What is the origin and composition of Callisto's 4-micron band?

Spectra collected by the Near Infrared Mapping Spectrometer (NIMS) onboard the Galileo spacecraft determined that a variety of near-infrared absorption bands overprint the dominantly 'dirty' H₂O ice spectral signature of Callisto's surface. One such absorption band centered near 4 µm has been attributed to the presence of SO₂ on Callisto, as well as on Europa and Ganymede. This 4-µm feature is thought to originate from S-rich species that are erupted from volcanoes on Io, ionized, and subsequently transported in Jupiter's co-rotating plasma to the three icy Galilean moons. This process can explain the presence of the 4-µm feature on the trailing hemispheres of Europa and Ganymede. On Callisto, however, the 4-µm band is stronger on its leading hemisphere, hinting at a different origin for this band and/or different contributing species, possibly including carbonates. Complicating matters, the often low signal-to-noise and low resolving power (R ~40-200) of spectra collected by NIMS has limited our ability to interpret the origin and composition of the 4-µm feature.

To further investigate Callisto's 4-µm band, we collected new near-infrared spectra with the SpeX spectrograph on NASA's Infrared Telescope Facility (~1.9-5.3 µm, R ~2500), representing a significant improvement over the quality of the available NIMS spectra. These SpeX spectra show that the 4-µm feature is significantly stronger on Callisto's leading hemisphere, supporting prior analysis of the distribution of the 4-µm feature detected by NIMS. The central wavelength position of the feature we detected is shifted to 4.02 µm, whereas the SO₂ combination band (ν₁ + ν₃) detected on Io is centered near 4.07 µm, casting doubt on SO₂ as the primary constituent contributing to Callisto's 4-µm band. Our results are more consistent with other S-bearing species like H₂S₂ or HS₂, which could be formed by charged particle radiolysis of hydrogen sulfide (H₂S). Additionally, the spectral signature of carbonates like Na₂CO₃ can also provide a good match to the 4-µm feature. The clear hemispherical asymmetry in the distribution of the 4-µm band is consistent with in-falling dust grains from Jupiter's retrograde irregular satellites, which should contribute material primarily to Callisto's leading side. Alternatively, the species contributing to Callisto's 4-µm band could be native to this moon and are exposed by dust particle collisions that spur regolith overturn, as well as larger impact events that sample greater depths into its subsurface.