Callisto is a large moon orbiting Jupiter (though not quite as big as Ganymede. It is outside Jupiter's radiation belt. The geology of Callisto is unusual, since it consists of an undifferentiated mix of rock and ice from the surface to the core. Since it is extremely unlikely that the ice should not have melted during the formation of Callisto, this implies that the rocks in Callisto are very good at absorbing water. This means that if Callisto is terraformed using extreme greenhouse gases in the outer solar system, the water pressure in the deep oceans would be likely to press much of the water into the mantle (just like much of Earth's water is in its mantle), resulting in a nice world with both land and sea, instead of an ocean world.