Io’s tidally driven global volcanism indicates widespread partial melting in its mantle. How this melt participates in the interior dynamics and, in particular, the role it plays in tidal dissipation, is poorly understood. We model Io’s tidal deformation by treating its mantle as a two-phase (solid and melt) system. By combining poro-viscous and poro-elastic compaction theories in a Maxwell framework with a consistent model of tidal and self-gravitation, we produce the first self-consistent evaluation of Io’s tidal heating rate due to shearing, compaction and Darcy flow. We find that Darcy dissipation can potentially exceed shear heating, but only for large (0.05–0.2) melt fractions, and if the grain size is large or melt viscosity ultra-low. Since grain sizes larger than 1 cm are unlikely, this suggests that Darcy dissipation is secondary to shear dissipation. Compaction dissipation is maximized when the asthenosphere is highly resistive to isotropic stresses, but contributes at most 1% of Io’s observed heating rate. This work represents a crucial step toward a self-consistent quantitative theory for the dynamics of Io’s partially molten interior.

The inferred density of Enceladus' core, together with evidence of hydrothermal activity within the moon, suggests that the core is porous. Tidal dissipation in an unconsolidated core has been proposed as the main source of Enceladus' geological activity. However, the tidal response of its core has generally been modeled assuming it behaves viscoelastically rather than poroviscoelastically. In this work, we analyze the poroviscoelastic response to better constrain the distribution of tidal dissipation within Enceladus. A poroviscoelastic body has a different tidal response than a viscoelastic one; pressure within the pores alters the stress field and induces a Darcian porous flow. This flow represents an additional pathway for energy dissipation. Using Biot's theory of poroviscoelasticity, we develop a new framework to obtain the tidal response of a spherically symmetric, self-gravitating moon with porous layers and apply it to Enceladus. We show that the boundary conditions at the interface of the core and overlying ocean play a key role in the tidal response. The ocean hinders the development of a large-amplitude Darcian flow, making negligible the Darcian contribution to the dissipation budget. We therefore infer that Enceladus' core can be the source of its geological activity only if it has a low rigidity and a very low viscosity. A future mission to Enceladus could test this hypothesis by measuring the phase lags of tidally induced changes of gravitational potential and surface displacements.