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

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.

Large sulfur isotope fractionation in lunar volcanic glasses reveals the magmatic differentiation and degassing of the Moon

Sulfur isotope variations in mantle-derived lavas provide important constraints on the evolution of planetary bodies. Here, we report the first in situ measurements of sulfur isotope ratios dissolved in primitive volcanic glasses and olivine-hosted melt inclusions recovered from the Moon by the Apollo 15 and 17 missions. The new data reveal large variations in 34S/32S ratios, which positively correlates with sulfur and titanium contents within and between the distinct compositional groups of volcanic glasses analyzed. Our results uncover several magmatic events that fractionated the primordial sulfur isotope composition of the Moon: the segregation of the lunar core and the crystallization of the lunar magma ocean, which led to the formation of the heterogeneous sources of the lunar magmatism, followed by magma degassing during generation, transport, and eruption of the lunar lavas. Whether the Earth’s and Moon’s interiors share a common 34S/32S ratio remains a matter of debate.

The end of the lunar dynamo

Magnetic measurements of the lunar crust and Apollo samples indicate that the Moon generated a dynamo magnetic field lasting from at least 4.2 until <2.5 billion years (Ga) ago. However, it has been unclear when the dynamo ceased. Here, we report paleomagnetic and 40Ar/39Ar studies showing that two lunar breccias cooled in a near-zero magnetic field (<0.1 μT) at 0.44 ± 0.01 and 0.91 ± 0.11 Ga ago, respectively. Combined with previous paleointensity estimates, this indicates that the lunar dynamo likely ceased sometime between ~1.92 and ~0.80 Ga ago. The protracted lifetime of the lunar magnetic field indicates that the late dynamo was likely powered by crystallization of the lunar core.

Precessing spherical shells: flows, dissipation, dynamo and the lunar core

SUMMARY Precession of planets or moons affects internal liquid layers by driving flows, instabilities and possibly dynamos. The energy dissipated by these phenomena can influence orbital parameters such as the planet’s spin rate. However, there is no systematic study of these flows in the spherical shell geometry relevant for planets, and the lack of scaling law prevents convincing extrapolation to celestial bodies. We have run more than 900 simulations of fluid spherical shells affected by precession, to systematically study basic flows, instabilities, turbulence and magnetic field generation. We observe no significant effects of the inner core on the onset of the instabilities. We obtain an analytical estimate of the viscous dissipation, mostly due to boundary layer friction in our simulations. We propose theoretical onsets for hydrodynamic instabilities, and document the intensity of turbulent fluctuations. We extend previous precession dynamo studies towards lower viscosities, at the limits of today’s computers. In the low viscosity regime, precession dynamos rely on the presence of large-scale vortices, and the surface magnetic fields are dominated by small scales. Interestingly, intermittent and self-killing dynamos are observed. Our results suggest that large-scale planetary magnetic fields are unlikely to be produced by a precession-driven dynamo in a spherical core. But this question remains open as planetary cores are not exactly spherical, and thus the coupling between the fluid and the boundary does not vanish in the relevant limit of small viscosity. Moreover, the fully turbulent dissipation regime has not yet been reached in simulations. Our results suggest that the melted lunar core has been in a turbulent state throughout its history. Furthermore, in the view of recent experimental results, we propose updated formulas predicting the fluid mean rotation vector and the associated dissipation in both the laminar and the turbulent regimes.