Scientists once believed that Titan, Saturn's largest moon, harbored a global ocean beneath its icy crust. However, a recent reanalysis of data from the Cassini spacecraft challenges this notion, suggesting a more complex and intriguing interior. The study, published in Nature, proposes a thick layer of slushy ice, close to melting, with pockets of liquid water near the rocky core, potentially reaching temperatures of 20°C. This discovery has significant implications for our understanding of Titan's geology and habitability.
For years, Titan was considered an ocean world, with its orbit around Saturn causing gravitational flexing that suggested the presence of a liquid layer. The Love number k2, a measure of the tidal response, was found to be significantly larger than expected, indicating a substantial deformation. This deformation was attributed to a global ocean, which could decouple the outer ice shell from the deeper interior, allowing for more flexibility.
However, the reanalysis of Cassini data reveals a different picture. The study's authors propose a thick outer ice shell, approximately 380 kilometers thick, above a layer of high-pressure ice that is hot enough to deform, convect, and contain scattered melt pockets. This high-pressure ice layer, with its unique crystal structures, forms under extreme pressures, challenging the idea of a global ocean.
The researchers combined tidal measurements with Titan's mass, moment of inertia, and other geophysical data to create a Bayesian inversion model. This model, without the assumption of a global ocean, provided a more accurate representation of Titan's interior. The preferred model suggests a low-pressure outer ice shell, about 170 kilometers thick, beneath which lies a high-pressure ice layer with a central thickness of 378 kilometers.
The term 'slushy ice' is used to describe this layer, which is not a single underground sea but a solid ice layer with small amounts of partial melt. The model estimates that Titan dissipates around 4 terawatts of orbital energy, with most of it coming from the high-pressure ice layer. This friction requires a high viscosity, similar to laboratory estimates for ice V and VI near their melting points.
The study also addresses the habitability of Titan. Even a small fraction of melt, estimated at 0.01%, across the moon's hydrosphere would equal the volume of the Mediterranean Sea. This suggests that separate pockets of liquid water could exist, potentially concentrating salts and organic molecules. However, the study emphasizes that there is no evidence to suggest that these pockets host life or are connected to the surface material.
The Dragonfly mission, set to launch in 2028, will provide an independent test of these findings. Equipped with a seismometer, it will measure seismic events and wave speeds across different ice phases, helping to constrain the hidden layering. While the mission's primary focus is on surface habitability, it may offer valuable insights into Titan's complex interior.
In conclusion, the reanalysis of Cassini data has revealed a more intricate and fascinating world beneath Titan's icy surface. The concept of a global ocean is challenged, and a new understanding of Titan's geology and potential habitability emerges. As we continue to explore this mysterious moon, we may uncover even more surprising discoveries that reshape our understanding of the solar system.
