Laboratory Analysis (Reflectance Spectroscopy) of Terrestrial Analogues

Abstract

clinopyroxene, olivine, and plagioclase, are the most important constituents of the lunar surface, associated with oxides and rare apatite (e.g., Papike et al. 1991). Though olivine and pyroxene show clear spectral signature and well-defined crystal field absorption bands in the visible and near-infrared (Burns 1993), plagioclase is difficult to recognize, due to very low iron content in its crystal structure. In fact, even if it is widely acknowledged that plagioclase is one of the most important constituents of the lunar surface (Heisenger and Head 2006), its presence has been usually related to featureless spectra and interpreted as shocked plagioclase (Spudis et al. 1984; Bussey and Spudis 2000). Only recently, the spectrometers on board lunar missions (Spectral Profiler (SP), onboard Selene, and Moon Mineralogy Mapper (M3), onboard Chandrayaan), with very high spectral (6–8 and 10 nm, respectively) and spatial (500 and 100 m, respectively) resolution, recognize regions composed of crystalline plagioclase, detecting the plagioclase absorption band in the 1,250 nm spectral region (Ohtake et al. 2009; Pieters et al. 2009; Cheek et al. 2012). Analyzing the plagioclase absorption band depth, Ohtake et al. (2009) recognized areas dominated by plagioclase (plagioclase >98 %), defined pure anorthosite (PAN) regions, mostly in crater central peaks. However, to relate plagioclase absorption band to modal abundance and mineralogical composition can be a difficult task. In fact, on the Moon, several factors such as the mineral chemistry, the presence of different minerals that absorb in a narrow spectral range, the particle size, the space weathering, etc., act in unpredictable ways on the reflectance spectra. For these reasons, studying terrestrial analogues can be fundamental in order to analyze separately the different factors and then superimpose effects to each other.