Breaking the Ice: Origins of Carbon Oxides on Uranian Moons

For decades, astronomers have tried to decipher the source of carbon oxides on Uranus’s largest moons from ground-based telescopes. A recent study has leveraged JWST spectroscopy and laboratory experiments to identify carbon oxides and their origins for Ariel, Umbriel, Titania, and Oberon.

Unknown Origins of Carbon Oxide Enrichment

Forty years ago, the Voyager 2 spacecraft flew past Uranus and its twenty-seven moons, sending back prized portraits of the planet’s large and tidally locked satellites Miranda, Ariel, Umbriel, Titania, and Oberon. These images revealed extensive valleys, smooth plains, deep canyons, and possible icy volcanic (cryovolcanic) activity across the surfaces of the icy moons — the telltale signs of intense geologic activity. While Voyager 2 could take spectacular pictures, the spacecraft was not capable of identifying the molecules and materials making up these active surfaces.

In the decades that followed, astronomers have used ground-based near-infrared spectroscopy of these moons to investigate their chemical compositions. These observations have detected weak carbon dioxide and hints of carbon monoxide ice features on the moons with clear spatial trends across the moons’ surfaces, but the origins of these carbon oxides remain unknown. There are two potential explanations for the presence of these molecules: 1) carbon oxides are native to the Uranus system, present from formation, or 2) they formed via radiolysis, in which charged particles trapped in Uranus’s magnetic field interact with water ice and carbon-bearing molecules on the moons’ surfaces.

Ground-based detections are limited due to contamination from Earth’s atmosphere as well as the overall weakness of the carbon dioxide spectral features, thus space-based near-infrared observations of the Uranian moons are necessary to better constrain the carbon oxides on the satellites. Luckily, JWST’s Near-Infrared Spectrograph (NIRSpec) now offers astronomers unimpeded access the moons’ carbon oxide spectral features.

Chemical Clues from Space and in the Lab

In 2024, a study acquired NIRSpec observations of Uranus’s moon Ariel, revealing prominent carbon dioxide features and confirming the presence of carbon monoxide ice hinted at by ground-based studies. Expanding on these observations to include Umbriel, Titania, and Oberon alongside Ariel, Richard J. Cartwright (Johns Hopkins University) and collaborators obtained JWST NIRSpec spectroscopy of Uranus’s four largest moons in hopes of disentangling the origin story of their carbon oxides.

In addition to the JWST spectra, Cartwright and team performed laboratory experiments measuring the spectral properties of carbon dioxide ice over a range of very low temperatures (tens of kelvin). These experiments provided a better understanding of the spectral properties and nature of carbon dioxide on the surfaces of the Uranian moons. Comparing the JWST spectra with the laboratory spectra, the authors found that Ariel, Umbriel, Titania, and Oberon exhibit a variety of spectral features resulting from carbon dioxide ice, carbon monoxide ice, and other carbon-bearing molecules. The presence of these molecules varies between the leading and trailing hemispheres of the moons, these variations appearing strongest in Ariel and Umbriel. The carbon dioxide and carbon monoxide spectral features are stronger on the trailing hemispheres, decreasing with increasing distance from Uranus for all four moons.

Carbon Oxide Origins

What does this mean for the origins of the carbon oxides on the Uranian moons? Spectral features of carbonate minerals and the widespread presence of carbon dioxide throughout the Uranus system suggest that these compounds exist natively. On the other hand, the concentration of carbon oxide features on the trailing hemispheres supports radiolytic production as these hemispheres are bombarded with more ionizing particles from Uranus’s magnetosphere.

Based on these results, the authors have postulated that both origin scenarios, native presence and radiolysis, are required to fully explain the presence of carbon oxides. Future laboratory studies and JWST observations will continue to reveal the nature and composition of bodies in the Uranus system, furthering our understanding of our solar system siblings.

Citation

“Tracing the Source of Carbon Oxides on the Large Moons of Uranus,” Richard J. Cartwright et al 2026 Planet. Sci. J. 7 199. doi:10.3847/PSJ/ae8818