Of the 16 known satellites orbiting Jupiter, all but four are smaller than 250 km diameter, and many are 80 km diameter or less and very irregular in shape. Most of these smaller bodies are believed to be captured asteroids. The four largest satellites, however, collectively known as the Galilean satellites, are almost a planetary system of their own, with two of them being comparable in size to Mercury. Like the Solar System, this satellite system exhibits a distinct change in composition from one end to the other: ice content increases, and density, reflectivity, and signs of geologic activity decrease with distance from Jupiter.Ganymede is the largest satellite in the Solar System. One-third water ice, Ganymede exhibits a geologically complex surface that is quite different from the geology of rocky bodies seen in the inner Solar System. Sections of old, dark, heavily cratered crust have survived as islands among younger, bright, grooved crust. Whether the darkness of the older terrain is due to silicate material being more easily entrained in Ganymede's mostly ice crust early in its history before differentiation occurred or whether it has merely darkened with age is unclear. Heating and softening of the crust early in Ganymede's history has caused the large, old craters to flatten out. The younger craters retain relief and have either bright or dark ejecta, indicating variations in subsurface geology. Lacking both an atmosphere and an adequate energy source to continue driving geologic processes at the surface, Ganymede appears to have ceased being active, though its strong magnetic field may indicate some internal activity.