 Io's eccentricity persists as the result of its orbital resonances with the Galilean moons Europa and Ganymede.The same mechanism has provided the energy to melt the lower layers of the ice surrounding the rocky mantle of Jupiter's next closest large moon, Europa.However, the heating of the latter is weaker, because of reduced flexing—Europa has half Io's orbital frequency and a 14% smaller radius; also, while Europa's orbit is about twice as eccentric as Io's, tidal force falls off with the cube of distance and is only a quarter as strong at Europa.