  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-micron feature in these spectra, which has been
attributed to sulfur dioxide (SO2), as well as carbonates, in previously
collected datasets of this moon. All eight spectra we collected display the
4-micron band. The four spectra collected over Callisto's leading hemisphere
display significantly stronger 4-micron bands compared to the four trailing
hemisphere spectra (> 3-sigma difference). We compared the central wavelength
position and shape of Callisto's 4-micron band to laboratory spectra of various
sulfur-bearing species and carbonates. Our comparison demonstrates that
Callisto's 4-micron band has a spectral signature similar to thermally-altered
sulfur, as well as a 4.025 micron 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-micron
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.
