Illustration of Jupiter and two of its largest satellites, Europa (right) and Io (left, smaller). Europa is the smallest of the four Galilean moons of Jupiter, and the second closest to the planet. Its surface is icy and relatively smooth. Impacting meteorites cause melting of the surface, allowing the water to smooth out before re-freezing. Io is closer in, a volcanic world, its interior heated by Jovian tidal forces.