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Internal structure of the Moon
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Having a mean density of 3,346.4 kg/m3,[2] the Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and planetary core. This structure is believed to have resulted from the fractional crystallization of a magma ocean shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust.
Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition,[3] consistent with the magma ocean hypothesis. In terms of elements, the lunar crust is composed primarily of oxygen, silicon, magnesium, iron, calcium, and aluminium, but important minor and trace elements such as titanium, uranium, thorium, potassium, sulphur, manganese, chromium[4] and hydrogen are present as well. Based on geophysical techniques, the crust is estimated to be on average about 50 km thick.[5]
Partial melting within the mantle of the Moon gave rise to the eruption of mare basalts on the lunar surface. Analyses of these basalts indicate that the mantle is composed predominantly of the minerals olivine, orthopyroxene and clinopyroxene, and that the lunar mantle is more iron-rich than that of the Earth. Some lunar basalts contain high abundances of titanium (present in the mineral ilmenite), suggesting that the mantle is highly heterogeneous in composition. Moonquakes have been found to occur deep within the mantle of the Moon about 1,000 km below the surface. These occur with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth. A few shallow moonquakes with hypocenters located about 100 km below the surface have also been detected, but these occur more infrequently and appear to be unrelated to the lunar tides.[5]
Core
[edit]
Several lines of evidence imply that the lunar core is small, with a radius of about 350 km or less.[5] The diameter of the lunar core is only about 20% the diameter of the Moon itself, in contrast to about 50% as is the case for most other terrestrial bodies. The composition of the lunar core is not well constrained, but most believe that it is composed of metallic iron alloy with a small amount of sulfur and nickel. Analyses of the Moon's time-variable rotations indicate that the core is at least partly molten.[6] Within the giant-impact formation scenario, the core formation of Moon could have occurred within the initial 100–1000 years from the commencement of its accretion from its moonlets.[7]
In 2010, a reanalysis of the old Apollo seismic data on the deep moonquakes using modern processing methods confirmed that the Moon has an iron rich core with a radius of 330 ± 20 km. The same reanalysis established that the solid inner core made of pure iron has a radius of 240 ± 10 km. The core is surrounded by the partially (10 to 30%) melted layer of the lower mantle with a radius of 480 ± 20 km (thickness ~150 km). These results imply that 40% of the core by volume has solidified. The density of the liquid outer core is about 5 g/cm3 and it could contain as much as 6% sulfur by weight. The temperature in the core is probably about 1600–1700 K (1330–1430 °C).[8]
Moon – Oceanus Procellarum ("Ocean of Storms")
Ancient rift valleys – rectangular structure (visible – topography – GRAIL gravity gradients) (October 1, 2014).
Ancient rift valleys – context.
Ancient rift valleys – closeup (artist's concept).
In 2019, a reanalysis of nearly 50 years of data collected from the Lunar Laser Ranging experiment with lunar gravity field data from the GRAIL mission, shows that for a relaxed lunar fluid core with non-hydrostatic lithospheres, the core flattening is determined as (2.2±0.6)×10−4 with the radii of its core-mantle boundary as 381±12 km.[9]
See also
[edit]
References
[edit]- ^ Maurice, M.; Tosi, N.; Schwinger, S.; Breuer, D.; Kleine, T. (1 July 2020). "A long-lived magma ocean on a young Moon". Science Advances. 6 (28) eaba8949. Bibcode:2020SciA....6.8949M. doi:10.1126/sciadv.aba8949. ISSN 2375-2548. PMC 7351470. PMID 32695879.
Text and images are available under a Creative Commons Attribution 4.0 International License.
- ^ Making it the second densest satellite in the Solar System after Io
- ^ P. Lucey and 12 coauthors, P. (2006). "Understanding the lunar surface and space-Moon interactions". Reviews in Mineralogy and Geochemistry. 60 (1): 83–219. Bibcode:2006RvMG...60...83L. doi:10.2138/rmg.2006.60.2.
{{cite journal}}: CS1 maint: numeric names: authors list (link) - ^ "What Chandrayaan 3 has found on moon so far: Oxygen, sulphur, iron, silicon". Hindustan Times. 2023-08-30. Retrieved 2023-11-15.
