As hypothesized by some:

[Frictional Heating Explains Plumes on Enceladus

05.16.07

Pasadena, Calif.--Rubbing your hands together on a cold day generates a bit of heat, and the same process of frictional heating may be what powers the geysers jetting out from the surface of Saturn's moon Enceladus.

Tidal forces acting on fault lines in the moon's icy shell cause the sides of the faults to rub back and forth against each other, producing enough heat to transform some of the ice into plumes of water vapor and ice crystals, according to a new study published in the May 17 issue of the journal Nature. ]

http://www.nasa.gov/mission_pages/cassini/media/cassini20070516.html