Io’s tidal heat budget as a function of melt fraction ϕ and the thickness d_melt of the partially molten layer(s). Three configurations are considered for the internal structure (see Fig. 1): with one partially molten layer at the top (case A, left column) or at the bottom of the mantle (case B, middle column), or with partially molten layers both at the top and bottom of the mantle (case C, right column). The color scale refers to the global tidal power P_bulk, including the contribution of bulk dissipation (top panel), and to the ratio (P_bulk − P_nobulk)∕P_nobulk, with P_nobulk designating the reference global tidal power produced with shear dissipation only (bottom panel). The red curves highlight the 100 TW value. The black curve indicates the parameters required to obtain this same value without bulk dissipation. For each configuration the red star denotes a reference case corresponding to a thickness d_melt = 100 km. The white dashed line indicates the value of ϕ_c.