first predicted shortly before the "voyager 1" flyby, the heat source for io's volcanism comes from tidal heating produced by its forced orbital eccentricity. this differs from earth's internal heating, which is derived primarily from radioactive isotope decay and primordial heat of accretion. io's eccentric orbit leads to a slight difference in jupiter's gravitational pull on the satellite between its closest and farthest points on its orbit, causing a varying tidal bulge. this variation in the shape of io causes frictional heating in its interior. without this tidal heating, io might have been similar to the moon, a world of similar size and mass, geologically dead and covered with numerous impact craters.