An Overlooked Galilean Satellite
Among the four large moons discovered by Galileo, Callisto often receives less attention because its surface appears heavily cratered and seemingly unchanged. Unlike Io’s constant volcanic resurfacing, Europa’s probable subsurface ocean, or Ganymede’s intrinsic magnetic field, Callisto shows little sign of recent geological activity and has been viewed as a preserved record of early solar system bombardment.
Infrared Observations with JWST
Using the Near-Infrared Spectrograph (NIRSpec) and Near-Infrared Camera (NIRCam) aboard the James Webb Space Telescope, scientists obtained reflectance spectra from 0.6 to 5 micrometres. The data reveal localized increases in reflectance consistent with fresh water ice deposits and possible alteration of the regolith in small patches. These signals appear in regions previously considered ancient and unmodified.
Implications for Planetary Science
The findings suggest that even heavily cratered moons can experience low‑level processes such as micrometeorite impacts or subtle thermal cycling. This challenges the notion of Callisto as a completely static world and provides valuable constraints for models of surface evolution on icy satellites throughout the outer solar system.