One of the moons of Saturn, Enceladus, has cracks and eruptions that couldn't be explained by heat. (It is much too small to have volcanic actiivty.) They think that the cracks might be caused by tidal forces from Saturn's gravity:
In 2005, the Cassini spacecraft flew by Enceladus and saw plumes of material erupting from the south pole of Enceladus. Scientists were surprised to see this because eruptions are powered by heat from an object's interior. Enceladus is tiny compared to most moons, only about 500 kilometers (310 miles) in diameter, so it should have lost its interior heat to the cold of space long ago.
A closer look by Cassini revealed a series of 120-kilometer (75-mile) long cracks in the south polar region of Enceladus, which were nicknamed "Tiger Stripes" because they resembled a tiger's distinctive marks. The stripes are warmer than their surroundings, so scientists believe they are the source of the eruptions. The Cassini observations also show the plumes consist of water vapor, so there is evidence for liquid water under the ice. Since liquid water is necessary to support known forms of life, Enceladus has become a promising place to look for extraterrestrial life.
Enceladus' 1.3-Earth-day orbit around Saturn is slightly elliptical (egg-shaped), so the moon's distance from Saturn changes regularly as it travels in its orbit. When Enceladus is closer to Saturn, the pull of Saturn's gravity is stronger, creating a larger tide; and when Enceladus is farther away, the pull is weaker, creating a smaller tide. Saturn's position in Enceladus' sky also changes slightly, moving the location of the tide on Enceladus' surface from east to west and back again with each orbit. These two effects combine to produce changing stress on the moon's icy surface. The team developed a computer model to calculate how the changing stress affects the Tiger Stripes.
"We found that because of the way the Tiger Stripes are oriented on the surface, when Enceladus is farthest from Saturn, the stresses in the region pull most of them open, and when Enceladus is closest to Saturn, the stresses force most of them to close," said Hurford. "Different stripes open at different times in the orbit. Assuming they erupt as soon as they open, exposing liquid water to the vacuum of space, we can predict which stripes will be erupting at certain times in the orbit. Also, because most of the stripes are open when Enceladus is farthest from Saturn, we expect the eruptive activity to be greatest at this time." (Emphasis mine.)
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