Powering the Galilean Satellites with Moon-Moon Tides

There is compelling evidence for subsurface water oceans among the three outer Galilean satellites and evidence for an internal magma ocean in the innermost moon, Io. Tidal forces from Jupiter periodically deform these bodies, causing heating and deformation that, if measured, can probe their interior structures. In addition to Jupiter-raised tides, each moon also raises tides on the others. We investigate moon-moon tides for the first time in the Galilean moons and show that they can cause significant heating through the excitation of high-frequency resonant tidal waves in their subsurface oceans. The heating occurs both in the crust and ocean and can exceed that of other tidal sources and radiogenic decay if the ocean is inviscid enough. The resulting tidal deformation can be used to constrain subsurface ocean thickness. Our understanding of the thermal-orbital evolution and habitability of the Jovian system may be fundamentally altered as a result. Plain Language Summary The three icy Galilean moons, Europa, Ganymede, and Callisto, are thought to contain liquid water oceans beneath their surface, while the innermost moon Io may contain an internal ocean of magma. Jupiter's gravity stretches and squeezes these moons as they orbit the gas giant, heating their interiors through friction. It is essential to understand this process, known as tidal heating, given the unique geophysical structure of ocean worlds and their potential for habitability. In addition to Jupiter, each moon also raises tides on the others, a process that is usually neglected as Jupiter's gravitational attraction is many times larger than that due to the adjacent moons. Here, we show that these moon-moon tides cannot in fact be neglected when considering tides as an energy source because they can excite these subsurface oceans near their natural frequencies. By modeling subsurface tidal currents, we find that the corresponding resonant response of the ocean manifests itself through the generation of fast flowing tidal waves, which can release significant amounts of heat into the oceans and crusts of Io and Europa. Our understanding of how ocean worlds in compact systems evolve over time may be altered by the existence of moon-moon tidal resonances.