A new analysis of Cassini mission data is fundamentally changing how we imagine the interior of Titan, Saturn's largest moon. Instead of a single global subsurface ocean, as has been assumed for years, Titan could be hiding a complex mosaic of slush-like ice layers and isolated pockets of liquid water in its depths that occasionally break through to the surface. Such a scenario not only changes the geological story of this icy world but also opens an entirely new debate about where and how we might look for signs of life there.

The results come from the latest research by a team from NASA's Jet Propulsion Laboratory (JPL), published in the journal Nature in December 2025. Scientists reprocessed precise radio measurements sent by the Cassini probe during ten close flybys of Titan. Instead of looking for a classic signature of a liquid global ocean in the data, they focused on subtle “lags” and tiny changes in Titan's gravitational field. These nuances point to something else: strong internal heating and layers of material that behave like a thick, semi-liquid mixture of ice and water.

Such a conclusion comes nearly two decades after Cassini entered orbit around Saturn in 2004 and began its systematic study of Titan. Combined with the historic descent of the Huygens probe to Titan's surface in 2005, the mission completely transformed our image of that world. By late 2025, we know that Titan has a thick, nitrogen-rich atmosphere, lakes and seas of liquid hydrocarbons, and an active “methane cycle” similar to Earth's water cycle – but at temperatures around –180 °C. Cassini also provided the first serious clues that Titan hides a deep internal layer of water or water mixed with ammonia, likely located beneath an ice crust a hundred or more kilometers thick.

The idea of a global ocean primarily arose from measurements of so-called tidal motion, the way Titan changes shape under the influence of Saturn's gravity. When the moon is closer to the planet on its slightly elliptical orbit, Saturn literally “squeezes” it; when it is further away, Titan stretches. If the interior is predominantly solid, this deformation will be small. If there is a liquid layer beneath the crust, the entire object will be more yielding, and the tidal bulge will increase. The first analysis of Cassini data showed that Titan “squishes” more than it should if it were entirely solid – and this was interpreted as evidence for a global ocean.

The key value in this story is the so-called Love number, a parameter describing how much a body deforms under the action of tidal forces. Early calculations for Titan suggested a very yielding interior, consistent with a large liquid zone beneath the crust. Later works, based on an expanded set of Cassini flybys and finer gravitational field models, emphasized that the internal layers are likely not a simple “sandwich” of rock, ocean, and ice. Instead, everything pointed increasingly toward a much more complex mixture of ice, liquid, and rock than assumed in the first models.

The latest NASA research goes a step further: instead of viewing Titan as a body with a clear boundary between a solid crust and a liquid ocean, scientists are developing a scenario in which multiple transition zones exist beneath the crust. In these zones, ice and water form a kind of slush – a mixture of ice crystals and tiny liquid-phase channels. Such material can deform slowly under the influence of tidal forces, much more easily than completely solid ice, but it does not flow as freely as pure liquid.

To test this possibility, researchers returned to the source of information itself: radio signals between Cassini and NASA's Deep Space Network antennas on Earth. When the probe flies past Titan, the uneven distribution of mass within the moon changes Titan's gravitational field, and thus the spacecraft's speed. These seemingly microscopic changes in speed leave a recognizable trace in the frequency of radio waves – known as the Doppler shift. By carefully analyzing these changes, it is possible to reconstruct variations in the gravitational field and the way Titan deforms during its orbit.

Previous analyses of Doppler data had already concluded that Titan exhibits a relatively high degree of tidal bulging. However, what was missing was a clear signature of enhanced energy dissipation in the interior – an additional “thermal cost” that a slushy, friction-rich structure of ice and water should leave behind. The new team managed to find this signature using advanced signal processing techniques: instead of standard filtering, they performed very aggressive noise reduction, searching the data for extremely tiny “jitters” in the frequency spectrum that would remain buried in standard processing.

The result was exactly what they expected from a multi-layer slush-like model: a strong signal of internal energy loss, consistent with layers of slushy ice beneath a thicker, relatively rigid ice crust. In such a scenario, Titan still deforms tidally almost as much as in the case of a global ocean, but part of the energy does not go only into the elastic “breathing” of the moon, but is converted into heat by the friction of ice crystals sliding and rubbing against each other.

This picture has an important consequence: if most of the interior is occupied by slushy zones of high-pressure ice, then a stable, continuous ocean may not exist at all. Instead of one vast aqueous shell, Titan could have a network of localized pockets of liquid water, formed where beams of tidal energy and heat concentrate enough to melt part of the ice. These pockets then slowly “travel” upwards through the ice layers until they encounter colder regions where they freeze again.

Although it might seem at first glance that Titan is “poorer” for lacking a global ocean, researchers emphasize the opposite: such a mosaic of water pockets could be even more exciting in an astrobiological sense. Each individual pocket would function like a small capsule in which chemical ingredients from the rocky core mix with organic molecules formed in deeper layers or material brought by meteorites. In some scenarios, water in these microenvironments could reach temperatures up to twenty degrees Celsius – values at which, at least on Earth, many biological processes occur.

On Earth, we know that even small, local systems – like hydrothermal vents on the ocean floor – can be hotspots of chemical diversity and perhaps cradles of life. If Titan truly hides hundreds or thousands of such “pockets” of liquid water, each with its own chemical signature and heating history, then this icy body becomes a laboratory for testing different scenarios for the origin of prebiotic molecules. The difference from Earth is that everything takes place deep beneath the surface, surrounded by an icy mantle and methane oceans, in an environment we might call a “cryochemical Jura.”

