Visible and near-infra-red transmission and reflectance measurements of the Luna 20 soil

Abstract

Visible and near-infra-red spectra of chemically analyzed grains of glass and minerals from the Luna 20 sample were compared with diffuse reflectance spectra of the bulk soil. As in the spectra of soil samples from other localities on the Moon, pyroxene contributes two broad absorption features near 1 μm and 2 μm. The soil has a high integral reflectance (or albedo) arising from plagioclase, which appears to be the dominant mineral in the lunar highlands. The Luna 20 soil curve is most similar to the reflectance curves of the non-rayed soils at Apollo 16, in agreement with the generally similar mineralogy of these samples. The average pyroxene composition in the Luna 20 soil, as determined from the absorption bands in the diffuse reflectance spectra, and analyses of single crystals, is more calcic than in the lithic fragments. Thus, the soil appears to have a few per cent of admixed material derived from mare basalts. Comparison of the soil spectrum with telescopic curves of nearby areas reveals a close similarity; however, the Luna 20 sample is slightly less mature than expected. Luna 20 may have sampled subsurface material that is fresher than the regional surface soil, or alternatively, the Luna 20 area may contain an admixture of relatively recently exposed material from a ray crater.