Even though it is smaller, Titan is close in size to Ganymede. It is also close in size to the slightly smaller Callisto, another of Jupiter's moons. Not only is Titan a big moon, it is even bigger than the planet Mercury, but it only has half as much mass (it is much lighter). Because Titan does not have a lot of mass, scientist think that Titan is made of matter that is not very heavy, specifically frozen water and ammonia. Some scientists think that there is a lot of liquid water and ammonia underneath the surface, enough to fill an entire ocean. These scientists think that there might be a form of life inside this ocean.