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Spectral calibration for deriving surface mineralogy of Asteroid (25143) Itokawa from Hayabusa Near-Infrared Spectrometer (NIRS) Data

We present spectral calibration equations for determining mafic silicate composition of near-Earth asteroid (25143) Itokawa from visible/near-infrared spectra measured using the Near Infrared Spectrometer (NIRS), on board the Japanese Hayabusa spacecraft. Itokawa was the target of the Hayabusa sample return mission and has a surface composition similar to LL-type ordinary chondrites. Existing laboratory spectral calibrations use a spectral wavelength range that is wider (0.75-2.5 microns) than that of the NIRS instrument (0.85-2.1 microns) making them unfit for interpreting the Hayabusa spectral data currently archived in the Planetary Data System. We used laboratory measured near-infrared reflectance spectra of ordinary (H, L and LL) chondrites from the study of Dunn et al. (2010), which we resampled to the NIRS wavelength range. Using spectral parameters extracted from these resampled spectra we established a relationship between band parameters and their mafic silicate composition (olivine and low-Ca pyroxene). We found a correlation >90% between mafic silicate composition (fayalite and forsterite mol. %) estimated by our spectral method and X-ray diffraction (XRD) measured values. To test the validity of the newly derived equations we blind tested them using nine laboratory-measured spectra of L and LL type chondrites with known composition. We found that the absolute difference between the measured and computed values is in the range 0.1 to 1.6 mol. %. Our study suggests that the derived calibration is robust and can be applied to Hayabusa NIRS data despite its limited spectral range. We applied the derived equations to a subset of uncalibrated NIRS spectra and the derived fayalite and ferrosilite values are consistent with Itokawa having a LL chondrite type surface composition.