Uranus’s moon Ariel has long fascinated astronomers, but recent research suggests it may be hiding something remarkable beneath its icy shell: a subsurface ocean. Evidence comes from carbon dioxide ice and other carbon-bearing molecules found on the surface, which likely originated from chemical processes deep inside the moon. The big question is: how did these materials make it to the surface?
A team of planetary scientists led by Chloe Beddingfield at the Johns Hopkins Applied Physics Laboratory (APL) believes they have found the answer. Their study suggests that medial grooves—trenches cutting through Ariel’s vast canyons—may be the pathways through which these materials emerge. These findings provide crucial insight into the moon’s hidden interior and raise the possibility of an active geological process at work.
Spreading Centers: A New Clue
The study proposes that Ariel’s medial grooves function similarly to spreading centers on Earth. Spreading centers are places where new oceanic crust forms as internal material rises and pushes the surface apart, much like what happens at Earth’s mid-ocean ridges.
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“If we’re right, these medial grooves are probably the best candidates for sourcing those carbon oxide deposits and uncovering more details about the moon’s interior,” said Beddingfield. “No other surface features show evidence of facilitating the movement of materials from inside Ariel, making this finding particularly exciting.”
By analyzing images captured by NASA’s Voyager 2—the only spacecraft to have flown past Uranus and its moons—the researchers determined that the grooves resemble spreading centers rather than volcanic fissures. One key clue is how the canyon walls on either side of the grooves fit together like puzzle pieces when digitally reconstructed, suggesting they were once a single structure before being pulled apart. Additionally, the canyon floors display regularly spaced ridges similar to patterns left by construction excavators, a sign of gradual material deposition over time.
Role of Tidal Forces
Ariel and several of Uranus’s other moons have experienced multiple periods of geological activity, likely driven by tidal forces. These forces stem from orbital resonances—precise gravitational interactions between moons that can generate internal heating. “It’s a fascinating situation—how this cycle affects these moons, their evolution, and their characteristics,” Beddingfield said.
Scientists believe that these resonances may have sustained subsurface oceans within both Ariel and its smaller neighbor, Miranda. A 2024 study co-authored by planetary scientist Tom Nordheim suggested that Miranda could still harbor an internal ocean, kept warm by these tidal forces.
A Possible Ocean Beneath Ariel
If Ariel does have an ocean, it could explain why carbon dioxide and other compounds are appearing on its surface. However, Beddingfield remains cautious, noting that while the presence of these compounds is intriguing, it is unclear whether they are directly linked to an internal ocean. “The size of Ariel’s possible ocean and its depth beneath the surface can only be estimated, but it may be too isolated to interact with spreading centers,” she explained.
Nordheim also pointed out that carbon-bearing compounds tend to have short lifespans in space due to radiation and other environmental factors. This suggests that the deposits on Ariel must have been placed there relatively recently, further supporting the idea of ongoing activity beneath the surface. “These new results suggest a possible mechanism for emplacing fresh material and short-lived compounds, including carbon monoxide and perhaps ammonia-bearing species on the surface,” he said.
The Need for Further Exploration
While these discoveries provide tantalizing clues about Ariel’s interior, much remains unknown. Voyager 2 was not equipped with instruments capable of mapping the distribution of ices on Ariel, leaving many questions unanswered. Richard Cartwright, another researcher involved in the study, emphasized the need for a dedicated mission to Uranus and its moons.
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“We need an orbiter that can make close passes of Ariel, map its medial grooves in detail, and analyze their spectral signatures for components like carbon dioxide and carbon monoxide,” he said. “If carbon-bearing molecules are concentrated along these grooves, then it would strongly support the idea that they’re windows into Ariel’s interior.”