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We investigated whether sulfur-bearing species are present on the icyGalilean moon Callisto by analyzing eight near-infrared reflectance spectracollected over a wide range of sub-observer longitudes. We measured the bandareas and depths of a 4-micron feature in these spectra, which has beenattributed to sulfur dioxide (SO2), as well as carbonates, in previouslycollected datasets of this moon. All eight spectra we collected display the4-micron band. The four spectra collected over Callisto's leading hemispheredisplay significantly stronger 4-micron bands compared to the four trailinghemisphere spectra (> 3-sigma difference). We compared the central wavelengthposition and shape of Callisto's 4-micron band to laboratory spectra of varioussulfur-bearing species and carbonates. Our comparison demonstrates thatCallisto's 4-micron band has a spectral signature similar to thermally-alteredsulfur, as well as a 4.025 micron feature attributed to disulfanide (HS2). Ouranalysis therefore supports the presence of S-bearing species on Callisto butis not consistent with the presence of SO2. The significantly stronger 4-micronband detected on Callisto's leading hemisphere could result from collisionswith H2S-rich dust grains that originate on Jupiter's retrograde irregularsatellites or implantation of magnetospheric S ions that originate fromvolcanic activity on Io. Alternatively, S-bearing species could be native toCallisto and are exposed by dust collisions and larger impacts that driveregolith overturn, primarily on its leading side.

Evidence for sulfur-bearing species on Callisto's leading hemisphere: Sourced from Jupiter's irregular satellites or Io?