A fresh look at data from NASA's Cassini mission indicates that Saturn's largest moon, Titan, likely lacks a vast subsurface ocean and instead features a slushy interior with isolated pockets of liquid water. This finding challenges earlier assumptions and could reshape the search for life on icy worlds. Researchers published their results on December 17 in the journal Nature.
For over a decade, scientists interpreted observations from NASA's Cassini spacecraft, which orbited Saturn from 1997 until nearly two decades later, as evidence of a deep ocean beneath Titan's icy crust. Titan, Saturn's largest moon, is unique in our solar system—besides Earth—for having stable surface liquids, though these are methane lakes and rain at temperatures around -297 degrees Fahrenheit, not water.
In 2008, researchers noted Titan's pronounced flexing as it orbits Saturn in an elongated path, attributing it to a subsurface ocean that would allow greater deformation under the planet's gravity. However, a reanalysis incorporating timing data has upended this view. The moon's shape changes lag about 15 hours behind Saturn's strongest gravitational pull, suggesting a thicker, more viscous interior that dissipates more energy than a free-flowing ocean would.
"The deformation we detected during the initial analysis of the Cassini mission data could have been compatible with a global ocean, but now we know that isn't the full story," said Baptiste Journaux, an assistant professor of Earth and space sciences at the University of Washington.
Lead author Flavio Petricca, a postdoctoral fellow at NASA's Jet Propulsion Laboratory, examined radio signals from Cassini's close fly-bys of Titan. The results point to layers of ice gradually turning into slushy pathways and isolated water pockets near the rocky core, rather than an open sea. "Nobody was expecting very strong energy dissipation inside Titan. That was the smoking gun indicating that Titan's interior is different from what was inferred from previous analyses," Petricca said.
Journaux contributed thermodynamic insights from his lab, which simulates extreme pressures where water behaves unlike on Earth. "The watery layer on Titan is so thick, the pressure is so immense, that the physics of water changes," he explained.
These slushy conditions may enhance prospects for life, with water pockets potentially reaching 68 degrees Fahrenheit and concentrating nutrients more than in a large ocean. "It expands the range of environments we might consider habitable," noted Ula Jones, a University of Washington graduate student.
The study, involving co-authors from NASA and international institutions, was funded by NASA, the Swiss National Science Foundation, and the Italian Space Agency. It will inform the Dragonfly mission to Titan, set to launch in 2028, where Journaux is involved.