The Constitution and Structure of the Lunar Interior

Abstract

until the acquisition of thorium surface concentrations by the Lunar Prospector spacecraft. It now appears that the Moon possesses a fundamental asymmetry, stemming from the time of initial differentiation, with KREEP-rich rocks and mare volcanism having been concentrated on the Moon's Earth-facing hemisphere. Curiously, the elevations of the nearside hemisphere are about 2 km less than that of the far side. No matter the origin of the asymmetric distribution of KREEP-rich rocks, it appears that their associated high heat production fundamentally affected the post-magma-ocean magmatic history of the Moon's near side. Figure 3.1. One possible interpretation of the Moon's internal structure (a) just after magma-ocean crystallization and (b) near the end of mare basaltic volcanism. With the exception of a hemispheric dichotomy in crustal thickness and mare basaltic volcanism, this model, like many pre-Lunar Prospector models, represents the crust and mantle as being laterally uniform in composition. (a) The lunar magma ocean is assumed to have a depth of 550 km, the lower mantle is composed of "primitive" unmelted materials, and a small core is assumed. The sequences of major mineralogy as a function of radius in the mantle (olivine → olivine + pyroxene → ilmenite + olivine + pyroxene) and crust (plagioclase-rich in the upper crust and plagioclase + pyroxene + olivine in the lower crust), as well as the existence of a global KREEP-rich layer at the crust-mantle boundary, are a result of a fractionally crystallizing magma ocean. Complex processes, such as mantle overturn and asymmetric solidification of the magma ocean, are not considered in this model. (b) The lunar interior at ~3 Ga, emphasizing the nearside-farside dichotomy in both the distribution of mare basalts and the thickness of the lunar crust. (CM and CF represent the center of mass and center of figure, respectively, which are offset from each other by about 2 km). Compare with the more recent interpretations presented in Figure 3.27. [Used by permission of Springer, from McCallum (2001), Earth Moon Planets, Vol. 85-86, Figs. 2 and 4, pp. 256 and 260.] 1738 km 65 km Depleted Mantle Crust Core 550 km Plag Pl � Ol�Px KREEP Ilm� Ol�Px Ol Ol Px Primitive Mantle (a) Moon at~4.4 Ga South Pole-Aitken Basin Mare basalt (FeO~15-22 wt.%) Farside Nearside (b) Moon at~3 Ga � � �� �� 90 km Anorthositic Crust (FeO~4.5 wt.%) 40-50 km 40 km Incr. Fe Incr. Fe 1738 km 65 km Depleted Mantle Crust Core 550 km Plag Pl � Ol�Px KREEP Ilm� Ol�Px Ol Ol Px Primitive Mantle (a) Moon at~4.4 Ga South Pole-Aitken Basin Mare basalt (FeO~15-22 wt.%) Farside Nearside (b) Moon at~3 Ga � � �� �� 90 km Anorthositic Crust (FeO~4.5 wt.%) 40-50 km 40 km

Ring faults and ring dikes around the Orientale basin on the Moon

“…GRAIL-derived global crustal thickness models excluded these small-scale anomalies by filtering the gravity data to a uniform resolution of ~68 km (the half-wavelength resolution at degree 80, at which the filter amplitude dropped below 0.5). The resulting models have higher resolution than pre-GRAIL models with filter magnitudes of 0.5 at degrees 30 (Wieczorek and Phillips, 1998;Wieczorek, 2006) and 50 (Ishihara et al, 2009). However, this limiting resolution nevertheless resulted in loss of signal from the crust-mantle interface at shorter wavelengths.…”

Lunar cryptomaria: Mineralogy and composition of ancient volcanic deposits

“…LP GRS pixels contained within the cryptomaria boundaries were used to calculate an average value for the entire deposit; LP GRS pixels are too large to measure the composition of individual DHCs. While VNIR data provide information about the spectrally dominant mineralogy, LP GRS Th measurements can provide information about the abundance of incompatible elements in the rock and will help to distinguish between lithologies with a high abundance of incompatible elements, such as the Mg-suite (o5 ppm) (Wieczorek et al, 2006) and KREEP basalts (5-15 ppm; Apollo 15 extrusive KREEP basalts have 10-11 ppm) (Lucey et al, 2006;Wieczorek et al, 2006), and those rock types with a lower concentration of incompatible elements (i.e., mare basalts, o1 ppm on average) (Korotev, 1998). FeO measurements can also provide more precise estimates of the iron content than the VNIR spectral data and will be useful for distinguishing between iron-and magnesiumrich rocks.…”

ARTEMIS Science Objectives

“…Laboratory conductivity-temperature measurements were used to translate the lunar conductivity profile to a selenotherm that is roughly equivalent with the background, global heat flow derived by Wieczorek et al (2006). The outer 500 km was poorly constrained due to the limited magnetometer bandwidth (higher frequencies are needed to resolve shallower depths).…”

“…Steady magnetizing fields of both external (solar or terrestrial) and internal (lunar dynamo) origin have been proposed, as have transient fields generated by impacts (for reviews, see Fuller 1974, andWieczorek et al 2006). Thermoremanent magnetization is likely the dominant process for igneous lunar samples; however, shock remanent magnetization may be significant in lightly metamorphosed breccias, which carry the strongest and most stable remanent magnetization of all lunar samples because they contain more metallic iron grains (likely produced by meteoritic impacts) (Fuller et al 1974;Fuller and Cisowski 1987).…”