Another important influence exerted by Jupiter on its moons comes from the giant planet’s magnetic field. Jupiter spins so fast, and contains so much mass, that the magnetic field generated by it engulfs the nearby moons and bathes them with ionization and charged particles. Meanwhile, powerful volcanoes that erupt on the surface of Io eject large amounts of small particles into space; many of them are swept up into Jupiter’s magnetosphere, forming a doughnut-shaped torus of volcanic particles that form an ethereal envelope around the Jovian environment. (This structure is called, appropriately, the Io torus.)

Callisto, the furthest away from Jupiter of the four Galilean moons, is scarred and pitted by ancient craters. Its surface may be the oldest of all the solid bodies in the solar system. There is evidence here, too (albeit weaker than that on Europa and Ganymede), that a magnetic field may exist around Callisto, which could be caused by a salty, liquid ocean far below its surface.