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.Europa is only slightly less dense than Io at 3.0 g/cm3, but could hardly be more different. Its surface is composed almost entirely of water ice (so far, no water has been detected at the surface of Io). Extremely flat, with relief usually no more than 100 m (mountains up to about 1,500 meters are very rare), the surface is scarred by a complex crack system that looks similar to that of polar sea ice on Earth, leading to speculation that an ocean lies beneath the ice crust, estimated to be about 20 km thick. Very few impact craters exist on Europa, indicating that some process must be acting to obliterate the craters that inevitably form on planetary surfaces. This further strengthens the theory that an ocean still exists below the crust. A different set of cracks a few kilometers wide and up to 1,000 km long criss-cross the surface. Reddish material flanking these cracks may be deposits from eruptions that sprayed out muddy or salty water (solar radiation or cosmic rays would darken the salts). Like Io, the energy to drive the resurfacing (and perhaps to keep an ocean from totally freezing at the frigid -150 oC temperatures there) is probably from tidal effects of Jupiter and the other satellites.