We start by considering the tidal dynamics of subsurface oceans. The investigations of extraterrestrial ocean tides have relied on the equations Pierre-Simon Laplace used to study the tides of Earth's oceans, the so-called Laplace tidal equations. The solutions of the Laplace tidal equations are given by two types of surface waves: surface gravity waves -where gravity acts as the restoring force-and Rossby-Haurwitz waves -where the Coriolis force is the restoring force. Ocean currents and tidal

more »

... dissipation due to tidal forcing are generally negligible unless the ocean resonates at the tidal frequency. Gravitywave resonances can be excited by eccentricity tides, however these resonances occur in oceans much thinner than those predicted for icy moons. In contrast, strong Rossby-Haurwitz waves can be excited in thick oceans by the obliquity tide. Rossby-Haurwitz waves can produce tidal dissipation above that resulting from solid tides provided the satellite has a high obliquity; as it is the case of the Neptunian moon Triton. Previous studies of ocean tides in subsurface oceans have relied on the assumption that the oceans are of constant thickness. However, oceans might be of variable thickness. The most evident example is Enceladus. Gravity and topography data indicates that Enceladus' ocean is not of uniform thickness but varies from ∼ 30 km at the equator to ∼ 50 km at the south pole. We investigate what are the effect of ocean thickness variations on the response of a subsurface ocean (Chapter 3). We show that the occurrence of gravity waves resonances is controlled by the equatorial ocean thickness. Moreover, we find that ocean thickness variation hinders the excitation of Rossby-Haurwitz waves. This is even more relevant for Triton, as it has been suggested that obliquity ocean tides might be the prevalent heating mechanism maintaining its subsurface ocean. Our results show that this only occurs if the moon's ocean is of nearly constant thickness. SUMMARY xiii circulate. Io's 'magma ocean' is another example of two phases (rock and magma) in one layer. In this thesis, new advances have been made on the modeling front; however, several xiv SUMMARY questions remain open. In Chapter 7, we revise the assumptions behind the models we use and underline how future space missions will inform modellers studying the tidal dynamics of moons with fluid layers. New missions to the outer planets like JUICE and Europa Clipper will provide a wealth of new data that will help constraining the bathymetry and composition of subsurface oceans, and thus allow to build more detailed ocean dynamics models than those used in this thesis. Furthermore, these missions might return the first remote measurements of extraterrestrial ocean currents against which the output of extraterrestrial ocean tidal models can be compared. We argue that a future Enceladan mission will bring similar benefits and resolve the outstanding question about how and where energy is dissipated within the moon. A dedicated Io mission would shed new light into the workings of moons experiencing high amounts of tidal heating, and particularly help understanding the unexplored regime that lays in-between solid and liquid tides. In the meantime, the James Webb Space Telescope (JWST) is on its way to L2. The discovery of a super-Io by the JWST would add a new member to the family of tidally active worlds, revealing what occurs in extreme instances of tidal heating, and bringing new insight into the interior of gas giants and the architecture of gas giant systems. We have good reason to believe that the future holds great promise for the topics explored in this thesis. 2 A SPECTRUM OF TIDALLY ACTIVE WORLDS We got our first close look of the outer Solar System moons thanks to Voyager 1 and Voyager 2. The Voyager mission was designed to exploit a very rare opportunity to visit the gas and ice giants in one go, an opportunity that will not take place in the next hundred years. Voyager 1 was put in a course designed to study up close the Jovian moon Io and