JWST peeks at Callisto’s ancient scars

Of Jupiter’s four Galilean moons, Callisto is the one that gets the least attention. Io is constantly being resurfaced by volcanoes. Europa has a giant liquid water ocean. And Ganymede has its own magnetic field that interacts with its parent planet in weird ways. Callisto, by comparison, seems sedate, with its ancient, crater-saturated surface seemingly frozen in time. But new data from the James Webb Space Telescope (JWST) shows that even this most benign of the Big Four moons is more active than previously realized.

The data, by an international team of astronomers led by Maria Camarca of Caltech, was based on JWST’s Near-Infrared Spectrograph (NIRSpec) instrument. It covered three major perspectives of Callisto itself—its trailing hemisphere, the giant Asgard impact basin and a view centered squarely on Valhalla, the largest multi-ring impact structure in the solar system. The paper was recently accepted for publication in the Planetary Science Journal and is available on the arXiv preprint server.

One notable feature in the data was water ice. NIRSpec picked up the 3.1 µm “Fresnel peak” indicative of ice crystals and mapped it onto the moon’s surface in unprecedented detail. The map revealed a notable divide between the leading and trailing hemispheres. On the leading hemisphere, the water ice directly tracks with geography. Bright impact basins like Valhalla and Asgard, along with younger craters like Lofn and Heimdall, show sharp spikes in water ice where ancient impacts excavated fresh, brighter material that contrasts sharply with the dark albedo of the rest of Callisto.

On the trailing hemisphere, on the other hand, the ice forms a distinct “bullseye” pattern. Its signature is weakest near the equatorial center and strengthens toward higher latitudes. Most likely, that pattern is shaped by plasma bombarding the moon’s surface from Jupiter’s magnetosphere. The charged particles that make up plasma sweep along behind Callisto, altering the structure of its ice in the process.

Another clear spectral feature was the 4.25 µm spectral band—representative of dry ice (i.e., frozen CO₂). The authors note that CO₂ shouldn’t naturally remain stable on Callisto’s surface without some sort of entrapment or replenishment mechanism. But the signal is there, clear as day, to anyone looking.

On the trailing hemisphere, solid CO₂ is concentrated at the center of the disk, forming the opposite pattern to that of the water ice. In fact, according to the paper, they were anti-correlated, supporting the idea that particle radiation actively converts water ice and carbon-rich grains into CO₂. On the leading hemisphere, the strongest CO₂ signal was found around the Lofn and Heimdall craters. Since these are relatively young, that deposit is most likely from the impact itself, making it the largest known reservoir of CO₂ not created by radiation.

CO₂ doesn’t just exist on Callisto’s surface, either. JWST detected a very faint atmosphere, primarily made up of CO₂, though it was extremely patchy. The highest concentration appeared around the Valhalla basin, but it doesn’t line up with the highest concentration of solid CO₂, nor does it align with the areas with the hottest temperatures. This illogical pattern mimics that seen on other icy moons such as Ganymede, highlighting the complex processes volatiles like water and CO₂ undergo and that we are still struggling to pin down in our models.

There was yet another signature of interest in the JWST data—a clear absorption feature at 4.57 µm. The current theory is that this is caused by carbon-nitrogen (CN)-bearing compounds, though its signal is notably stronger on Callisto than on other Galilean moons. How specifically those get to Callisto is also a matter for debate, though the leading theory is that the other irregular satellites in the system regularly rain down dust on Callisto’s leading hemisphere, where the nitrogen-rich minerals react with the native carbonaceous material to create organic compounds.

Obviously, even with data from the most powerful space-based telescope, we are still lacking in our understanding of some fundamental processes happening on this quietest of Galilean moons. But soon, we might get some better insight, as the Jupiter Icy Moons Explorer (JUICE) mission is on its way to the system. During planned flybys in the 2030s, JUICE will use its high-resolution camera and spectrometer to capture more detailed images of Callisto’s surface than have ever been possible before. As we continue to shine even more light on this moon, it could offer up even more insight into the differences among four of the most interesting moons in the solar system.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

Full profile →

Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

Full profile →