2020ApJ...902L..38C

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.