Juno Constraints on Io’s Interior: Tidal Response and Melt Stability

Jupiter’s moon Io is the most volcanically active body in the Solar System, powered by intense internal heating due to tidal dissipation. Although tidal friction is widely accepted as the main energy source, how this heat is distributed within Io and how it shapes the moon’s internal structure remain open questions. In this study, we use Io’s tidal response, quantified through the degree-2 Love number (k₂)_, to constrain its interior, using recent estimates derived from Juno observations (Park et al., 2025).

We model Io with a three-layer structure consisting of a fluid core, a viscoelastic mantle, and a crust, using an adapted version of the California Planetary Geophysics Code (CPGC). Tidal dissipation is self-consistently coupled to mantle rheology through an Andrade model, with viscosity and shear modulus updated as functions of the local melt fraction. We explore two end-member scenarios that differ in the treatment of the Andrade parameter β: in the first, β is held constant, representing a uniform dissipation regime dominated by deep-mantle heating; in the second, β varies with depth, allowing dissipation to be preferentially localized in the upper mantle. In both scenarios, viscosity and shear modulus evolve with melt fraction.

Our results identify several partially molten mantle configurations whose real part of k₂ is consistent with Juno constraints. In all acceptable models, melt fractions remain below the threshold required to form a global magma layer. To test the physical viability of these states, we compare thermodynamic melt production with the capacity for melt migration. We find that melt transport is efficient enough to prevent long-term melt accumulation, favoring a stable, partially molten “magma sponge” rather than a global magma ocean. These results provide new constraints on Io’s thermal state and are consistent with independent estimates of its global volcanic output.