Analysis of Meteorite Spectra in the Mid-infrared

Introduction: Minerals have a variety of absorption features in the visible, near-infrared, and midinfrared wavelength regions [1,2]. Except for a few spectral studies in the mid-infrared such as the one done by Lim et al. [3], most asteroids have been only observed in the visible and near-infrared spectral regions due to the difficulty in obtaining reflectance spectra at longer wavelengths. The use of the Spitzer Space Telescope now makes it possible to collect and study asteroid spectra with very sensitive instruments over a broader wavelength range than can normally be observed on Earth [4]. To try to determine how well meteorites can be differentiated in the mid-infrared wavelength region, we are analyzing the spectral properties of meteorites from 0.3 to 25 μm. Samples: The meteorites in this study were mainly from the Smithsonian Institution’s Analyzed Meteorite Powder Collection (USNM 7073). The meteorites were originally ground into a powder so that they could be analyzed by wet chemistry for elemental concentrations [5]. Most of the meteorites are ordinary chondrites [6]. The samples were measured at Brown University’s Keck/NASA Reflectance Experiment Laboratory (RELAB). The resulting reflectance data is a combination of spectra taken with a bidirectional reflectance spectrometer for visible to near-infrared wavelengths and a Nicolet 870 Nexus FTIR spectrometer for nearto mid-infrared wavelengths. All spectra are publicly available on the RELAB website. Computational Technique: A computer program developed by Jonathan Leachman, an Associate Software Specialist in the Computer Science Computing Facility at the University of Massachusetts at Amherst, was used to determine the band minima in each meteorite spectrum. It was designed to search for ascending and descending number sequences and to determine the minimum band in each. The program accepts a number of parameters that narrow the minima result set, one of which is a “depth” parameter that defines the minimum magnitude of descension of a band. Figure 1. H, L, and LL chondrite spectral features. The band minima for H, L and LL chondrites occur at slightly different wavelengths.