Composition, Structure and Origin of the Moon

TL;DR

This review synthesizes geophysical inversions and geochemical data to test Moon origin theories. By combining a small, likely Fe–Ni-rich core with Earth-like major-element mantle signatures and pronounced volatile depletion, the work constrains interior structure and volatile histories, then assesses several dynamical formation scenarios, including canonical giant impact and alternatives like synestia and half-Earth collisions. The isotopic and trace-element similarities between Earth and Moon challenge Earth-impactor models and favor scenarios involving shared reservoir material or thorough Earth–Moon equilibration, while volatile-loss signatures point to low-temperature processes. Overall, the paper demonstrates that no single dynamical model currently satisfies all observational constraints, highlighting the need for new data (e.g., farside seismicity, sample returns) to definitively resolve the Moon’s origin and its coupling to Earth's formation. The findings have broad implications for planetary accretion and differentiation in the inner Solar System, informing how terrestrial planets acquire their volatile inventories and isotopic signatures.

Abstract

Here we critically examine the geophysical and geochemical properties of the Moon in order to identify the extent to which dynamical scenarios satisfy these observations. New joint inversions of existing lunar geophysical data (mean mass, moment of inertia, and tidal response) assuming a laterally- and vertically homogeneous lunar mantle show that, in all cases, a core with a radius of 300$\pm$20 km ($\sim$0.8 to 1.5 % the mass of the Moon) is required. However, an Earth-like Mg# (0.89) in the lunar mantle results in core densities (7800$\pm$100 kg/m$^3$) consistent with that of Fe-Ni alloy, whereas FeO-rich compositions (Mg# = 0.80--0.84) require lower densities (6100$\pm$800 kg/m$^3$). Geochemically, we use new data on mare basalts to reassess the bulk composition of the Moon for 70 elements, and show that the lunar core likely formed near 5 GPa, 2100 K and $\sim$1 log unit below the iron-wüstite buffer. Moreover, the Moon is depleted relative to the Earth's mantle in elements with volatilities higher than that of Li, with this volatile loss likely having occurred at low temperatures (1400$\pm$100 K), consistent with mass-dependent stable isotope fractionation of moderately volatile elements (e.g., Zn, K, Rb). The identical nucleosynthetic (O, Cr, Ti) and radiogenic (W) isotope compositions of the lunar and terrestrial mantles, strongly suggest the two bodies were made from the same material, rather than from an Earth-like impactor. Rb-Sr in FANs and Lu-Hf and Pb-Pb zircon ages point Moon formation close to $\sim$4500 Ma. Taken together, there is no unambiguous geochemical or isotopic evidence for the role of an impactor in the formation of the Moon, implying perfect equilibration between the proto-Earth and Moon-forming material or alternative scenarios for its genesis.

Figures (23)

• Figure 1: Schematic diagram of the internal structure of the Moon as viewed from geophysics. The Moon is differentiated into crust, mantle, and core. A mid-mantle seismic discontinuity -- a potential relic of magma ocean crystallisation in the form of a compositional boundary -- is not required by data. Geophysical evidence suggests the presence of a deep-seated partially molten layer, although whether the layer is located in the deep mantle or belongs the core is unclear. The core is small and likely consists of a liquid metallic Fe-Ni alloy, plausibly together with a light element component (e.g., S or C, see section \ref{['sec:geophys_inv_core']}). The locations of the Apollo lunar seismic stations 12, 14, 15, and 16 on the lunar nearside are indicated by numbers. Shallow and deep (DMQ) moonquakes occur in the depth ranges 50--200 km and 800--1100 km, respectively. Farside DMQs have yet to be confirmed. The selenographical location of the indicated moonquakes is not accurate. Variations in crustal thickness are real but not to scale. Modified from wieczorek2006 and khan_etal2014.
• Figure 2: Inverted seismic P-wave (A) and S-wave (B) speed and density (C, D) profiles that fit the lunar tidal response and mean mass and moment of inertia for the range of compositional models listed in Table \ref{['tab:inv_com']}. The inverted profiles correspond to Dauphasetal2014 (blue), Earth's mantle-like after palmeoneill2014 (light blue), khan2006earth (cyan), oneill1991origin (light green), Taylor1999 (light yellow), warren2005 (orange), and variable composition (red). Core P- and S-wave speeds are unconstrained and therefore not shown. For comparison, models obtained from the Apollo lunar seismic data are shown as yellow khan2006earth and black solid garcia_etal2019 and dashed lines nakamura1983seismic, respectively. For the khan2006earth model, uncertainties are included in the form of upper and lower bounds. Core density is not well constrained by these models and therefore omitted. See main text for details.
• Figure 3: The relationship between Mg# of the lunar mantle and the density of its core required to satisfy its bulk density and moment of inertia. Mantle compositions are fixed, except for the 'Variable Composition' case (see Table \ref{['tab:inv_com']}). Note that the Mg#-core density distribution of the Variable Composition case is not Gaussian. Vertical lines correspond to densities of potential core-forming materials at 1900 K and 5.5 GPa, Yellow = Fe$_{73}$Ni$_{10}$S$_{17}$ (in at %) liquid, terasaki_etal2019; Blue = Fe$_{88}$Ni$_{9}$C$_{3}$ (in at %) liquid, zhu2021density; Orange = Fe liquid, andersonahrens1994; Green = Ni liquid, naschmanghnani1998.
• Figure 4: Calculated evolution of density of the lunar core (at 5.5 GPa) assuming it is produced from an ideal (i.e., mechanical) mixture of Fe-Ni alloy of density 8000 kgm$^{-3}$ (black point) and Fe-S eutectic liquid (77 wt. % Fe, 23 wt. % S) of density 5171 kgm$^{-3}$ (red point) and a constant total radius of 330 km (see Table \ref{['tab:inv_com']}). The total core mass fraction thus varies from 1.74 % (pure Fe-Ni) to 0.46 % (pure Fe-S). The solid black curve refers to the S content of the bulk Moon (in ppm) while the red dashed line denotes the S content of the bulk core (in wt. %). Dashed vertical lines represent the modelled percentages of Fe-S eutectic composition that constitute the core. The S content of the bulk silicate Earth palmeoneill2014 is shown as a horizontal bar.
• Figure 6: Data for K (ppm) vs. K/La compiled from mare basalts neal2007, subdivided into those from the Apollo 11, 12, 15 and 17 missions.