The James Webb Space Telescope has resolved a 40-year-old puzzle about the icy moons of Uranus, finding that carbon dioxide on their surfaces comes from two distinct sources rather than one. A team from Johns Hopkins University analyzed the data and found that roughly half the carbon dioxide is created by radiation from Uranus's magnetosphere, while the other half appears to originate from inside the moons.
The mystery dates back 40 years, to when Voyager 2 sent back images of Uranus's icy moons, Ariel, Umbriel, Titania and Oberon, showing canyons, smooth plains and possible signs of cryovolcanism. The probe could not determine what the surfaces were made of, and ground-based observatories spent decades trying and failing to pin down the moons' chemistry, because Earth's atmosphere blocks the key carbon dioxide spectral band needed to study it.
A spectrograph aboard the James Webb Space Telescope was able to observe that band without interference for the first time, giving scientists a clear answer.
Observing leading and trailing hemispheres
The Johns Hopkins team studied James Webb observations made in September 2023, examining the leading hemisphere, which always faces the direction of a moon's orbital motion, and the trailing hemisphere separately for each moon. The data showed that carbon dioxide is concentrated on the trailing sides, which absorb most of the flow of charged particles from Uranus's magnetosphere. Electrons, protons and heavy ions bombard water ice and carbon-bearing material in the moons' regolith, triggering a chemical process called radiolysis that produces carbon dioxide. The inner moons, Ariel and Umbriel, turned out to be far richer in carbon dioxide than the outer moons, Titania and Oberon.
Signs of an internal source
James Webb also picked up something ground-based telescopes had never detected. On the leading hemispheres of all four moons, where radiolysis is known to be weaker, the telescope clearly registered the key 4.27-micrometer carbon dioxide band. The spectra of Ariel and Umbriel also showed distinct lines of crystalline carbon dioxide ice, which forms only in thick deposits, along with a broad band at 4.02 micrometers that corresponds to carbonate minerals, the same kind found on the dwarf planet Ceres and Jupiter's moon Callisto. Carbonates form when carbon dioxide reacts with silicate rock in the presence of liquid water. On Ariel and Umbriel, the spectrometer also found signs of carbon dioxide clathrates, compounds in which carbon dioxide molecules are trapped inside cages of water ice that typically form under high pressure deep within a body.
A two-part cycle and hints of past water
Together, the findings point to a full cycle of carbon dioxide formation: a thin crust of fine-grained carbon dioxide frost covers a thicker layer of coarse-grained carbon dioxide ice mixed with clathrates. Some of the carbon dioxide migrates across the surface with seasonal heating, building up in cold traps near the equator. The researchers concluded that radiolysis alone cannot account for all the carbon dioxide observed, since the carbonates and clathrates are not confined to the trailing hemispheres alone, meaning a second source, carbon dioxide escaping from inside the moons, must also be at work.
If that interpretation holds, the interiors of Ariel and Umbriel once held liquid water, raising the possibility that conditions for prebiotic chemistry once existed in the Uranus system. A definitive answer would require an orbital mission carrying a mapping infrared spectrometer and a mass spectrometer, but until such a mission flies, the James Webb observations remain the best evidence for how the process works.
