Using LROC NAC Photometry to Study the Moon’s Feldspathic Crust

Abstract:

The primary lunar crust is feldspathic; it was formed by crystallization and upward accumulation of plagioclase during solidification of the lunar magma ocean. Although some samples of anorthosite from Apollo collections contain very high percentages of plagioclase, implying extremely efficient separation that leaves little or no trapped mafic material, the extent to which trapped melt was expelled is still debated.

We use photometric analysis of high-resolution images from the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Cameras (NAC) to investigate compositional characteristics of regolith in areas where hyperspectral data indicate plagioclase and a near absence of mafic silicates. In particular, we use single scattering albedo (SSA), determined using the Hapke photometric model, combined with topographic data corrected for local incidence and emission angles at every pixel (5 mpp) to quantify the extent of highly pure anorthosite (≥98% plagioclase, i.e., “purest anorthosite” or “PAN”). This analysis bears on the efficiency of plagioclase separation from the magma ocean and the composition of the Moon’s primary crust.

Using LROC NAC data, we have analyzed locations in the Inner Rook Ring of Orientale basin, the Inner Ring of Hertzsprung basin, the crater Korolev-M, and an area of highlands northeast of Nectaris basin to determine the location and extent of PAN in these regions. We use a relationship between composition and SSA, calibrated using Apollo landing site soils, and find that SSA varies approximately linearly with the plagioclase vs. mafic mineral contents. We use FeO estimated from multispectral, data coupled with SSA at sites of PAN identification, to assess the percentage of plagioclase in regolith developed on uplifted structures associated with craters and basins. Regolith maturity also has a strong effect on SSA: where the surface is immature, SSA is higher than expected from compositional correlations. Thus, we restrict analyses to areas that are optically mature using Kaguya OMAT. In our study areas, we find local patches of regolith with SSA indicative of PAN. In most areas, however, SSA values are consistent with regolith developed from anorthosite with plagioclase <96%, suggesting that anorthosite of the lunar crust mainly contains more realistic percentages of trapped melt.