Inferring Io's Internal Properties by Combining Gravity Measurements and Volcanic Activity

Abstract

With its extreme geological activity, Io represents an archetype for tidally heated exoplanets/moons and provides insight into early stages of terrestrial planet evolution. Decades of ground-based observations and multiple space missions have refined our understanding of Io, yet it remains debated where inside Io the tidal heating takes place. Here, we synthesize several independent data sets in a Bayesian approach to provide an updated view of Io's interior. Tidal response data constrain the bulk mechanical properties of the silicate envelope but do not resolve the radial profile. We propose adding constraints based on the observed distribution of volcanic activity, thereby introducing sensitivity to the radial viscosity profile. The combined observations are consistent with the majority of the dissipation occurring in a low-viscosity asthenosphere approximately 200 km thick, with a maximum viscosity of (Formula presented.) Pa s, which is also valid under a more conservative estimate of spatially uniform heat flux. This low viscosity implies either enhanced viscous dissipation under Io's conditions due to poorly understood rheological laws, an asthenosphere near the disaggregation threshold, corresponding to a melt fraction of (Formula presented.), or a combination of both. Together, these results provide a coherent interior structure and thermal state that can be used in future Io studies.