This is a diagram of the nine different kinds of ice.
Click on image for full size
Image from: Bridgeman, [1937]

How do we know there is water?

A cube of ice can take on different shapes and sizes, and can even change to a liguid or a gas. Some of the shapes the ice crystal can assume are shown in the diagram to the left. There are nine different kinds of shapes the ice crystal can assume. A basic ice cube from your refrigerator is known as Ice I. (There is a science fiction story about a mad scientist who tried to freeze the earth by turning the ocean into Ice Nine). If it becomes warm enough, the ice can change to water, so the pertinent question is, whether it ever became warm enough in some region of a moon, for there to be a water layer.

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