Titan, Saturn's largest moon, has long been considered an ocean world, with a global ocean hidden beneath its frozen crust. However, a recent reanalysis of Cassini data challenges this notion, suggesting a very different interior structure. Instead of a liquid ocean, the data points to a thick layer of slushy ice, close to melting and likely containing pockets of liquid water near the rocky interior.
The original ocean interpretation made sense due to Titan's eccentric orbit, which causes Saturn's gravity to squeeze and stretch the moon, resulting in a tidal response. This response was measured using the Love number k2, with a 2012 analysis finding a response roughly twice as large as pre-Cassini predictions. A buried global ocean offered a persuasive mechanical explanation for this deformation.
However, the new reanalysis, published in Nature, proposes a different interior structure. The preferred model has an outer ice shell and hundreds of kilometers of high-pressure ice that is hot enough to deform, convect, and contain scattered melt pockets. This model best fits the reprocessed spacecraft data, with the imaginary component of k2 being 0.135 ± 0.035, which is three to four times the maximum value produced by models containing an ocean.
The 380-kilometer figure for the high-pressure ice layer has wide uncertainty, with a range of about 228 to 529 kilometers. The entire water-rich hydrosphere is approximately 600 kilometers thick, and the inferred rocky core has a radius of 2,026 kilometers, with an uncertainty of roughly 150 kilometers in either direction. None of these boundaries has been imaged, and they are the internal arrangement that best reconciles several geophysical constraints within the authors' framework.
The new measurement implies that Titan dissipates about 4 terawatts of orbital energy inside itself, with most of the heating occurring in the high-pressure ice. This requires the layer to have a high viscosity, allowing for convection and partial melt. The term 'slushy ice' is used to describe this condition, where solid ice can convect and small amounts of partial melt can collect between or within crystals.
The habitability story also becomes less simple, with the paper estimating that even a 0.01 percent melt fraction across Titan's hydrosphere would equal the Mediterranean Sea's volume. Separate pockets could concentrate salts and organic molecules, but no evidence shows that they host life or that all the modeled pockets are connected to material from the surface.
The Dragonfly mission, targeted to launch in 2028, may provide an independent test of the new model. With a seismometer on board, it could help constrain the hidden layering by measuring differences in wave speed across ice phases. However, the mission is primarily focused on surface habitability, and a definitive interior answer is not guaranteed.
In conclusion, the reanalysis of Cassini data challenges the long-held belief of a global ocean on Titan, suggesting a much stranger world of hot, slowly moving ice. This new understanding raises questions about the moon's habitability and the potential for life in its subsurface environment.
