Characterization of D‐Type Spectra Based on Hyperspectral Remote Sensing of the Lunar Surface

We surveyed dark, red‐sloped, and featureless visible and near‐infrared spectra on the lunar surface, similar to those of D‐type asteroids. Using the hyperspectral data obtained by Spectral Profiler onboard SELENE, we examined the distribution of sites showing low albedo and high spectral redness, irrespective of absorption features. We found that regions around several dark mantle deposits, such as Sinus Aestuum, Rimae Bode, Vaporum, Sulpicius Gallus, Oppenheimer, and Taurus Littrow, are the darkest, reddest sites on the lunar surface. In addition, dark mare regions (albedo < 4%) are shown to be high in TiO 2 abundance, while red mare regions (spectral redness > 2.3) are low in TiO 2 abundance. Interestingly, according to the criterion of asteroidal taxonomic classification used in previous studies, most of these geological features are classified as D‐type, which are characterized by featureless spectra, although they show absorption bands at visible, 1 μm, and/or 2 μm regions. This is because the asteroidal taxonomic classification relies on a small number of spectral bands of scaled reflectances, while the presence of absorption bands should be identified based on the continuum‐removed reflectance of the hyperspectral data. Thus, our results suggest that the D‐type group classified in previous studies consists not only of primitive objects of carbonaceous chondritic materials but also differentiated objects composed of rocky materials that may be found on the Moon. To provide better constraints for D‐type bodies regarding their composition from remote‐sensing data, a quantitative assessment based on an analysis of the continuum‐removed reflectance of hyperspectral data may be required.