Final answer:
Jupiter's moons are mainly composed of a mixture of rock and ice. The three largest Jovian moons are roughly half water ice and half rocky materials, differing from inner solar system bodies which are predominantly rocky and metallic.
Step-by-step explanation:
The moons of Jupiter are not all captured asteroids, nor are they all made of rock, all captured comets, or all made of ice. The correct characterization of Jupiter's moons is that they are made of a mixture of rock and ice. This is evidenced by the three largest moons in the Jovian system, Ganymede and Callisto, as well as Titan in the Saturnian system, which are approximately half frozen water and half rocks and metals. Most of these moons have differentiated, which means they have developed internal layers, typically having rocky or metallic cores and icy crusts.
In contrast, the terrestrial Moon is chemically and structurally similar to the terrestrial planets and lacks the ice component that is significant in the composition of the outer moons. Additionally, while Mars' moons Phobos and Deimos resemble captured asteroids, this is not necessarily the standard for all moons. In the colder outer solar system, where temperatures are lower, ice has survived without evaporating, which is why objects further from the Sun, like many of Jupiter's moons, are composed of both ice and rock.