Strong ocean tidal flow and heating on moons of the outer planets

Moons under water There is mounting evidence for liquid oceans on several moons of the outer Solar System planets. How these oceans manage to remain liquid is something of a mystery, considering the low surface temperatures and meagre radiogenic heat sources on these moons. Taking Jupiter's moon Europa as a model, Robert Tyler calculates that a previously unconsidered tidal force caused by obliquity, the axial tilt of the moon with respect to its orbital plane, could be inducing waves within the ocean itself. Such waves could be the dominant source of ocean heat. There is support for the idea that there are liquid oceans on several moons of the outer planets, with Jupiter's moon Europa having received the most attention. But it is unclear how these oceans remain liquid. This paper describes strong tidal dissipation (and heating) in the liquid oceans of such moons, and shows that a previously unconsidered tidal force due to obliquity has the right form and frequency to resonantly excite large-amplitude Rossby waves. Data from recent space missions have added strong support for the idea that there are liquid oceans on several moons of the outer planets, with Jupiter's moon Europa having received the most attention 1 , 2 , 3 , 4 . But given the extremely cold surface temperatures and meagre radiogenic heat sources of these moons, it is still unclear how these oceans remain liquid. The prevailing conjecture is that these oceans are heated by tidal forces that flex the solid moon (rock plus ice) during its eccentric orbit, and that this heat entering the ocean does not rapidly escape because of the insulating layer of ice over the ocean surface. Here, however, I describe strong tidal dissipation (and heating) in the liquid oceans; I show that a subdominant and previously unconsidered tidal force due to obliquity (axial tilt of the moon with respect to its orbital plane) has the right form and frequency to resonantly excite large-amplitude Rossby waves in these oceans. In the specific case of Europa, the minimum kinetic energy of the flow associated with this resonance (7.3 × 10 18 J) is two thousand times larger than that of the flow excited by the dominant tidal forces, and dissipation of this energy seems large enough to be a primary ocean heat source.