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 expo sure 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, con sistent with spectral behavior in the VNIR and the dominant optical ef fect of increasing amounts of finely divided metallic iron in more mat ure soils. However, in the 5-8 mu m region soils tend to show higher r eflectances with greater exposure age, which suggests some unanticipat ed change in the optical properties of fine metallic iron at those wav elengths. The most useful spectral feature for compositional remote se nsing is the Christiansen reflectance minimum (emissivity maximum), th e spectral contrast of which is enhanced by the lunar environment, and the wavelength position of which can be related to composition withou t being much affected by exposure age. The vacuum environment at the l unar surface not only enhances the spectral contrast of the Christians en feature, but also shifts it slightly to shorter wavelength, an effe ct that must be compensated for in inferring composition. By contrast with the Christiansen feature, the weak and relatively few overtone/co mbination tone absorption bands in the volume scattering region betwee n 3 and 8 mu m appear to be of limited usefulness. The reststrahlen ba nds are also very weak in absolute emissivity spectra, and are evident ly not enhanced by the lunar environment in the same fashion as the Ch ristiansen feature. Thus, they can only be used for remote sensing wit h measurements of extraordinarily high signal-to-noise (1000/1). Howev er, these features, as well as the transparency feature (which is part icularly prominent in spectra of feldspathic soils), do contain import ant mineralogical information, such as the relative abundances of plag ioclase and pyroxene, and can be used for laboratory studies of lunar soils. More certain and more quantitative mineralogical analyses of lu nar soils appear feasible after additional spectral analysis of soil s eparates, and additional mineralogical analysis of soil samples for wh ich spectral data are available. (C) 1997 Academic Press.