Titan’s Strong Tidal Dissipation Precludes a Subsurface Ocean

Introduction: The Cassini spacecraft orbited Saturn from 2004 to 2017, performing over a hundred flybys of Titan, Saturn’s largest moon. Ten of these flybys were dedicated to gravity measurements, yielding unprecedented insight into the moon’s interior. The first four flybys revealed a weakly differentiated deep interior, consisting of a large (~2000 km, Titan’s radius is 2575 km) and low-density (~2600 kg/m3) rocky core and a ~600 km thick hydrosphere (Iess et al., 2010). Subsequent additional radio tracking data allowed the first measurement of Titan’s response to the gravitational tides exerted by Saturn, quantified by the real part of the complex tidal Love number, Re(k₂) (Iess et al., 2012; Durante et al., 2019). The inferred large value (Re(k₂) ~0.6) was 2-3 times larger than pre-Cassini predictions (Rappaport et al., 2008) and indicated strong deformability over the tidal timescale. This finding was interpreted as evidence of the existence of a global subsurface ocean beneath Titan’s ice shell but escaped complete explanation. A large Re(k₂) can be also generated by a viscoelastic and oceanless interior (Rappaport et al., 2008). This configuration would also produce strong tidal dissipation through shear friction, which is quantified by the imaginary part Im(k₂). The detection of Im(k₂) was thus indicated as a criterion to break the degeneracy between models with and without a subsurface ocean (Rappaport et al., 2008), but earlier analyses of Cassini radio tracking data could not measure the Im(k₂) contribution to Titan’s gravity field.

A recent analysis derived the imaginary part of k₂ from Titan’s rotation state as observed by Cassini’s RADAR images, revealing a large value of Im(k₂) = 0.120 ± 0.027 (Downey and Nimmo, 2025). This value corresponds to a low tidal quality factor Q ~ 5 (the Q of solid Earth is ~300), indicating strong tidal dissipation in Titan’s interior.

We reanalyzed Cassini radio tracking data with improved techniques, including processing of open loop data and phase compression, to improve the assessment of Titan’s gravity field and tidal response and to confirm the recent observation of Im(k₂) (Petricca et al. 2025).

Figure 1: Posterior distributions for Titan’s tidal Love number k2 compared to the observations for models with and without a subsurface ocean.

Results: We succeeded in measuring Titan’s gravity field and tidal response with reduced uncertainties compared to previous studies. The measured Re(k₂) = 0.608 ± 0.048 confirmed the earlier value. The improved precision allowed us to detect for the first time the contribution of tidal dissipation to Titan’s gravity field, resulting in Im(k₂) = 0.135 ± 0.035, consistent with results derived from Titan’s rotation (Downey and Nimmo, 2025). Because the presence of an ocean reduces the tidal dissipation generated below it, these new measurements indicate the absence of a global ocean inside Titan (Figure 1). Instead, the observations are explained by a model in which the dissipation is concentrated in high-pressure ice layer that is close to its melting point globally, as inferred from our inversion of the measurements, and is thus “slushy” (Figure 2). In addition to explaining the tidal response, the oceanless model that we introduce is the first model of Titan’s interior that can also reproduce Titan’s static gravity field and obliquity, reconciling all the geophysical observations acquired by Cassini, while requiring a geologically recent event as the source of Titan’s orbital eccentricity.

The presence of a slushy layer instead of a global ocean might have profound implications for Titan’s astrobiological potential. The absence of a global ocean in Titan, despite strong tidal heating, suggests that ocean worlds may be less common than has been supposed in recent years. Although a global ocean has been considered ideal for supporting habitability, the presence of slushy layers potentially makes this world even more interesting. The interior configuration inferred from the data implies the widespread presence of melt pockets throughout the hydrosphere, potentially creating sites with highly concentrated organic and saline aqueous solutions. These solutions could be transported upward by strong convection in the ice shell, which is both indicated by the data and required to prevent the ice from melting into a global ocean. Multiple Dragonfly investigations will help constrain the physical structure of Titan’s interior, testing the ocean-free model introduced here with an independent dataset.

 

Figure 2: The strong tidal response amplitude and dissipation preclude a global subsurface ocean and indicate a slushy high-pressure ice layer, comprising ice III (light green), ice V (light blue), ice VI (light purple) and small amounts of partial melting (fuchsia).

References

Babin, M. et al. 2025, Life in the frozen ocean. Ann. Rev. Marine Sci.

Downey and Nimmo (2025), Titan’s spin state as a constraint on tidal dissipation, Science Advances, 11, eadl4741

Durante et al. 2019, Titan’s gravity field and interior structure after Cassini, Icarus, 326, 123–132

Iess et al. 2010, Gravity field, shape, and moment of inertia of Titan, Science, 327, 1367–1369

Iess et al. 2012, The tides of Titan Science, 337, 457–459

Petricca et al. 2025, Titan’s strong tidal dissipation precludes a subsurface ocean, Nature, 648, 556–561’

Rappaport et al. (2008), Can Cassini detect a subsurface ocean in Titan from gravity measurements?, Icarus 194, 711–720.