Physical Properties and Internal Structure of the Central Region of the Moon

Abstract

One of the pivoting problems of the geochemistry and geophysics of the Moon is the structure of its central region, i.e., its core and adjacent transition layer located at the boundary between the solid mantle and liquid or partially molten core. The chemical composition of the mantle and the internal structure of the central region of the Moon were simulated based on the joint inversion of seismic, selenophysical, and geochemical parameters that are not directly interrelated. The solution of the inverse problem is based on the Bayesian approach and the use of the Markov chain Monte Carlo algorithm in combination with the method of Gibbs free energy minimization. The results show that the radius of the Moon’s central region is about 500–550 km. The thickness of the transition layer and the radii of the outer and inner cores relatively weakly depend on the composition models of the bulk silicate Moon with different contents of refractory oxides. The silicate portion of the Moon is enriched in FeO (12–13 wt %, FeO ~ 1.5 × BSE) and depleted in MgO (Mg# 79–81) relative to the bulk composition of the silicate Earth (BSE), which is in conflict with the possibility of the formation of the Moon from the Earth’s primitive mantle and does not find an adequate explanation in the current canonical and non-canonical models of the origin of the Moon. SiO2 concentrations in all zones of the lunar mantle vary insignificantly and amount to 52–53 wt %, and the predominant mineral of the upper mantle is low-Ca orthopyroxene but not olivine. With respect to Al2O3, the lunar mantle is stratified, with a Al2O3 content higher in the lower mantle than in all overlying shells. The partially molten transition layer surrounding the core is about 200–250 km thick. The radii of the solid inner core are within 50–250 km, and the most probable radii of the liquid outer core are ~300–350 km. The physical characteristics of the lunar core are compared with experimental measurements of the density and speed of sound of liquid Fe(Ni)–S–C–Si alloys. If the seismic model of the liquid outer core with VP = 4100 ± 200 m/s (Weber et al., 2011) is reasonably reliable, then this uncertainty range is in the best agreement with the VP values of 3900–4100 m/s of liquid Fe(Ni)–S alloys, with sulfur content up to ~10 wt % and a density of 6200–7000 kg/m3, as well as with the inverted values of density and velocity of the outer core. The VP values of liquid Fe–Ni–C and Fe–N–Si alloys at 5 GPa exceed seismic estimates of the speed of sound of the outer lunar core, which indicates that carbon and silicon can hardly be dominant light elements of the lunar core. The inner Fe(Ni) core (possibly with an insignificant content of light elements: sulfur and carbon) is presumably solid and has a density of 7500–7700 kg/m3. The difference in density between the inner and outer cores Δρ ~ 500–1000 kg/m3 can be explained by the difference in their composition.

Local Structure and Density of Liquid Fe‐C‐S Alloys at Moon's Core Conditions

“…The existence of a solid inner core puts more constraints on the core's temperature since it must be below the liquidus. If we assume a core temperature of about 1850 K, the amounts of S and C required to match the outer core density proposed in model E by Kuskov et al (2021) make the existence of a solid inner core rather unlikely. As the liquidus temperature at the Fe-rich side decreases with increasing S or C content, a cooler core can accommodate more light elements in the liquid outer core while having an fcc-Fe solid inner core, in qualitative agreement with the model of Kuskov21.…”

“…Many Moon models were built by integrating various independent observables, including seismic, electromagnetic, geodetic, and geochemical data. Great efforts have been made to interpret these observables in terms of composition, but discrepancies still exist among studies, particularly concerning the core (Garcia et al., 2019; Kuskov et al., 2021; Viswanathan et al., 2019 and references therein). To discuss the possible content of sulfur and carbon in the Moon's core, two sets of density contours assuming a hotter (1850 K) and cooler (1600 K) core are plotted in Figure 4, where the densities proposed by three of the latest Moon models (see Table 3 and associated references for more details) are correlated with sulfur and carbon content based on here‐presented results.…”

“…On the other hand, if the core were not fully molten (e.g., model E in Kuskov et al. (2021)), in the Fe‐C‐S system, a solid inner core would be made of Fe or Fe 3 C depending on whether the bulk C content is on the C‐poor or C‐rich side of the eutectic. The currently limited knowledge of the phase diagram and melting properties of the ternary Fe‐C‐S system does not allow an entirely quantitative discussion.…”

“…In this case the liquid outer core would have a sulfur and 447 carbon content ranging from 13 to 5 and from 0 to 2.3 wt%, respectively. Garcia19 (Garcia et al, 2019), Viswanathan19 (Viswanathan et al, 2019), and Kuskov21 (Kuskov et al, 2021). The shaded triangle shows the composition range for which the solid Fe phase (+C at solid solubility limit) is expected to be in coexistence with Fe-C-S liquid.…”

“…Based on joint inversion of available independent observations from Apollo missions, LLR (Lunar Laser Ranging) and GRAIL (Gravity Recovery and Interior Laboratory) data, and comparative geochemical analysis of Moon samples vs . bulk silicate Earth, several models have been put forward, which however show significant density spreading in the core, ranging from 4,200 to 7,000 kg/m 3 (e.g., Garcia et al., 2011; Kuskov et al., 2021; Weber et al., 2011).…”