The Cassini mission provided unprecedented insights into Saturn's largest moon, Titan, from its atmosphere to the deep interior¹. The moon's large measured response to the tides exerted by Saturn was interpreted as evidence of the existence of a subsurface ocean^2,3. This response, twice the value predicted in pre-Cassini studies, has escaped complete explanation. Here we show that the signature of tidal dissipation in Titan's gravity field is not consistent with the presence of an ocean. Our results arise from the detection of this signature through a reanalysis of the radiometric data acquired by Cassini with improved techniques. We found that substantial energy is being dissipated in the interior (approximately 3-4 TW, corresponding to a tidal quality factor Q ≈ 5), consistent with recent studies of Titan's rotational state⁴. Because the presence of a liquid layer reduces the tidal dissipation generated below it⁵, these new measurements preclude the existence of a subsurface ocean on Titan and are explained by a model in which dissipation is concentrated in a high-pressure ice layer close to its melting point. This model also reproduces Titan's observed rotational state and static gravity field self-consistently, reconciling all available geophysical measurements. Efficient ice shell convection can prevent widespread melting and ocean formation, but a slushy high-pressure ice layer is consistent with expectations⁶, indicating that it probably hosts liquid water pockets. The forthcoming Dragonfly mission to Titan will provide a further test of whether a subsurface ocean exists.

Several bodies in the outer solar system are believed to host liquid water oceans underneath their icy surfaces. Knowledge of the hydrosphere properties is essential for understanding and assessing their habitability. We introduce a methodology based on Bayesian inference that enables a robust characterization of the hydrosphere through the combination of gravity and magnetic induction data. The interior models retrieved are consistent with the geophysical observations, leading to probability distributions for the relevant interior properties. We apply this joint inversion approach to constrain Europa's hydrosphere with gravity and magnetic field measurements acquired by the Galileo mission. Our results indicate that the combination of these datasets allows simultaneous constraints on the ice shell and ocean thickness, enhancing our knowledge of the hydrosphere structure. This methodology is valuable for synergistic interior science investigations of several missions in development or in planning, including Europa Clipper, JUICE and the Uranus Orbiter and Probe.

The gravity fields of celestial bodies that possess an atmosphere are periodically perturbed by the redistribution of fluid mass associated with atmospheric dynamics. A component of this perturbation is due to the gravitational response of the body to the deformation of its surface induced by the atmospheric pressure loading. The magnitude of this effect depends on the relation between the loading and the response in terms of geopotential variations measured by the load Love numbers. In this work, we simulate and analyze the gravity field generated by the atmospheres of Venus and Mars by accounting for different models of their internal structure. By precisely characterizing the phenomena that drive the mass transportation in the atmosphere through general circulation models, we determine the effect of the interior structure on the response to the atmospheric loading. An accurate estimation of the time-varying gravity field, which measures the atmospheric contribution, may provide significant constraints on the interior structure through the measurement of the load Love numbers. A combined determination of tidal and load Love numbers would enhance our knowledge of the interior of planetary bodies, providing further geophysical constraints in the inversion of internal structure models.