JWST peeks at Callisto's ancient scars

The James Webb Space Telescope (JWST) has uncovered surprising details about Callisto's surface, revealing that it is more active than previously thought. The data, collected by an international team led by Maria Camarca of Caltech, used the JWST's Near-Infrared Spectrograph (NIRSpec) instrument. The study focused on three key areas: the trailing hemisphere, the giant Asgard impact basin, and the Valhalla multi-ring impact structure. The findings included the presence of water ice and dry ice (CO2) on Callisto, with distinct patterns on the leading and trailing hemispheres.

The data showed that water ice is concentrated on the leading hemisphere, where it tracks with the geography, and is found in bright impact basins like Valhalla and Asgard. On the trailing hemisphere, the ice forms a "bullseye" pattern, with the strongest signal near the equatorial center and strengthening toward higher latitudes. The study also detected CO2, which is concentrated at the center of the trailing hemisphere and appears to be formed by particle radiation. The CO2 signal is anti-correlated with the water ice signal, suggesting that radiation converts water ice and carbon-rich grains into CO2.

The JWST data also revealed a clear absorption feature at 4.57 µm, possibly from carbon-nitrogen (CN)-bearing compounds. The signal is stronger on Callisto than on other Galilean moons, though the exact mechanism of how these compounds reach Callisto is still debated. The study highlights the complex processes involving volatiles like water and CO2 on Callisto, and suggests that the JUICE mission will provide more detailed images and data to further understand these processes.

The findings indicate that Callisto's surface is more dynamic than previously thought, with active processes involving water and CO2. The data also shows that the moon's surface is not uniformly covered in ice or CO2, with patterns that suggest the influence of particle radiation and the presence of other volatiles. The study underscores the need for further research to better understand the complex interactions between the moon's surface and its environment.

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