Ground truth constraints and remote sensing of lunar highland crust composition

Abstract

We review constraints on the magnitude and possible causes of discrepancies, or at least major disparities, among global and near‐global data sets for lunar highland surface composition. When compared with data from other sources, reported mafic mineral abundance results from the Kaguya Spectral Profiler (Kaguya SP) spectral reflectance method for four Apollo 16 soils appear systematically low by a factor of 0.6, or an even more extreme factor (~1/3) if viewed in relation to the soils’ nonglass or CIPW mineralogy. Also, whether evaluated on a global median basis or on the basis of site‐by‐site comparison (for Apollo 16, Luna 20, and Apollo 17), the compositions found by the Kaguya SP technique show discrepancy, or at least disparity, versus other mafic abundance observations by that same factor of ~1/3. Spectral reflectance does not supply a simple bulk analysis of the target soil. The reflectance mineralogical signal is preponderantly determined by the nonglass fraction, and especially the masswise subordinate 10–20 µm grain size fraction. Literature data show that in anorthositic lunar soil, chemical composition is fractionated, more extremely anorthositic, for the nonglass component compared to the glass component. Also, the grain size fraction (10–20 μm) that most closely matches bulk reflectance has a significantly higher abundance of impact/agglutinitic glass than does the coarser material that dominates the soil mass. The Kaguya SP mafic abundance calibration needs adjustment by a factor of nearly 3 if results are to be interpreted as indicative of the mineralogy of the underlying crust. A claimed detection of several hundred lunar 500 m scale purest anorthosite (PAN; ≥98 vol% plagioclase) locales among millions of spectral reflectance observations is dubious, in part because with large data sets, compositional extremes are inevitably exaggerated as a byproduct of analytical uncertainty. Preponderance of PAN composition is rare among terrestrial layered intrusive anorthosites and is neither required nor expected for the flotation crust of a global magma ocean. Buoyant flotation and compaction would not suffice to yield pure plagioclase unless adcumulus growth was negligible, and trace element contents of ferroan anorthosites show that their mafic silicate components are for the most part of adcumulus, not “trapped melt,” derivation. A PAN‐dominated crust would imply a curiously fractionated (low) thorium/aluminum ratio for the crust, an implausibly high mantle/crust Th concentration ratio, and an oddly low Th/Al for the bulk Moon. Remote sensing techniques for planetary regolith composition are not easy to calibrate, particularly near the extremes of composition‐space and sensitivity.

Experimentally Induced Thermal Fatigue on Lunar and Eucrite Meteorites—Influence of the Mineralogy on Rock Breakdown

“…While solid rocks with a coherent structural integrity such as magmatic or possibly also metamorphic rocks react to thermal fatigue with the formation of cracks as shown by the basaltic eucrite NWA 11050, rocks which are highly reworked in the regolith of planetary bodies (i.e., the Moon) and are of higher maturity such as the lunar anorthositic breccia NWA 11273, contribute to fine‐grained, tenth‐of‐µm‐sized soil as shown by the micro‐flaking observed on the sample. This agrees with observations of the size‐frequency distribution of lunar soil (e.g., Warren & Korotev, 2022). Additionally, the specific mineralogy of exposed rocks and how easy it is to comminute them by impacts and introduce planes of relative weaknesses (such as cleavages of plagioclase and pyroxene and mosaicism in olivine) by excavating them can also affect the rate of decomposition by thermal fatigue.…”

“…(2010) studied lunar highland soils collected during Apollo 14 and 16 with respect to their grain size and composition. They stated that for the 10–20 μm size fraction of anorthositic Apollo 16 soils with higher glass abundances, the plagioclase abundance will be increased relative to the “mafic silicates fraction.” This bears important implications for remote sensing and especially for reflectance measurements, since these measurements are dominated by the 10–20 μm fraction (e.g., Warren & Korotev, 2022). The observed micro‐flaking on lunar anorthosite breccia NWA 11273 is acting on exactly this grain size fraction (Figures 3 and 4a), and since its mineralogy is dominated by plagioclase and glass (Figure 3a), it is possibly one reason for the observed characteristics of Apollo 16 soils.…”

Where Is the Lunar Mantle and Deep Crust at Crisium? A Perspective From the Luna 20 Samples

“…Because band depths are generally very low across the Crisium rim (Figure 11b), this suggests that the widespread Pure Anorthosite Ratio elevation across the rim is probably due to crystalline plagioclase, although minor (<∼5%) amounts of olivine cant be completely ruled out. Further, Warren and Korotev (2022) argued that the evolution of anorthositic regolith may preferentially lead to an overestimation of the "pure-anorthosite ratio" values due to the importance of the 10-20 micron size fraction in bulk reflectance measurements and the importance of impact/agglutinitic glass in this size fraction. This process may provide artificial enrichment at the rim of Crisium, but this cannot rule out the difference between the rim and the nHFT.…”

MoonIndex, an Open‐Source Tool to Generate Spectral Indexes for the Moon From M3 Data

“…The surface of the Moon has a limited mineralogical diversity, it has been broadly divided into two types of terrains, the “highlands” which are anorthosite‐rich and relatively light‐toned, and “maria,” dark‐toned plains of effusive lavas enriched in mafic and opaque minerals (Hiesinger & Head, 2006; Taylor, 1976). In the highlands, the dominant minerals are calcium plagioclases (Taylor, 1972; Warren & Korotev, 2022), while in the maria mafic compositions become important showing higher abundances of clinopyroxene (CPX), orthopyroxene (OPX), and olivine (Agrell et al., 1970; Albee, 2003). The clear definition of the lunar mineralogy has been driven by the samples returned by space missions (Prissel & Prissel, 2021), but due to their limited coverage of the lunar surface, the use of remote sensing techniques is still the only way to assess the mineralogy of the Moon at a global level.…”