceladus takes 33 hours, exactly half that time. Orbital resonance effects are caused by mutual gravitation between celestial objects, such as moons, when they both orbit around a common, larger object, such as a planet. This resonance effect is believed to be responsible for tidal forces on Enceladus that cause it to generate heat from inside.

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