Io's tidal response precludes a shallow magma ocean.

Io experiences tidal deformation as a result of its eccentric orbit around Jupiter, which provides a primary energy source for Io's continuing volcanic activity and infrared emission. The amount of tidal energy dissipated within Io is enormous and has been suggested to support the large-scale melting of its interior and the formation of a global subsurface magma ocean. If Io has a shallow global magma ocean, its tidal deformation would be much larger than in the case of a more rigid, mostly solid interior. Here we report the measurement of Io's tidal deformation, quantified by the gravitational tidal Love number k, enabled by two recent flybys of the Juno spacecraft. By combining Juno and Galileo Doppler data from the NASA Deep Space Network and astrometric observations, we recover Re(k) of 0.125 ± 0.047 (1σ) and the tidal dissipation parameter Q of 11.4 ± 3.6 (1σ). These measurements confirm that a shallow global magma ocean in Io does not exist and are consistent with Io having a mostly solid mantle. Our results indicate that tidal forces do not universally create global magma oceans, which may be prevented from forming owing to rapid melt ascent, intrusion and eruption, so even strong tidal heating-such as that expected on several known exoplanets and super-Earths-may not guarantee the formation of magma oceans on moons or planetary bodies.