For years, scientists envisioned Titan, Saturn's largest moon, as a world with a vast subsurface ocean, similar to other icy moons. However, a recent study overturns this perspective by suggesting the existence of isolated pockets of liquid water, rather than a global sea, beneath its frozen crust.
This moon is a remarkable object in our Solar System. Larger than the planet Mercury, it possesses a thick atmosphere and liquid expanses on its surface, composed mainly of methane. This landscape makes it a prime study area for understanding distant worlds.
Artist's impression of a possible internal structure for Titan based on Cassini data.
Credit: A. D. Fortes/UCL/STFC
To refine this knowledge, a team of researchers scrutinized the radio data from NASA's Cassini mission using novel methods. These techniques allowed them to reduce uncertainties regarding Titan's internal structure, revealing unexpected details about its response to Saturn's gravitational forces.
The results show that Titan's interior resists deformation more strongly than expected. This observation rules out the hypothesis of a global ocean, replacing it with the idea of a layer of ice near its melting point, kept solid by high pressure. This mushy ice could contain localized reservoirs of liquid water.
This discovery transforms our perception of ocean worlds in space. Indeed, subsurface oceans might be less common than previously estimated. Titan's habitability, with its water pockets, remains an open question that future space missions will need to explore.
NASA's Dragonfly mission, scheduled to launch to fly over Titan, will help scan its surface and subsurface. These observations will better define the conditions necessary for life, by refining our understanding of the geology and presence of liquid water on this moon.
The large amplitude and dissipation of tidal effects rule out a global subsurface ocean and indicate a high-pressure melting ice layer, comprising ice III (light green), ice V (light blue), ice VI (light purple) and small amounts of partial melt (fuchsia). The role of pressure in preserving water
The pressure exerted by the upper layers of a celestial body can alter the state of ice. Near Titan's core, high pressure prevents the ice from melting completely, even when the temperature is near its melting point. This phenomenon generates a zone where the ice takes on a mushy consistency, with inclusions of liquid water.
In such environments, pressure stabilizes the internal structure. It acts as a barrier that keeps the ice in an intermediate state, preventing the formation of a continuous ocean. This scenario differs from that observed on other moons, where internal heat allows water to remain liquid over large areas.
Geophysical studies use these principles to model the interiors of planets and moons. Understanding the interaction between pressure and temperature helps anticipate areas where water could persist in a liquid state. This opens new avenues for the search for environments conducive to life elsewhere in the Solar System.