the tidal heating of io is dependent on its distance from jupiter, its orbital eccentricity, the composition of its interior, and its physical state. its laplace orbital resonance with europa and ganymede maintains io's eccentricity and prevents tidal dissipation within io from circularizing its orbit. the eccentricity leads to vertical differences in io's tidal bulge of as much as as jupiter's gravitational pull varies between the periapsis and apoapsis points in io's orbit. this varying tidal pull also produces friction in io's interior, enough to cause significant tidal heating and melting. unlike earth, where most of its internal heat is released by conduction through the crust, on io internal heat is released via volcanic activity and generates the satellite's high heat flow (global total: 0.6–1.6 w). models of its orbit suggest that the amount of tidal heating within io changes with time, and that the current heat flow is not representative of the long-term average. the observed release of heat from io's interior is greater than estimates for the amount presently generated from tidal heating, suggesting that io is cooling after a period of greater flexing.