REMOTE DETERMINATION OF EXPOSURE DEGREE AND IRON CONCENTRATION OF LUNAR SOILS USING VIS-NIR SPECTROSCOPIC METHODS

On the Moon, space weathering processes such as micrometeorite bombard ment alter the optical properties of lunar soils. As a consequence, lu nar soil optical properties are a function not only of composition, bu t of degree of exposure on the lunar surface as well. In order to accu rately assess the compositional properties of the lunar surface using remotely acquired visible and near-infrared spectroscopic data, it is thus necessary either (1) to compare optical properties only of soils characterized by similar degrees of exposure or (2) to otherwise norma lize or remove the optical effects due to exposure. Laboratory spectro scopic data for lunar soils are used to develop and test remote spectr oscopic methods for determining degree of exposure and for distinguish ing between the optical effects due to exposure and those due to compo sition. A method employing a ratio between reflectances within and out side of the 1 mu m Fe2+ crystal field absorption band was developed fo r remotely identifying highland soils that have reached a steady-state maturity. The relative optical properties of these soils are a functi on solely of composition and as such can be directly compared. Spectro scopic techniques for accurate quantitative determination of iron cont ent for lunar highland soils are investigated as well. It is shown tha t approximations of the 1 mu m Fe2+ absorption band depth using few to several channel. multispectral data or spectroscopic data of inadequa te spectral range cannot be used with confidence for compositional ana lysis. However, band depth measurements derived from continuum-removed high spectral resolution data can be used to calculate the weight per cent FeO and relative proportion of iron-bearing silicates in mature l unar highland and mare/highland mixture soils. A preliminary effort to calibrate telescopic band depth to laboratory soil measurements is de scribed. (C) 1994 Academic Press, Inc.