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.Callisto may be the Solar System's most heavily cratered object, attesting to its billions of years of geologic inactivity. Its most prominent features are impact in origin and are no-table only for their size and the concentric ring fractures that formed when the crust around them was fractured by adjustment to the huge impact. The rings extend out to 2,000 km from the centers of the impact sites. The central regions of the impact sites are bright, perhaps from "cleaner" ice that was excavated by the impacts, or flowed out from impact-generated melting of the subsurface. Small craters are extremely numerous, and their only characteristic is that they have thrown out brighter ice, likewise indicating that the surface has been coated with dark material that may have accumulated from billions of years of meteorite bombardment.