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Using the James Webb Space Telescope (JWST), astronomers discovered that Ariel, the moon of Uranus, may be hidden in an ocean of liquid water underground.
The discovery could provide an answer to a mystery related to this Uranian moon that has puzzled scientists: that Ariel’s surface is covered with a significant amount of carbon dioxide ice. This is strange because Uranus and its moons are so far from the sun, 20 times further from the sun than Earth, that carbon dioxide turns to gas and is lost to space. This means that some process must renew the carbon dioxide on Ariel’s surface.
Previous theories 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 known as “radiation.”
However, new evidence from the JWST suggests that the source of this carbon dioxide could not come from outside of Ariel but rather from within, possibly from a deep underground ocean.
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Because chemical elements and molecules absorb and emit light at characteristic wavelengths, they leave individual “fingerprints” on spectra. The team behind this discovery used the JWST to collect light spectra from Ariel, which helped them paint a picture of the chemical composition of the moon Uranus.
Comparing this with simulated spectra from a chemical mixture in the laboratory here on Earth, the team was told that Ariel has some of the richest deposits of carbon dioxide in the solar system. Not only did this add an extra 10 millimeters (0.4 inches) to the ice on the tide-locked side of Ariel that is permanently moving away from Uranus, but it also revealed clear deposits of carbon monoxide for the first time.
“It shouldn’t be there. You have to reach 30 kelvin [minus 405 degrees Fahrenheit] before carbon monoxide stall,” team leader Richard Cartwright of the Johns Hopkins Applied Physics Laboratory (APL) said in a statement.
That’s because Ariel’s surface temperature is, on average, about 65 degrees Fahrenheit (18 degrees Celsius) warmer than this key temperature.
Cartwright acknowledges that some of this replenishment may involve radiology. However, observations from Voyager 2’s 1986 flyby of Uranus and its moons and other recent findings have suggested that the interactions behind the radiology may be limited because the axis of Uranus’ magnetic field and the orbital plane of its moons offset about 58 degrees apart.
That means that most of the carbon/oxygen compounds seen on Ariel’s surface could have been created by chemical processes in an ocean of liquid water trapped under ice on Ariel.
Cool customer Ariel may have a volcanic temper
Once formed in Ariel’s walking water ocean, these carbon oxides could then escape through cracks in the icy shell of the Uranian moon or even be ejected explosively in powerful pressure plumes.
Scientists have suspected for some time that Ariel’s cracked and scarred surface could indicate the presence of active cryovolcanoes, volcanoes that push out plumes of icy ice rather than lava. These plumes could be so powerful that they send matter into Uranus’ magnetic field.
Most of the cracks and grooves seen on Ariel’s surface are located on the side of the moon facing Uranus. If carbon dioxide and carbon monoxide are leaking from these features to the surface of the Uranian moon, this could explain why these compounds are found more on this side of the icy body.
The JWST also picked up more chemical evidence of a subsurface liquid water ocean. Spectral analysis revealed the presence of carbonate minerals, salts formed when rock meets and interacts with liquid water.
“If our interpretation of that carbonate feature is correct, that’s a pretty big find because it means it had to form internally,” Cartwright explained. “That is something we need to fully confirm, through future observations, modeling, or through certain techniques.”
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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 Planetary Science and Astrobiology decennial survey highlighted the need to prioritize a dedicated mission to the Uranian system.
Cartwright believes such a mission would provide an opportunity to gather valuable information about Uranus and Neptune, the other ice giants of the solar system. Such a mission could also provide vital data about the other moons that may be oceans of these systems. This information could then be applied to extrasolar planets, or “exoplanets,” outside the solar system.
“All these new insights emphasize just how robust the Uranian system is,” said NASA Applied Physics Laboratory staff member and scientist Ian Cohen. “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 life, many of us in the science community planetary looking forward to a future mission to explore Uranus.”
The team’s research was published on Wednesday (July 24) in The Astrophysical Journal Letters.
