Io’s tidal response precludes a shallow magma ocean

Io experiences tidal deformation due to its eccentric orbit around Jupiter, which provides a primary energy source for Io’s ongoing volcanic activity and infrared emission. The amount of tidal energy dissipated within Io is enormous and has been hypothesized to support the large-scale melting of Io’s interior and the formation of a global subsurface magma ocean. Tidal heating is believed to be a primary cause of water and possibly magma oceans both within our solar system and in recently discovered exoplanetary systems. Io is the only object within our solar system to learn about tidal heating with respect to the formation of global magma oceans. 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 k2, enabled by two recent flybys of the Juno spacecraft. By combining Juno and Galileo 5-7 Doppler data from the Deep Space Network and astrometric observations, we recover Re(k2) of 0.125±0.047 (1s) and the tidal dissipation parameter Q of 11.4±3.6 (1s). 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 due to rapid melt ascent, intrusion, and eruption8,9 , so even strong tidal heating – like that expected on several exoplanets and super-Earths10 – may not guarantee the formation of magma oceans on moons or planetary bodies.