Global classification of lunar reflectance spectra obtained by Kaguya (SELENE): Implication for hidden basaltic materials

Abstract

A global classification map of lunar absorption spectra is developed under unified classification criteria by applying the K-means and ISODATA unsupervised classification methods. The spectra were obtained by the Spectral Profiler (SP) onboard the Kaguya (SELENE) lunar orbiter spacecraft. The spectra are mostly categorized into seven spectral groups by the K-means method: K1, K2, K3, K4, K5, K6, and K7. Then, each K-means group is divided into 66 total classes by the ISODATA method. The K1 and K3 groups are classified into 21 and 14 classes, respectively, for which the average spectra are high-calcium pyroxene (HCP)-abundant compared to the other groups. Both groups are mainly located on maria and pyroclastic deposits. However, while the K1 group can be found primarily near the center of large maria, the K3 group can be found mainly at small maria and the margins of large maria. The K2 group contains eight classes and shows average spectra, such as low-calcium pyroxene (LCP), in abundance, which are found mainly on the South Pole-Aitken (SPA) basin and on the coast of Mare Frigoris. The K5 (eight classes), K6 (two classes) and K7 (three classes) groups indicate average spectra with shallower absorption (between 0.9 µm and 1.2 µm wavelength) than those of the K1, K2, and K3 groups. These groups are located in highland regions, and the K5 group is located in high-albedo regions, such as fresh craters. The K6 group is found on nearside highland areas and the K7 group is found on farside highland areas. The K4 group with nine classes shows average spectra, such as mixtures of K1/K2/K3 and K5/K6/K7, and these classes are located around boundaries between mare/SPA and highland. In addition, one class is independently defined without using K-means and ISODATA because its spectra are unsuitable for automatic unsupervised classification. However, this class is still geologically meaningful.The average spectra of the ISODATA classes show not only the characteristics of the major host material belonging to a spectral group, but they also show the small influence of materials from other groups in nearby regions on that host material. This influence is considered to be caused by horizontal material mixing of materials in that location with the ejecta from nearby regions. However, occasionally the ISODATA classes of the K4, K5, and K6 groups influenced by the K1/K3 groups are clustered geographically in regions far from the K1/K3 regions. Some locations of the clusters correspond to locations of the well-known cryptomaria and small pyroclastic deposits. On the other hand, some locations of them also contain candidates of hidden subterranean basaltic materials, such as undiscovered cryptomare, dike, and sill estimated based on recent lunar gravitational anomalies. This agreement suggests to be appeared hidden basaltic materials by the material mixing occurring vertically. The total area of the clusters except for known cryptomaria is approximately two times larger than that of the known cryptomaria, and this area reaches approximately 20% of the total area of exposed maria. The total volume of hidden basaltic materials, except for the known cryptomaria, is estimated to be 104 to 106 km3, depending on assumptions of the relative abundance between cryptomare and dike/sill. This volume is comparable to that of the known cryptomaria and approximately 10% of that of the exposed mare.