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 emission¹. 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 interior². Here we report the measurement of Io's tidal deformation, quantified by the gravitational tidal Love number k₂, enabled by two recent flybys of the Juno spacecraft. By combining Juno^3,4 and Galileo^5-7 Doppler data from the NASA Deep Space Network and astrometric observations, we recover Re(k₂) 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 mantle². 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^8,9, so even strong tidal heating-such as that expected on several known exoplanets and super-Earths¹⁰-may not guarantee the formation of magma oceans on moons or planetary bodies.