Io’s tidal heat budget P_glob as a function of melt fraction of the partially molten layer. Configurations for the interior structure involve: a top partially molten layer (case A, red), a bottom partially molten layer (case B, green), and a combination of the two, both layers being of equal thickness (case C, blue). The layers thickness is d_melt = 100 km. Cases involving bulk dissipation correspond to solid curves. Cases where bulk dissipation is not included correspond to dash-dotted curves. The shaded rectangle gives Io’s heat budget range (65–125 TW; see, e.g., Lainey et al. 2009).