Thermal infrared spectra of lunar soils

We have measured the infrared (2.08-14 mu m) directional hemispherical reflectance spectra of lunar soils representing the major lithologic units so far sampled on the lunar surface, and soils of different exposure ages within those units. Such reflectance (R) spectra can be used to calculate absolute emissivity (E) using Kirchhoff's Law (E = 1 - R). The effects of exposure age vary with wavelength region. In the 2-5 mu m and 8-14 mu m regions, lunar soils darken with exposure age, consistent with spectral behavior in the VNIR and the dominant optical effect of increasing amounts of finely divided metallic iron in more mature soils. However, in the 5-8 mu m region soils tend to show higher reflectances with greater exposure age, which suggests some unanticipated change in the optical properties of fine metallic iron at those wavelengths. The most useful spectral feature for compositional remote sensing is the Christiansen reflectance minimum (emissivity maximum), the spectral contrast of which is enhanced by the lunar environment, and the wavelength position of which can be related to composition without being much affected by exposure age. The vacuum environment at the lunar surface not only enhances the spectral contrast of the Christiansen feature, but also shifts it slightly to shorter wavelength, an effect that must be compensated for in inferring composition. By contrast with the Christiansen feature, the weak and relatively few overtone/combination tone absorption bands in the volume scattering region between 3 and 8 mu m appear to be of limited usefulness. The reststrahlen bands are also very weak in absolute emissivity spectra, and are evidently not enhanced by the lunar environment in the same fashion as the Christiansen feature. Thus, they can only be used for remote sensing with measurements of extraordinarily high signal-to-noise (1000/1). However, these features, as well as the transparency feature (which is particularly prominent in spectra of feldspathic soils), do contain important mineralogical information, such as the relative abundances of plagioclase and pyroxene, and can be used for laboratory studies of lunar soils. More certain and more quantitative mineralogical analyses of lunar soils appear feasible after additional spectral analysis of soil separates, and additional mineralogical analysis of soil samples for which spectral data are available. (C) 1997 Academic Press.