Ariel’s Hidden Ocean Could Explain Its CO2-Rich Surface and Clues About Cryovolcanism Beneath Uranus’s Moon
TL;DR
Using the James Webb Space Telescope, astronomers discovered that Ariel, one of Uranus’s moons, may have a hidden subsurface ocean of liquid water, which could be replenishing the moon’s carbon dioxide-rich surface. Light spectra analysis revealed Ariel’s chemical makeup, including carbon dioxide and carbon monoxide, which should not be stable given the moon’s temperatures. This suggests that these compounds might originate from a liquid ocean beneath the surface, possibly leaking through cracks or expelled by cryovolcanic activity. Carbonate minerals found in Ariel’s ice shell further hint at the presence of liquid water, supporting the need for a dedicated mission to explore Uranus and its moons.
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Using the James Webb Space Telescope (JWST), astronomers have found that Ariel, one of Uranus’s moons, might have a hidden subsurface ocean of liquid water.
This discovery could help solve a long-standing mystery about Ariel, specifically why its surface has so much carbon dioxide ice. Given that Uranus and its moons are 20 times farther from the Sun than Earth, carbon dioxide should turn to gas and escape into space, suggesting that something must be replenishing the carbon dioxide on Ariel’s surface.
Earlier theories suggested that interactions between Ariel’s surface and charged particles in Uranus’s magnetosphere, which cause ionizing radiation and break down molecules into carbon dioxide, a process known as “radiolysis,” could be responsible for this replenishment. However, new evidence from the JWST suggests that this carbon dioxide might come from inside Ariel, potentially from a hidden ocean beneath the surface.
By analyzing light spectra, which show specific “fingerprints” of elements and molecules based on their light absorption and emission, the JWST allowed scientists to study Ariel’s chemical composition. Comparing this data with simulated spectra from a lab chemical mix, researchers found that Ariel has some of the most carbon dioxide-rich deposits in the solar system. This analysis not only added around 10 millimeters (0.4 inches) of ice thickness on Ariel’s side facing away from Uranus but also, for the first time, revealed clear signs of carbon monoxide.
“It just shouldn’t be there. You’ve got to get down to 30 kelvins [minus 405 degrees Fahrenheit] before carbon monoxide’s stable,” said Richard Cartwright, team leader from the Johns Hopkins Applied Physics Laboratory (APL). “The carbon monoxide would have to be actively replenished, no question.”
This is because Ariel’s surface temperature is typically around 65 degrees Fahrenheit (18 degrees Celsius) warmer than the temperature required for carbon monoxide to remain stable. While Cartwright acknowledges that radiolysis might account for some replenishment, observations from Voyager 2’s 1986 flyby and other recent data suggest that radiolysis might be limited because of the 58-degree offset between Uranus’s magnetic field axis and the orbital plane of its moons.
This implies that most of the carbon and oxygen compounds seen on Ariel’s surface could originate from chemical processes occurring within a liquid water ocean beneath its icy shell.
Ariel might seem cool, but it could have a fiery side
These carbon oxides, formed in Ariel’s subsurface ocean, could leak through cracks in the moon’s ice shell or be explosively expelled by powerful cryovolcanoes, which erupt icy slush instead of lava. Some scientists have long speculated that Ariel’s cracked and scarred surface might indicate active cryovolcanism, with plumes that could even send material into Uranus’s magnetic field.
Most of the cracks and grooves on Ariel’s surface are located on the side facing away from Uranus. If carbon dioxide and carbon monoxide are escaping from these features, this could explain why these compounds are more abundant on Ariel’s trailing side.
The JWST also detected more chemical evidence for a hidden subsurface ocean. Spectral analysis pointed to the presence of carbonate minerals, salts that form when rock interacts with liquid water.
“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 explained. “That’s something we absolutely need to confirm, either through future observations, modeling, or some combination of techniques.”
Uranus and its moons have not been visited by a spacecraft since Voyager 2’s flyby nearly four decades ago, which was not even its main mission. The 2023 Planetary Science and Astrobiology decadal survey emphasized the need for a dedicated mission to the Uranian system.
Cartwright believes that such a mission would be a chance to gather crucial information about Uranus and Neptune, the solar system’s other ice giant, and could also provide essential data on other moons in these systems that may also harbor oceans. These findings could also have implications for studying exoplanets, or planets beyond our solar system.
“All these new insights underscore how compelling the Uranian system is,” said Ian Cohen, a team member and NASA Applied Physics Laboratory scientist. “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.”
The team’s research was published on July 24 in The Astrophysical Journal Letters.
