Enceladus is mostly rock. Or at least it is more rocky than Saturn's other icy satellites. The (tidally melted?) rock is believed to heat up the covering icy layer forming pockets of liquid under the south pole. The water then escapes as the observed geysirs.

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman