Callisto: New Insights from Galileo Disk-resolved UV Measurements

The entire set of observations from the Galileo Ultraviolet Spectrometer (UVS) is analyzed to look for spectral trends across the surface of Callisto, and to probe the spectral shapes in the near-UV. At low resolution, the leading hemisphere is slightly redder than the trailing hemisphere at λ > 280 nm; this has been interpreted by past researchers to indicate the presence of SO₂ on the leading hemisphere. Here we point out that such an "absorption feature" can be induced when ratioing hemispherical spectra. High-resolution observations are used to detect the presence of an absorption band at high southern latitudes, interpreted to be due to some organic species that is weathered away (carbonized) at lower latitudes. The presence of CO₂ in the surface and in the atmosphere of Callisto and the dark nature of the surface suggest that carbon-based species are present across the surface associated with either endogenic or delivered organics. These organics experience chemical modification by UV radiation and are mixed into the regolith by meteoritic bombardment.