The new study does not entirely erase earlier works suggesting the existence of a global ocean but puts them into a broader context. Analyses published in recent years, based on Cassini's gravitational measurements and precise rotation models, show that Titan's interior almost certainly contains a significant amount of liquid water or water mixed with ammonia. However, the question is how this liquid is distributed: as a single continuous shell or as multiple layers and pockets within a complex structure of high-pressure ice. The new NASA team's work leans strongly toward the second possibility.

Titan is not alone in this. Over the past decade, scientists have discovered an entire “family” of ocean-worlds in the outer Solar System: Europa and Ganymede around Jupiter, Enceladus and Mimas around Saturn, and likely several other icy moons hiding oceans beneath their crusts. Some of these bodies, like Enceladus, eject water geysers that reveal a direct link between the internal ocean and the surface. Titan, on the other hand, does not have such a dramatic signature – its surface is covered with complex organic layers, hydrocarbon sand dunes, and methane lakes. This is why the interpretation of Cassini's interior measurements is so demanding and so important.

Another interesting aspect of the new picture of Titan is how internal heating fits into the broader story of the moon's evolution. Saturn's tidal forces have been “massaging” Titan's interior for millions of years, converting orbital energy into heat. If slushy layers are efficient at dissipating energy, it means that Titan could go through phases of enhanced and weakened heating over geological periods. In these phases, liquid water pockets form and disappear, merge and separate. A long history of such cycles could leave traces on the surface – in the form of tectonic cracks, craters with altered morphology, or unexpected relief anomalies.

Cassini previously discovered a series of unusual structures on Titan's surface: from massive methane seas at the north pole to river valleys and potential cryovolcanic areas where ice and ammonia mixtures may have once erupted from underground. The new interpretation of the internal structure provides them with additional context: perhaps some of these areas are surface reflections of deeper, temporary water reservoirs that touched the upper layers of the ice crust in the past or even briefly reached very close to the surface.

Despite the impressive amount of data Cassini collected during its 13-year stay in the Saturn system, there are things that radio measurements simply cannot resolve. The detailed internal structure, the thickness of different layers, and even the exact arrangement of water pockets remain subjects of modeling and indirect estimates. That is precisely why the next big step in exploring Titan is linked to a completely different type of mission – one that will not just fly over the moon but will land on its surface and move from one place to another.

That mission is called Dragonfly. It is a unique NASA rotorcraft-type spacecraft – a sort of nuclear-powered drone with eight rotors – whose launch window is currently planned for July 2028. After about six years of travel, Dragonfly is expected to reach Titan in 2034 and spend at least 3.3 Earth years flying between different locations on its surface. Unlike classic rovers, which are limited to a few kilometers around the landing site, this rotorcraft will be able to cover tens of kilometers in a single “hop,” targeting dunes, ancient impact craters, and potentially cryovolcanic areas.

Dragonfly carries a full suite of instruments: from high-resolution cameras and a meteorological package to spectrometers and a seismometer. The seismometer is key to testing the new models of Titan's interior. If the mission successfully records quakes or other seismic events, their propagation through the moon can be compared with different scenarios of internal structure – including the one with layers of slushy ice and liquid water pockets. In other words, Dragonfly could offer a “field” test of hypotheses that currently exist only in computer models and radio data.

On the other hand, geochemical instruments on Dragonfly will focus on what makes Titan so attractive for astrobiology: rich organic chemistry. Titan's atmosphere is full of complex molecules formed by the breakdown of methane and nitrogen under the influence of solar and cosmic radiation. These molecules settle on the surface, where they mix with ice and rocks. If there are indeed pockets of warm liquid water in the depths that occasionally come into contact with this organic material, Dragonfly could find traces of chemical processes reminiscent of the early stages of the chemistry of life on Earth.

The new NASA study thus shifts the focus: instead of looking for one large ocean, Titan is increasingly seen as a complex, “multi-story” structure where water, ice, and rock intertwine at different depths. In such an environment, the question of “is there an ocean or not” is perhaps less important than the question of the diversity of conditions in which liquid water can exist at least temporarily. It is this diversity – from deep pockets near the core to transition zones closer to the surface – that makes Titan a special laboratory for exploring the limits of habitability in the Solar System.

Equally important is the message this story sends about the nature of space missions themselves. Cassini concluded its Grand Finale in 2017 with a dramatic entry into Saturn's atmosphere. Yet, the data it collected there continues to yield new discoveries nearly a decade later. More advanced signal processing methods, new models, and comparisons with data from other missions turn telemetry archives into a “gold mine” for researchers. Titan's slushy interior may be just one of many hidden insights still waiting for us in the vast databases created during Cassini's stay at Saturn.

For scientists studying ocean worlds, Titan is at the center of two key debates in 2025. The first concerns the very definition of an ocean: should it necessarily be understood as a continuous global layer, or is the existence of deeper zones of liquid water, even in fragmented form, sufficient. The second deals with the question of conditions for life: are stability and longevity crucial, or can a series of shorter-lived episodes of liquid water in different pockets be enough to spark prebiotic chemistry.

We likely won't get the answers to these questions from a single observation or a single model. A combination of everything is needed: detailed analysis of Cassini data, new observations from Earth and space, laboratory experiments simulating Titan's extreme conditions, and bold missions like Dragonfly that will descend to the surface itself and collect samples where the story unfolds. But it is already clear that the latest NASA study has taken an important step: it has broken the simple image of “one large ocean” and replaced it with a richer, branched story of layers, pockets, and water cycles within an icy world on the edge of the Solar System.