Uranus's moon, Ariel, may have a hidden ocean under its frozen crust

Last year, a study suggested that deposits of carbon dioxide ice and other carbon-bearing molecules on Uranus’s moon Ariel likely originated from internal chemical processes – possibly even from a subsurface ocean.

Now, new research provides insight into how these materials may have reached, or may still be reaching, the moon’s surface. 

Led by APL planetary geologist Chloe Beddingfield, the research points to medial grooves – trenches that cut through Ariel’s massive canyons – as the likely pathways for this movement.

Mysteries of Ariel’s interior 

The findings indicate that these medial grooves function similarly to spreading centers on Earth, where new oceanic crust forms as internal material rises to the surface.

“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,” Beddingfield said. “No other surface features show evidence of facilitating the movement of materials from inside Ariel, making this finding particularly exciting.”

Ariel’s grooves are among the youngest known features on its surface. Scientists have long debated whether they formed through fissures caused by tectonic forces or through volcanic activity. 

The team analyzed images captured by NASA’s Voyager 2 – the only spacecraft to have flown past Uranus and its moons – and concluded that these features more closely resemble spreading centers rather than volcanic conduits.

The spreading center theory

One key piece of evidence comes from the way the canyon walls on either side of the medial grooves align. When the central canyon floors are digitally removed, the walls fit together like puzzle pieces, suggesting they were once joined before being pulled apart. 

Additionally, the canyon floors feature regularly spaced ridges that resemble excavation tracks – patterns consistent with successive depositions of material, a hallmark of spreading centers.

Beddingfield explained that spreading centers arise from convection cells beneath the crust. As heat from Ariel’s interior pushes material upward, the surface cracks open, spreading apart as new material emerges and gradually cools.

The influence of tidal forces 

Ariel, along with several other Uranian moons, has undergone multiple periods of geological activity, likely due to tidal forces. 

These forces arise from orbital resonances – precise ratios in the orbital periods of moons that generate repeated gravitational interactions. The interactions create cycles of internal heating, potentially melting parts of the icy interior and then refreezing them over time.

“It’s a fascinating situation – how this cycle affects these moons, their evolution, and their characteristics,” Beddingfield said.

Some scientists believe that these resonances could have helped maintain subsurface oceans beneath Ariel and its smaller neighboring moon, Miranda. 

A recent 2024 study, co-authored by APL’s Tom Nordheim, proposed that such tidal forces might have created an ocean within Miranda’s interior – one that could still exist today.

Ariel’s internal structure

Ariel’s medial grooves could provide important clues about the moon’s internal structure, particularly regarding the presence of an ocean. 

Nordheim noted that carbon-bearing compounds do not last long on planetary surfaces due to space weathering, suggesting that these materials must have been deposited relatively recently.

“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,” Nordheim said.

However, Beddingfield cautioned against making direct connections between Ariel’s possible ocean and the medial grooves.

“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. 

“There’s just a lot we don’t know. And while carbon oxide ices are present on Ariel’s surface, it’s still unclear whether they’re associated with the grooves because Voyager 2 didn’t have instruments that could map the distribution of ices.”

Future exploration of Uranus

Study co-author Richard Cartwright emphasized that further exploration is necessary to confirm the role of these medial grooves in transporting material from Ariel’s interior.

“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,” said Cartwright. 

“If carbon-bearing molecules are concentrated along these grooves, then it would strongly support the idea that they’re windows into Ariel’s interior.”

With Uranus gaining attention as a priority for future space exploration, Ariel and its enigmatic grooves may soon receive closer scrutiny. These investigations could unlock critical insights into the moon’s geological past – and potentially even into ongoing internal activity today. 

The study is published in The Planetary Science Journal.

Image Credit: NASA/Jet Propulsion Laboratory

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