Complex sentence: Ganymede, Callisto, and Europa (excluding the more volcanically active Io) all of which are thought to have significant bodies of liquid water beneath their surfaces, making them potentially habitable environments.
Split into simpler sentences: Ganymede, Callisto, and Europa (excluding the more volcanically active Io). It will characterise these three worlds, all of which are thought to have significant bodies of liquid water beneath their surfaces, making them potentially habitable environments.