2 Moons as Small Worlds There are several ways we could group the moons of our Solar System. Some schemes are based on the sequence of the moons in their orbits around the parent planets: others are based on the sizes or compositions of the moons. For example, we have already noted that a few moons are pre- dominantly rocky objects, some are mostly ices, and most appear to be mixtures of ice and rock. 11a Moons as Small Worl