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# Saturn's Major Moons Help (page 3)

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## Tethys

Tethys has a diameter of 1,060 km (660 mi) and orbits Saturn at a distance of 290,000 km (180,000 mi). Like Dione and Rhea, this satellite is believed to consist mainly of water ice with some rocky material mixed in.

Tethys is noted for its long surface canyon and for a crater that is gigantic compared with the size of the moon itself. The size of this crater and the presence of the fracture suggest that a large asteroid smashed into Tethys and nearly broke the moon in two. Gravity, however, pulled the object back together again. According to one theory, Tethys was liquid at one time, and if this was the case when the violent impact took place, it might have saved the moon from being pulverized.

Like Dione, Tethys is involved in an orbital resonance with one of Saturn’s minor moons, Mimas . Mimas orbits the planet twice for every orbit of Tethys. There are also two tiny moons that orbit in exactly the same path around Saturn as Tethys but 60 degrees of arc (one-sixth of a circle) ahead and behind it. This is a common phenomenon for major satellites of planets and stars that have nearly circular orbits and is a result of gravitational interaction between the moon and its parent planet or between the planet and its parent star. The points 60 degrees ahead and behind an object in a nearly circular orbit are known as the Lagrange points .

Figure 10-4 shows the five major satellites of Saturn, along with the Earth and the curvature of Saturn itself for size comparison.

Figure 10-4. The five largest satellites of Saturn, along with Earth, as they would appear if placed in close proximity. They are all tiny compared with Saturn (gray curve).

Practice problems of this concept can be found at: Major Moons of Outer Planets Practice Problems

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