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

TL;DR

This study develops spectral calibration equations tailored for Hayabusa NIRS data to accurately determine the surface mineralogy of asteroid Itokawa, overcoming previous wavelength range limitations and confirming its LL chondrite-like composition.

Contribution

The paper introduces new spectral calibration equations specifically designed for Hayabusa NIRS data, enabling precise mineralogical analysis of Itokawa's surface from limited spectral range data.

Findings

Calibration equations show >90% correlation with XRD measurements.

Blind tests yield composition estimates within 0.1 to 1.6 mol. %.

Application confirms Itokawa's LL chondrite surface composition.

Abstract

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.