Hobbies And Interests
Ganymede
With a diameter of 3,269 miles (5,261 km), Ganymede is the largest moon in the solar system. It is larger than the planet Mercury and so far is the only moon known to have its own magnetic field. Like the Earth, it has a layered structure with an iron core, a rocky mantle and an outer surface made of water ice. Its surface appears very old and is covered with craters.
Callisto
Callisto orbits Jupiter further away than the other three Galilean moons. It ranks second in size after Ganymede, and third-largest in the solar system after Saturn's moon, Titan. Like Ganymede, it has a heavily cratered, icy surface. Underneath Callisto's hard ice crust may lie a salty ocean.
Io
The surface of Io is covered with volcanoes. Its red and orange colors come from sulfur sprayed onto the surface from volcanic eruptions. Tidal forces from its orbit around Jupiter produce tremendous heat inside Io, making it one of the most geologically active bodies in the solar system. Because Io's surface is constantly being remade from eruptions, any meteor impact craters soon become covered over with fresh volcanic material.
Europa
Like the other Galilean moons, Europa is made of layers. Its outermost layer is a 60-mile (100 km) deep ocean, the outermost few miles of which is frozen solid. Tidal forces from Europa's orbit have left this surface with a fractured appearance. Scientists think that the same tidal forces can produce enough heat to keep the water under the ice liquid. If this is true, it may have twice as much water as Earth has in its oceans, and may even have zones inhabited by alien forms of life.