How deep is Io's asthenosphere?

Io's extreme volcanism is only possible if its mantle is partially molten but the extension of the partial melting zone is unknown. Magnetic data support the presence of a subsurface magma layer thicker than 50 km, without constraining its maximum thickness. The concentration of volcanoes near the sub- and anti-Jovian points is often interpreted as evidence for a thin asthenosphere (say 100 km thick) since such a model dissipates more in the equatorial zone. Tidal dissipation in the whole mantle instead leads to more surface heating at the poles. The asthenospheric model, however, also predicts that the distribution of volcanoes should have a significant component of harmonic degree four, which is not found in the data. Possible solutions to this problem are averaging by convection and influence of lithosphere on heat piping. We propose another explanation by showing that the degree-four component is greatly reduced if the asthenosphere is several hundreds of kilometers thick. The observed distribution of volcanoes thus favors a deep asthenosphere, at least if the inferred heat flux pattern is primarily determined by Io's sublithospheric structure.