We have utilized telescopic near‐infrared spectra and multispectral images of the Moon provided by the Galileo and Clementine missions to determine the distribution and modes of occurrence of pure anorthosite. Anorthosites have now been identified in all portions of the nearside, including the site of the putative Procellarum basin. Anorthosite is associated with the rings of Orientale, Grimaldi, Humorum, Nectaris, Nubium, Mutus‐Vlaq, and Balmer basins. Major portions of the inner rings of Grimaldi, Humorum, Crisium, Orientale, and Nectaris are composed of pure anorthosite. The large spatial extent of these anorthosites appears to rule out an origin in the upper portions of discrete differentiated Mg‐suite plutons. In every instance, the anorthosites were exposed from beneath a shallower near‐surface layer of more pyroxene‐rich material. More mafic material also occurs beneath the pure anorthosite unit. Large expanses of the northern farside exhibit very low FeO values. This region contains abundant anorthosite and stands in stark contrast to the mafic composition exhibited by the interior of the South Pole‐Aitken basin (SPA). The distribution of compositional units on large portions of the lunar farside as well as the southern portion of the lunar nearside appears to be largely attributable to the SPA impact event. The distribution and modes of occurrence of anorthosites clearly indicate that a thick, global layer of anorthosite is present at various depths beneath most portions of the lunar surface. This anorthosite layer dominated the upper portion of the primordial crust and was produced by plagioclase flotation in the global magma ocean.

Abstract. We have used near-inftarcd reflectance spectra and Earth-based radar data to investigate the composition and origin of the various geologic units northwest of Humorum basin as well as the stratigraphy of the Humorum pre-Ímpact target site. The results of our spectral analysis indicate that at least a portion of the inner, m are-bounding ring is composed of pure anorthosite. Other highlands units in the region are dominated by noritic anorthosite.

We have acquired both an extensive data base of visible and near‐infrared spectra and multispectral images (ultraviolet and visible) of the Orientale region of the Moon. Our results show that the eastern Inner Rook Mountains are composed of anorthosite. Portions of the main ring of the Humorum basin and the inner ring of Grimaldi are also composed of anorthosite. Other deposits within the Orientale basin and the major ejecta unit outside the basin are dominated by noritic anorthosites; mature surfaces have spectra nearly identical to those taken of areas in the vicinity of the Apollo 16 site. Thus, it appears that Orientale ejecta are more mafic than materials that are thought to have originated at depth (i.e., the anorthosites in the Inner Rook Mountains). There are large areas that have mare basalt signatures mixed with highland rock types, indicating the existence of ancient, pre‐Orientale mare volcanism. Present mare surfaces in the Orientale region contain basalts of intermediate to very low TiO2 content.