The possibility of a subsurface magma ocean on Io

Io experiences intense tidal heating and has active silicate volcanism, which has led to a debate on its internal structure. The recent reanalysis of the Galileo magnetometer data by Kuhrana et al. (2011) suggested the existence of a high melt fraction layer with >50 km thickness in the subsurface region of Io. Whether this layer is a "magmatic sponge" with interconnected solid or a rheologically liquid "magma ocean" would influence the interpretation of various observations. To this end, we estimated the amount of internal heating necessary to sustain such a layer with a high degree of melting. Our results show that the predicted rate of tidal dissipation within Io (~100-200 TW) is insufficient to sustain a partial melt layer that is consistent with the magnetometer data for a wide range of parameters. This indicates that a high-melt fraction layer would swiftly separate into two phases, resulting in a magma ocean. Unless melt and/or solid viscosities are in the higher end of the plausible range, a magmatic sponge would be unstable, and thus a high melt fraction layer suggested in Khurana et al. is likely to be a subsurface magma ocean.