Webb Telescope Finds Carbon Dioxide Near Callisto Craters

A team of scientists at the California Institute of Technology (Caltech) has produced new infrared maps of the surface of Callisto, the outermost of Jupiter’s four large moons, based on observations made with the NIRSpec spectrograph aboard the James Webb Space Telescope. The maps reveal a reserve of solid carbon dioxide around two younger craters that, by all indications, was not produced by radiation, along with a finding that water ice is distributed in completely different patterns across the moon’s two hemispheres. The research is led by Maria Camarca, and the paper has been accepted for publication in the Planetary Science Journal.

Of the four Galilean moons, Callisto has long been considered the quietest. Io’s surface is constantly reshaped by volcanoes, Europa hides a liquid water ocean beneath its ice, and Ganymede has its own magnetic field. Callisto, its surface blanketed in ancient craters, has looked frozen in time by comparison. The preprint of the study, posted to arXiv on August 4, 2026 under the designation 2608.04272, builds on earlier measurements of carbon dioxide and spectral signatures published in 2024 by a team led by Richard Cartwright, now adding a new observation centered on Valhalla, the largest multi-ring impact basin in the solar system. The new paper’s co-authors also include Katherine de Kleer, Geronimo Villanueva, Bryan Holler, Zachary Milby, Kevin Hand and Lorenz Roth.

The observations covered three main views of the moon: the trailing hemisphere, which faces backward relative to Callisto’s motion around Jupiter, the large impact basin Asgard, and a view centered on Valhalla. Water ice was mapped using the so-called Fresnel peak at a wavelength of 3.1 microns, under the simplifying assumption that the ice is in crystalline form. Solid carbon dioxide was tracked using the absorption band at 4.25 microns.

Ice and Carbon Dioxide in Opposite Patterns

On the leading hemisphere, which faces the direction of the moon’s orbital motion, water ice follows the terrain. The bright impact basins Valhalla and Asgard, along with the younger craters Lofn and Heimdall, show pronounced ice signatures where older impacts exposed fresh, brighter material beneath Callisto’s dark surface.

On the trailing hemisphere, the pattern is different. There, ice forms a bullseye-like ring, weakest at the equator and strengthening toward higher latitudes. The authors link this distribution to plasma from Jupiter’s magnetosphere, which bombards the moon’s trailing side as it orbits and alters the structure of the ice.

Carbon dioxide behaves in the opposite way to water ice on the trailing hemisphere: solid CO2 is most pronounced at the center of the disk, precisely where the ice is weakest. According to the paper’s data, the two distributions are anticorrelated, supporting the idea that particle radiation converts water ice and carbon-rich material into carbon dioxide. On the leading hemisphere the picture is different: the strongest solid CO2 signal was found around the craters Lofn and Heimdall. Since those craters are relatively young, this reserve most likely came directly from the impacts themselves, making it the largest known deposit of carbon dioxide on Callisto that was not produced by radiolysis.

A Faint Atmosphere That Doesn’t Match the Ice

Carbon dioxide is not confined to the surface. NIRSpec also detected a very faint atmosphere around Callisto, made up mostly of CO2 and distributed in patches. The highest concentration of gaseous carbon dioxide was measured near the Valhalla basin, but it does not line up with either the areas of greatest solid CO2 or the warmest parts of the surface. A similar, hard-to-explain pattern has previously been recorded on Ganymede, which the authors say shows how poorly existing models still capture the movement of volatile compounds like water and carbon dioxide.

The data also revealed an absorption line at 4.57 microns, attributed to compounds containing a carbon-nitrogen bond. That signal is stronger on Callisto than on the other Galilean moons. How the compound reaches the surface is still debated, and one hypothesis is that Jupiter’s irregular satellites continually shed dust onto Callisto’s leading hemisphere, where nitrogen-rich minerals react with carbon already present on the surface to form organic compounds.

What These Measurements Don’t Prove

The sample behind this study consists of three NIRSpec observations, aimed at three different parts of Callisto’s surface, not continuous coverage of the entire moon. The water-ice map relies on a simplified assumption of crystalline ice, so the ice’s actual composition may be more complex than the spectrum alone suggests. The link between plasma from Jupiter’s magnetosphere and the ring-shaped ice pattern on the trailing hemisphere is a hypothesis the authors consider the most likely explanation, not a proven mechanism, since the study does not directly measure the particle flux that would confirm that connection. The same caveat applies to the origin of the nitrogen compounds at 4.57 microns, where the authors list dust from the irregular moons as the leading possibility but not the only one.

The mismatch between the highest concentration of gaseous CO2 and the warmest parts of the surface remains unexplained, something the authors openly note as a sign that models of gas evaporation and binding on icy moons are still incomplete. More detailed answers may have to wait for the European Space Agency’s Jupiter Icy Moons Explorer mission, which is expected to reach the system and, during flybys in the 2030s, image Callisto’s surface with a camera and spectrometer of higher resolution than has been possible so far.