The surface composition of Enceladus: clues from the Ultraviolet

The reflectance of Saturnʹs moon Enceladus has been measured at far ultraviolet (FUV) wavelengths (115-190 nm) by Cassiniʹs UltraViolet Imaging Spectrograph (UVIS). At visible and near infrared (VNIR) wavelengths Enceladusʹ reflectance spectrum is very bright, consistent with a surface composed primarily of H2O ice. At FUV wavelengths, however, Enceladus is surprisingly dark - darker than would be expected for pure water ice. We find that the low FUV reflectance of Enceladus can be explained by the presence of a small amount of NH3 and a small amount of a tholin in addition to H2O ice on the surface.