Uranus’ moon Ariel may have a hidden ocean

New research has revealed that the surface of Uranus’ moon Ariel is coated with a significant amount of carbon dioxide ice, particularly on its trailing hemisphere, which always faces away from the moon’s direction of orbital motion. 

This discovery is surprising because even in the frigid environment of the Uranian system, carbon dioxide typically turns to gas and escapes into space.

Uranus’ moon may have molecules within

Scientists have theorized that something must be supplying the carbon dioxide to Ariel’s surface. 

One popular hypothesis suggests that interactions between the moon’s surface and charged particles in Uranus’ magnetosphere create carbon dioxide through radiolysis, a process where molecules are broken down by ionizing radiation.

However, a new study published in the Astrophysical Journal Letters supports an alternative theory: that carbon dioxide and other molecules are emerging from within Ariel, possibly from a subsurface liquid ocean. 

Carbon dioxide deposits on Uranus’ moon

The research, led by Richard Cartwright from Johns Hopkins Applied Physics Laboratory (APL), used NASA’s James Webb Space Telescope to collect chemical spectra of Ariel’s surface and compared them with spectra of simulated chemical mixtures in the lab. 

The experts found that Ariel has some of the most carbon dioxide-rich deposits in the solar system, with an estimated thickness of at least 10 millimeters (0.4 inches) on the moon’s trailing hemisphere.

Carbon monoxide on Ariel’s surface

Additionally, the study detected clear signals of carbon monoxide on Ariel’s surface. “It just shouldn’t be there. You’ve got to get down to 30 Kelvin [minus 405 degrees Fahrenheit] before carbon monoxide’s stable,” Cartwright said.

Ariel’s surface temperature averages around 65 degrees Fahrenheit warmer, suggesting that carbon monoxide must be actively replenished.

Radiolysis could contribute to this replenishment. Laboratory experiments have demonstrated that radiation bombardment of water ice mixed with carbon-rich material can produce both carbon dioxide and carbon monoxide, indicating a potential source for the rich abundance of these molecules on Ariel’s trailing hemisphere.

Many questions remain unanswered 

However, many questions remain about Uranus’ magnetosphere and its interactions with the planet’s moons. 

Even during Voyager 2’s flyby of Uranus nearly 40 years ago, scientists suspected that these interactions might be limited due to the 58-degree offset between Uranus’ magnetic field axis and the orbital plane of its moons. Recent models have supported this prediction.

Instead, the bulk of the carbon oxides on Ariel may come from chemical processes occurring in a subsurface water ocean, with these molecules escaping through cracks in Ariel’s icy exterior or even through eruptive plumes. 

Does Ariel have liquid water?

The new spectral observations also hint that Ariel’s surface might contain carbonate minerals, which can only form through interactions between liquid water and rocks. 

“If our interpretation of that carbonate feature is correct, then that is a pretty big result because it means it had to form in the interior,” Cartwright said. “That’s something we absolutely need to confirm, either through future observations, modeling or some combination of techniques.”

Geological activity on Uranus’ moon

Ariel’s surface features, including gash-like canyons, crisscrossing grooves, and smooth spots believed to result from cryovolcanic spills, have led researchers to suspect that the moon may have been or still might be geologically active. 

A 2023 study led by APL’s Ian Cohen even suggested that Ariel and/or its sister moon Miranda could be emitting material into Uranus’ magnetosphere, potentially through plumes.

“All these new insights underscore how compelling the Uranian system is,” Cohen said. “Whether it’s to unlock the keys to how the solar system formed, better understand the planet’s complex magnetosphere or determine whether these moons are potential ocean worlds, many of us in the planetary science community are really looking forward to a future mission to explore Uranus.”

Dedicated missions to Uranus are needed

In 2023, the planetary science community, through its Planetary Science and Astrobiology decadal survey, prioritized the first dedicated mission to Uranus, raising hopes for a future scientific voyage to the turquoise ice giant.

Cartwright views this as an opportunity to gather valuable data about the solar system’s ice giants and their potentially ocean-bearing moons. These findings could have implications for understanding similar worlds in other stellar systems. 

Additionally, a mission to Uranus could provide concrete answers that are only possible through direct exploration. Most of Ariel’s observed grooves, which are suspected to be openings to its interior, are on its trailing side. If carbon dioxide and carbon monoxide are leaking through these grooves, it could explain their abundance on Ariel’s trailing side.

“It’s a bit of a stretch because we just haven’t seen much of the moon’s surface [Voyager 2 captured only about 35% of Ariel’s surface during its brief flyby],” Cartwright cautioned. “We’re just not going to know until we perform more dedicated observations.”

Image Credit: NASA/Jet Propulsion Laboratory

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