December 17, 2025
4 min read
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New Views of Solar System Moons Complicate Ocean Worlds Theory
Oceans hiding within the crusts of distant moons are tantalizing targets for scientists looking for life beyond Earth
Titan (left) and Europa (right).
Left: NASA/JPL/University of Arizona/University of Idaho. Right: Image data: NASA/JPL-Caltech/SwRI/MSSS, Image processing: Kevin M. Gill CC BY 3.0
Among the solar system’s hundreds of known moons, few are as enticing as the handful that scientists believe have a global ocean lurking below their crusty surface. But two of these hidden oceans may not be as promising as scientists have hoped, leaving new questions for those interested in the search for life beyond Earth.
Those results come from unrelated research papers, both of which were published on December 17. In one, scientists reexamined data gathered by NASA’s Cassini mission, which ended in 2017, about Saturn’s largest moon, Titan. This research suggests that the moon’s hypothesized internal ocean of water is more of an icy slush, with only pockets of liquid, according to the paper, which was published in Nature. In the other work, scientists used new observations from NASA’s Juno mission about Jupiter’s intriguing world Europa. Their findings suggest that the moon’s icy shell is quite thick, reducing the possibility of the ocean and the surface interacting, according to research published in Nature Astronomy and presented at the annual meeting of the American Geophysical Union.
Titanic Secrets
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First, a jaunt to Titan. Of all the worlds scientists have seen up close to date, it’s among the most intriguing. On the surface, Titan is an Earth-like world with a twist—liquid hydrocarbons sculpt a landscape of water ice the way terrestrial seas flow over rock crust. That dynamic geology makes it the only moon with any hope of habitable surface conditions.
And scientists have also wondered whether Titan hid an ocean layer of water and ammonia below that changeable surface. Some interpretations of the Cassini mission’s data have supported that idea: scientists noticed certain surface features moved much more than would be expected if the icy shell lay directly on a solid interior.
But overall, the mission’s data haven’t quite all fallen in line with this theory, leaving scientists dissatisfied. So Flavio Petricca, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL), and other researchers came together to take a new look at the entire collection of data. They now propose that Titan doesn’t have a global subsurface ocean after all—instead it’s filled with slush, punctured by occasional pockets of full melt. That would be a sea change in how scientists think about Titan, Petricca admits. “Sometimes it’s still shocking to think of Titan without an ocean,” he says.
But the new story does manage to pull together all the existing data better than any previous attempt, says Jani Radebaugh, a planetary scientist at Brigham Young University, who was not involved in the new research. And even if that subsurface ocean layer was a mirage all along, the moon itself remains tantalizing, both scientists agree.
Titan’s next robotic visitor will be NASA’s Dragonfly mission, due to launch in July 2028. This car-sized octocopter will whir over Titan’s icy dunes and give scientists an unprecedented look at the moon’s surface. But the mission will also carry a seismometer that will listen to the grumbles and shakes of Titan’s interior—data that scientists will be able to translate into a much better understanding of the moon’s structure.
As tantalizing as Dragonfly’s observations will be, there’s no need to wait for the mission’s interplanetary touchdown to keep delving into Titan’s secrets, Radebaugh says—not when observations from previous missions offer findings like these.
“We need to keep examining existing spacecraft data,” she says. “There is always going to be something new that we can find out if we look carefully.”
Europa’s Mystery
Although Titan may have lost its global ocean, that of Jupiter’s moon Europa remains scientifically plausible—but it may be trapped under a thicker ice shell than scientists hoped.
To date, researchers’ estimates for the thickness of the moon’s ice crust have ranged from less than two miles to as much as 20 miles (three to 30 kilometers). But those have been only estimates. And the specifics matter because Europa’s crust is riddled with cracks; if the shell is thin enough and the cracks go deep enough, that could let material travel between the icy moon’s surface and its hidden ocean.
New observations of Europa gathered by NASA’s Juno mission, however, suggest that the ice shell is on the thicker side of scientists’ estimates, closer to 20 miles deep—although the exact depth depends on the ocean’s saltiness.
“There have been theoretical arguments, but this is the first pretty much direct physical measurement,” says Steven Levin, project scientist of the Juno mission and an astrophysicst at JPL.
That calculation is based on data from Juno’s microwave radiometer, an instrument that was designed to peer deep into Jupiter’s atmosphere but that is now turned toward analyzing the planet’s largest moons’ internal structure, too. Remarkably, the measurement was based on only about five minutes of data because of the constraints of the Juno spacecraft’s existing orbit around Jupiter, Levin notes.
The researchers emphasize that their work remains an estimate and that the analysis was limited to a swath of Europa’s equator, which means that the ice shell could still be thinner or thicker in other regions.
And our knowledge of Europa, too, will benefit from future missions—two are already on route to study it and its neighbors. NASA’s dedicated Europa Clipper mission will arrive at Jupiter in 2030, and pinpointing the thickness of Europa’s ice shell is one of its top three agenda items. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE) will arrive at the giant planet in the following year and will provide additional observations.
Editor’s Note (12/17/25): This article was edited after posting to include comments from Steven Levin.
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