The outside surface is ice. [Evidence: look at the impact craters.] Under that it appears [see pictures ] that there was once water that flowed up in a kind of plate tectonics. Maybe there is still a water layer.
One hemisphere of Ganymede. The bright spots are relatively recent impact craters. There are dark regions separated by light regions. We can interpret the dark regions as the most ancient, covered with lots of meteoritic dust. The light regions are more recent.
It appears that the dark regions were pushed by water flows in the mantle underneath, bringing new water to the surface along cracks. Then the new water froze.
The older dark region. The big dark region. Note the impact craters in various stages of degradation. It appears that the ice flows slowly to return to a flat surface under the pull of gravity.
Impact crater. The fresh ice is lighter colored. Note again the disappearing craters.
