Discussion of the 1.04-mu m water ice absorption band in the Europa NIMS spectra and a new NIMS calibration

Measurements made by the Galileo Near Infrared Mapping Spectrometer (NIMS) have been used to calculate reflectance spectra for regions of Europa. Many of these spectra show features indicative of water ice. Others show asymme tric H2O absorption features suggesting hydrated non-ice constituents such as hydrated sulfates and carbonates. Often, both types of features are pres ent. However, some previously published NIMS spectra with strongly develope d ice features did not show the expected 1.04-mu m ice absorption band. Thi s discrepancy has led to questions regarding earlier interpretations of Eur opa's spectral features. Recent work reveals that unexpected changes have o ccurred in the spectral range covered by each NIMS detector and that an add itional calibration is required. Preliminary calibration values are given. When this new calibration is applied, the 1.04-mu m feature emerges clearly in those spectra where it was expected but absent previously, thereby reso lving the 1.04-mu m band discrepancy.