Evidence for Sulfur-bearing Species on Callisto’s Leading Hemisphere: Sourced from Jupiter’s Irregular Satellites or Io?

We investigated whether sulfur-bearing species are present on the icy Galilean moon Callisto by analyzing eight near-infrared reflectance spectra collected over a wide range of sub-observer longitudes. We measured the band areas and depths of a 4 μm feature in these spectra, which has been attributed to sulfur dioxide (SO2), as well as carbonates, in previously collected data sets of this moon. All eight spectra we collected display the 4 μm band. The four spectra collected over Callisto's leading hemisphere display significantly stronger 4 μm bands compared to the four trailing hemisphere spectra (>3σ difference). We compared the central wavelength position and shape of Callisto's 4 μm band to laboratory spectra of various sulfur-bearing species and carbonates. Our comparison demonstrates that Callisto's 4 μm band has a spectral signature similar to thermally altered sulfur, as well as a 4.025 μm feature attributed to disulfanide (HS2). Our analysis therefore supports the presence of S-bearing species on Callisto but is not consistent with the presence of SO2. The significantly stronger 4 μm band detected on Callisto's leading hemisphere could result from collisions with H2S-rich dust grains that originate on Jupiter's retrograde irregular satellites or implantation of magnetospheric S ions that originate from volcanic activity on Io. Alternatively, S-bearing species could be native to Callisto and are exposed by dust collisions and larger impacts that drive regolith overturn, primarily on its leading side.