A subsurface magma ocean on Io: Exploring the steady state of partially molten planetary bodies

TL;DR

This paper investigates Io's subsurface structure, concluding that a high melt fraction layer is likely a magma ocean rather than a magmatic sponge, based on tidal heating constraints and stability analysis.

Contribution

It provides a steady-state model analysis showing that a high melt fraction layer on Io is more consistent with a magma ocean than a magmatic sponge, considering tidal dissipation limits.

Findings

Tidal dissipation is insufficient to sustain a magmatic sponge with high melt fraction.

A high melt fraction layer is likely a subsurface magma ocean.

Magmatic sponge stability is unlikely unless viscosities are very high.

Abstract

Intense tidal heating within Io produces active volcanism on the surface, and its internal structure has long been a subject of debate. A recent reanalysis of the Galileo magnetometer data suggested the presence 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 alter the distribution of tidal heating and would also influence the interpretation of various observations. To this end, we explore the steady state of a magmatic sponge and estimate the amount of internal heating necessary to sustain such a layer with a high degree of melting. Our results show that the rate of tidal dissipation within Io is insufficient to sustain a partial melt layer of $\phi>0.2$ for a wide range of parameters, suggesting that such a layer would swiftly separate into two phases. Unless melt and/or solid viscosities are at the higher end of the estimated range, a magmatic sponge would be unstable, and thus a high melt fraction layer suggested in Khurana et al. (2011) is likely to be a subsurface magma ocean.