shortly before the "voyager 1" encounter, stan peale, patrick cassen, and r. t. reynolds published a paper in the journal "science" predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. they based this prediction on models of io's interior that took into account the massive amount of heat produced by the varying tidal pull of jupiter on io caused by its slightly eccentric orbit. their calculations suggested that the amount of heat generated for an io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. this effect would be even greater with a differentiated io.