JWST peeks at Callisto's ancient scars

The James Webb Space Telescope (JWST) has provided new insights into the ancient surface of Jupiter's moon Callisto, revealing that even this seemingly benign moon is more active than previously thought. The data, collected using the Near-Infrared Spectrograph (NIRSpec) instrument, covers Callisto's trailing hemisphere, the Asgard impact basin, and the Valhalla multi-ring structure.

One of the most notable findings is the presence of water ice, which is more abundant on the leading hemisphere where recent impacts expose fresh, brighter material. In contrast, the trailing hemisphere shows a distinct bullseye pattern of ice formation, likely shaped by plasma bombarding the moon's surface from Jupiter's magnetosphere.

Additionally, the presence of dry ice (frozen CO2) on the trailing hemisphere suggests that this moon undergoes complex processes involving the conversion of water ice and carbon-rich grains into CO2. The JWST also detected a weak atmosphere primarily composed of CO2, with the highest concentrations found around the Valhalla basin.

Furthermore, a clear absorption feature at 4.57 µm indicates the presence of carbon-nitrogen (CN)-bearing compounds, which could be derived from the dust raining down on Callisto from other irregular moons in the system. While these findings provide valuable insights into Callisto's surface ice and atmospheric composition, much remains unknown about the fundamental processes occurring on this moon.

Further observations from the upcoming Jupiter Icy Moons Explorer (JUICE) mission in the 2030s may offer even more detailed information about Callisto's surface, potentially shedding light on the differences among the most fascinating moons in our solar system.