Using the James Webb Space Telescope (JWST), astronomers have discovered that Ariel, a moon of Uranus, may be hiding in a buried ocean of liquid water.
The discovery could provide an answer to a mystery surrounding this moon of Uranus that has perplexed scientists: Ariel’s surface is covered in a significant amount of carbon dioxide ice. This is a surprising discovery because at the distance between Uranus and its moons from the Sun, 20 times farther than Earth, carbon dioxide turns into gas and is lost into space. This means that a process must be renewing the carbon dioxide on Ariel’s surface.
Previous theories have suggested that this occurs as a result of interactions between Ariel’s surface and charged particles trapped in Uranus’ magnetosphere that provide ionizing radiation, breaking down molecules and leaving carbon dioxide, a process called “radiolysis.”
However, new evidence from JWST suggests that the source of this carbon dioxide may not be from outside Ariel but from inside it, perhaps from a buried subsurface ocean.
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a blue and white disk surrounded by concentric white, green and blue rings
Uranus and its rings seen by the James Webb Space Telescope in 2023. (Photo credits: NASA, ESA, CSA, STScI)
Chemical elements and molecules absorb and emit light at characteristic wavelengths, allowing them to leave individual “fingerprints” on spectra. The team behind the discovery used JWST to collect spectra of light from Ariel, which allowed them to build a picture of the chemical composition of Uranus’ moon.
By comparing these results to simulated spectra from a chemical mixture made in a laboratory on Earth, the team discovered that Ariel has some of the richest deposits of carbon dioxide in the solar system. Not only did this add an extra 10 millimeters of thickness to the ice on the tidally locked side of Ariel that permanently faces away from Uranus, it also revealed clear deposits of carbon monoxide for the first time.
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“It just shouldn’t be there. You have to go down to 30 kelvins [minus 405 degrees Fahrenheit] “The carbon monoxide would have to be regenerated before the carbon monoxide would be stable,” Richard Cartwright, team leader at the Johns Hopkins Applied Physics Laboratory (APL), said in a statement. “There is no question that the carbon monoxide would have to be actively replenished.”
That’s because Ariel’s surface temperature is, on average, about 18 degrees Celsius (65 degrees Fahrenheit) warmer than this key temperature.
Cartwright acknowledges that radiolysis may be partly responsible for this regeneration. However, observations of the 1986 Voyager 2 flyby of Uranus and its moons and other recent discoveries suggest that the interactions causing radiolysis may be limited because the axis of Uranus’s magnetic field and the orbital plane of its moons are offset from each other by about 58 degrees.
This means that the majority of carbon/oxygen compounds observed on Ariel’s surface could be created by chemical processes in an ocean of liquid water trapped beneath the ice on Ariel.
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Once created in Ariel’s seepage ocean, these carbon oxides could then escape through cracks in the Uranian moon’s icy shell or could even be explosively ejected by powerful eruptive plumes.
A very shaded white and gray sphere
The most detailed photo of Ariel, a moon of Uranus, taken by Voyager 2 in 1986. (Image credit: NASA/JPL-Caltech)
Scientists have suspected for some time that Ariel’s cracked and scarred surface could indicate the presence of active cryovolcanoes, volcanoes that emit plumes of melting snow rather than lava. These plumes could be so powerful that they hurl material into Uranus’ magnetic field.
Most of the cracks and grooves observed on Ariel’s surface are on the side of the moon that does not face Uranus. If carbon dioxide and carbon monoxide are escaping from these elements onto the surface of Uranus’ moon, this could explain why these compounds are found in greater abundance on this back side of the icy body.
JWST also collected other chemical evidence for the existence of a subsurface liquid water ocean. Spectral analysis revealed the presence of carbonite minerals, salts created when rock meets and interacts with liquid water.
“If our interpretation of this carbonate feature is correct, then this will be a pretty important result because it means that the formation must have happened internally,” Cartwright said. “This is something we definitely need to confirm, either through future observations, modeling, or a combination of techniques.”
Uranus and its moons have not been visited by a spacecraft since Voyager 2 nearly four decades ago, and it wasn’t even the spacecraft’s primary mission. In 2023, the Planetary Science and Astrobiology Decadal Survey highlighted the need to prioritize a dedicated mission to the Uranian system.
Cartwright believes that such a mission would provide an opportunity to gather valuable information about Uranus and Neptune, the other ice giants in the solar system. Such a mission could also provide vital data about the other potentially oceanic moons in those systems. This information could then be applied to extrasolar planets, or “exoplanets,” beyond the solar system.
“All of these new discoveries underscore just how fascinating the Uranian system is,” said team member Ian Cohen, a scientist at NASA’s Applied Physics Laboratory. “Whether it’s unlocking the keys to the formation of the solar system, better understanding the planet’s complex magnetosphere, or determining whether these moons are potential ocean worlds, many of us in the planetary science community are eagerly awaiting a future mission to explore Uranus.”
The team’s research was published Wednesday (July 24) in The Astrophysical Journal Letters.
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