Rethinking Titan: New Study Casts Doubt on Water Ocean

For anyone who has known me over the years, you’ll know I’ve spent a large amount of time talking about the five moons in our Solar System that harbour subsurface oceans of liquid water: Europa, Enceladus, Callisto, Ganymede, and Titan. I explored this topic in depth in my first book, Exploring the Ocean Worlds of Our Solar System (now in its second edition), where I examined every line of evidence for these remarkable oceans. In the process, I had the privilege of speaking with leading planetary scientists worldwide, and in the case of Saturn's biggest moon, Titan, with the very researchers responsible for uncovering that evidence (Chapter 7 in the book). Taken together, the data returned by the Cassini spacecraft that orbited Saturn as well as the Huygens probe that landed on Titan in 2005 pointed to a single compelling explanation: a hidden ocean of liquid water beneath the surface. That Titan, the Solar System’s second‑largest moon, should resemble Ganymede (1st) and Callisto (3rd) in supporting a subsurface ocean was no shock; if anything, it seemed to align with what was expected from such large icy moons.

Over the years though, planetary scientists have revisited Cassini and Huygens’ data and refined their models. Already, the interpretation of the data from Huygens 'Permittivity, Waves, and Altimetry' sensor data has been challenged.* Now, a new paper (link here) published just a few weeks ago and authored by an impressive roster of leading scientists, including Vance and Lunine (both of whom I interviewed for my book), as well as Nimmo and others, has re‑evaluated Cassini's data. The authors argue that the moon's gravity-field signature, as measured by Cassini, does not align with the presence of a subsurface ocean of liquid water.

From 2004 to 2017, the Cassini spacecraft repeatedly flew past Titan to study its gravity, rotation, and internal structure, and measurements showed that the moon responded strongly to Saturn’s gravitational pull, which researchers took as evidence for such an ocean. Since then, improved data processing techniques have prompted scientists to look again at Cassini's data. This time they were able to better analyse not just at how much Titan flexes under Saturn’s tides, but also at how delayed that response is. A delay means that energy is being lost inside the moon as heat, revealing where and how the interior deforms. Using improved analysis of Cassini’s radio-tracking data, researchers were able for the first time to measure this delay directly. They found that Titan dissipates tidal energy far more strongly than expected if it had a subsurface ocean. In fact, the amount of energy loss is several times greater than what an ocean-bearing Titan could produce.

Detailed modelling shows that this strong energy loss is best explained not by a liquid ocean, but by a thick layer of “high-pressure ice” deep inside Titan. This ice (Ice III, V, and VI) is squeezed so intensely that it behaves slowly and squishily, almost like warm wax. Through convection, heat is carried upward efficiently enough that it prevents a global ocean from forming, even though Titan is tidally active; this model also implies that this convection could also carry upwards minerals and other componds from the lower silicate mantle.

Thus, while Titan appears to lack a global subsurface ocean, it likely contains something as intriguing: a deep, slushy ice layer where pockets of "warm" liquid water exist. This opens a new possibility: liquid environments that are mixed with nutrient-rich fluids from below and complex organic molecules from above. To grasp the potential scale of these meltwater "pockets", it is worth recalling that Titan holds so much water that melting a mere 1% of it would produce the volume of the entire Atlantic Ocean, while 0.01% would be enough to fill the Mediterranean.

This new study paints Titan’s interior as a chemically rich liquid environment constantly stirred by slow ice convection. Niches of melted water could resemble Earth’s polar sea-ice ecosystems and may offer more interesting conditions for prebiotic chemistry or exotic life than a global subsurface ocean cut off from the silicate mantle (as was proposed in the ocean worlds model). More broadly, the findings suggest that even in the presence of substantial tidal heating, true ocean worlds may be less common than previously assumed.

Regardless, the new findings make Saturn’s largest moon even more compelling than before, and I’m eagerly anticipating 2034, when NASA’s Dragonfly mission is scheduled to land and deploy its DraGMet seismometer to better study the moon's interior. As we toast 2026, let us take a moment to appreciate Titan’s remarkable complexity and its tantalising promise of habitability.

Happy New Year to you all. As always, onwards and upwards.

*Lorenz, R. D. & Le Gall, A. Schumann resonance on Titan: a critical re-assessment. Icarus 351, 113942 (2020) https://doi.org/10.1016/j.icarus.2020.113942