New evidence suggests that Ariel, one of Uranus' moons, may once have had a massive ocean hidden beneath its icy crust. A study published in the journal Icarus explores how this ocean might have changed over time, estimating it could have been more than 100 miles (170 kilometers) deep—far deeper than Earth's average ocean depth of about 2.5 miles (4 kilometers). Ariel is the second-closest moon to Uranus and the fourth-largest in the system, despite its relatively small size of about 720 miles (1,159 kilometers) across. Its surface is strikingly complex, with a mix of ancient impact craters and younger, smoother regions that may have formed through cryovolcanism, a process where water, ice, or other volatile materials erupt from an icy body instead of molten rock. Ariel's surface is also marked by extensive fractures, ridges, and grabens—areas where the crust has shifted and dropped. These features are on a scale rarely seen elsewhere in the solar system. Researchers believe that Ariel's orbit may have once been significantly more eccentric—about 40 times greater than it is today—leading to stronger tidal forces that could have shaped its landscape. This orbital eccentricity would have been about four times greater than that of Jupiter’s moon Europa, which is known for its heavily fractured icy shell due to tidal forces. These forces may have played a key role in creating the dramatic surface features observed today. Alex Patthoff, a senior scientist at the Planetary Science Institute, explains that the massive fractures on Ariel's surface suggest the presence of a subsurface ocean. "In order to create those fractures, you have to have either a really thin ice on a really big ocean, or a higher eccentricity and a smaller ocean," he said. "But either way, we need an ocean to be able to create the fractures that we are seeing on Ariel's surface." This study is part of a series examining the potential for subsurface oceans on Uranus' moons. Last year, the same team found similar evidence on Miranda, another icy moon of Uranus. The findings suggest that more than one moon in the Uranus system may have once harbored a substantial subsurface ocean. "We are finding evidence that the Uranus system may harbor twin ocean worlds," said coauthor Tom Nordheim of the Johns Hopkins University Applied Physics Laboratory. However, scientists still do not know exactly when Ariel's ocean may have existed or how long it lasted. A future mission to Uranus could help answer these questions by imaging the unexplored northern hemispheres of Ariel and Miranda, searching for predicted surface features like fractures and ridges. This research provides a crucial foundation for understanding how subsurface oceans on distant icy worlds form, evolve, and potentially vanish over time.