Using the James Webb Space Telescope (JWST), astronomers discovered that Ariel, a moon of Uranus, may be hiding in a buried ocean of liquid water.
The discovery may provide an answer to a mystery surrounding this moon of Uranus that has baffled scientists: the fact that Ariel’s surface is covered in a significant amount of carbon dioxide ice. This is puzzling because at the distance Uranus and its moons exist from the sun, 20 times farther from the sun than Earth is, carbon dioxide turns to gas and is lost to space. This means that some process must freshen 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, which provide ionizing radiation, breaking down the molecules and leaving carbon dioxide, a process called “radiolysis.”
But new evidence from JWST suggests that the source of this carbon dioxide may not come from outside Ariel, but from inside it, possibly from a buried subsurface ocean.
Connected: “Traffic jams” around Uranus may reveal the mystery of its faint radiation belts
Because chemical elements and molecules absorb and emit light at characteristic wavelengths, they leave individual “fingerprints” on spectra. The team behind this discovery used JWST to collect spectra of light from Ariel, which helped them paint a picture of the chemical composition of Uranus’ moon.
Comparing this to simulated spectra of a chemical mixture in the laboratory here on Earth revealed to the team that Ariel has some of the richest carbon dioxide deposits in the Solar System. Not only did this add an extra 10 millimeters (0.4 in) of thickness to the ice on the side of tidally locked Ariel that permanently faces Uranus, it also revealed clear carbon monoxide deposits for the first time.
“It just shouldn’t be there. You need to get down to 30 kelvin [minus 405 degrees Fahrenheit] before the carbon monoxide stabilizes,” team leader Richard Cartwright of the Johns Hopkins Applied Physics Laboratory (APL) said in a statement. “The carbon monoxide will have to be actively replenished, no doubt.”
That’s because Ariel’s surface temperature averages about 65 degrees Fahrenheit (18 degrees Celsius) higher than this key temperature.
Cartwright acknowledges that radiolysis may explain some of this replenishment. However, observations from the Voyager 2 flyby of Uranus and its moons in 1986 and other recent discoveries suggest that the interactions behind the radiolysis may be limited because the axis of Uranus’ magnetic field and the orbital plane of its moons are shifted by one relative to another by about 58 degrees.
This means that most of the carbon/oxygen compounds observed on Ariel’s surface could have been created by chemical processes in a liquid water ocean trapped beneath Ariel’s ice.
Cool customer Ariel can have a volcanic temper
Once created in Ariel’s seeping water ocean, these carbon oxides could then escape through cracks in the icy mantle of Uranus’ moon, or could even be explosively ejected by powerful eruptive jets.
For some time, scientists have suspected that Ariel’s cracked and scarred surface may indicate the presence of active cryovolcanoes, volcanoes that spew jets of icy slush rather than lava. These jets may be so powerful that they eject material into Uranus’ magnetic field.
Most of the cracks and furrows seen on Ariel’s surface are located on the side of the moon that faces Uranus. If carbon dioxide and carbon monoxide are leaking from these elements onto the surface of Uranus’s moon, this may explain why these compounds are found in greater abundance on this back side of the icy body.
JWST also gathered more chemical evidence for a subsurface liquid water ocean. Spectral analysis hinted at 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 is a pretty big result because it means it must have formed in the interior,” Cartwright explained. “This is something we absolutely need to confirm, either through future observations, modeling or some combination of techniques.”
Uranus and its moons haven’t been visited by a spacecraft since Voyager 2 nearly four decades ago, and that wasn’t even the spacecraft’s primary mission. In 2023, the Decadal Survey of Planetary Science and Astrobiology highlighted the need to prioritize a dedicated mission to the Uranus system.
Cartwright believes such a mission would provide an opportunity to gather valuable information about Uranus and Neptune, the other ice giant in the Solar System. Such a mission could also deliver vital data on the other potentially oceanic moons of these systems. This information can then be applied to extrasolar planets or “exoplanets” outside the Solar System.
“All of these new insights underscore just how fascinating the Uranus system is,” said team member and NASA Applied Physics Laboratory scientist Ian Cohen. “Whether it’s unlocking the keys to how the solar system formed, 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 really looking forward to future mission to explore Uranus.”
The team’s research was published Wednesday (July 24) in The Astrophysical Journal Letters.
