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C.A. Peterson, B.R. Hawke, P.G. Lucey, J.G. Taylor, D.T. Blewett (U. of Hawaii/HIGP), P.D. Spudis (LPI)
Evidence strongly suggests that Earth’s Moon, early in its history, was covered with a deep magma ocean. As the magma cooled and crystallized, pyroxene and olivine grains settled to the bottom, and eventually plagioclase feldspar formed a floatation crust. At first, the continuing flux of impactors easily breached the thin but growing floatation crust and mixed it with magma from below. Crust thick enough to resist further penetration became underplated with pure anorthosite (rock containing at least 90
Analysis of Earth-based near-IR reflection spectra of features on the Moon's nearside has revealed the locations of outcrops of pure anorthosite that may be remnants of the flotation crust. Nearside anorthosite deposits very commonly are found on or very near basin rings. Anorthosites in the inner rings of basins such as Humorum, Grimaldi, and Orientale were derived from beneath more mafic-rich layers in the pre-impact target sites. In addition, the anorthosites associated with the outer rings of Nectaris and other basins are found in the central peaks and walls of large impact craters and were derived from layers deep beneath the crater target sites.
In the early 1990s, two spacecraft, Galileo and Clementine, returned multispectral data sets that allow further investigation of the distribution of anorthosite on the Moon, including areas of the farside for which no previous multispectral data existed. Iron maps derived from these data suggest that pure anorthosite may be quite abundant on certain farside surfaces. Farside anorthosite distribution may be largely related to the emplacement of the giant South Pole-Aitken (SPA) basin. Near the basin rim, thick blankets of SPA ejecta appear to cover the original crust, while in more distal regions the thinner cover of SPA ejecta (and mafic-contaminated upper crust) has been completely scattered away by subsequent impacts to reveal the underlying anorthosite.