- ^ a b c Mark Wieczorek and 15 coauthors, M. A. (2006). "The constitution and structure of the lunar interior" (PDF). Reviews in Mineralogy and Geochemistry. 60 (1): 221–364. Bibcode:2006RvMG...60..221W. doi:10.2138/rmg.2006.60.3. Archived from the original (PDF) on 2014-12-21.
{{cite journal}}: CS1 maint: numeric names: authors list (link) - ^ J. G. Williams; S. G. Turyshev; D. H. Boggs; J. T. Ratcliff (2006). "Lunar laser ranging science: Gravitational physics and lunar interior and geodesy". Advances in Space Research. 37 (1): 67–71. arXiv:gr-qc/0412049. Bibcode:2006AdSpR..37...67W. doi:10.1016/j.asr.2005.05.013. S2CID 14801321.
- ^ S. Sahijpal; V. Goyal (2018). "Thermal evolution of the early Moon". Meteoritics and Planetary Science Journal. 53 (10): 2193–2211. arXiv:2001.07123. Bibcode:2018M&PS...53.2193S. doi:10.1111/maps.13119. S2CID 134291699.
- ^ Weber, R. C.; Lin, P.-Y.; Garnero, E. J.; Williams, Q.; Lognonne, P. (2011). "Seismic Detection of the Lunar Core". Science. 331 (6015): 309–312. Bibcode:2011Sci...331..309W. doi:10.1126/science.1199375. PMID 21212323. S2CID 206530647.
- ^ Viswanathan, V.; Rambaux, N.; Fienga, A.; Laskar, J.; Gastineau, M. (9 July 2019). "Observational Constraint on the Radius and Oblateness of the Lunar Core-Mantle Boundary". Geophysical Research Letters. 46 (13): 7295–7303. arXiv:1903.07205. Bibcode:2019GeoRL..46.7295V. doi:10.1029/2019GL082677. S2CID 119508748.
External links
[edit]- Moon articles in Planetary Science Research Discoveries, including articles about internal structure of the Moon
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Internal structure of the Moon
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Methods of investigation
Seismic studies
The Apollo Passive Seismic Experiment (PSE) involved the deployment of four seismometer stations on the lunar nearside during the Apollo 12, 14, 15, and 16 missions between November 1969 and December 1972, with the network operating until September 1977.[5] These stations recorded over 34,000 seismic events in total, including over 22,000 newly identified in 2024 reanalyses, providing the primary dataset for probing the Moon's internal structure through wave propagation analysis.[6] The instruments were sensitive to both compressional (P-) and shear (S-) waves, enabling the mapping of velocity discontinuities and attenuation patterns across the lunar interior.[7] Moonquakes detected by the PSE fall into four main categories, each with distinct depths, frequencies, and magnitudes that reveal different internal processes. Deep moonquakes, occurring at depths of 700–1100 km near the core-mantle boundary, were the most numerous, with approximately 7,400 events recorded, often in swarms triggered by tidal stresses from Earth's gravity; these typically had low magnitudes below 1 on the Richter scale, though some reached up to 3.[8] Shallow moonquakes, at depths less than 200 km, with approximately 74 well-located events including 46 newly discovered in 2024, were rare but more energetic, registering magnitudes from 2.0 to 5.5 and lasting up to 10 minutes due to prolonged scattering in the low-velocity regolith.[9][10] Thermal moonquakes, caused by diurnal temperature variations cracking the brittle outer layer, were frequent (over 12,000 identified, mostly small-amplitude signals below magnitude 1) and confined to the upper few meters.[11] Meteoroid impacts, both natural and from discarded spacecraft, produced around 1,700 events with magnitudes up to 4, simulating artificial sources for velocity calibration.[7] Analysis of arrival times from these events yielded seismic velocity profiles showing a gradual increase from the crust into the upper mantle. In the crust (0–50 km depth), P-wave velocities range from approximately 6–8 km/s and S-wave velocities from 3.5–4.5 km/s, reflecting a heterogeneous, impact-fractured layer.[12] These velocities rise sharply at the crust-mantle boundary to about 8.1 km/s for P-waves and 4.7 km/s for S-waves in the upper mantle (50–400 km), indicating a transition to more competent, olivine-rich material, though models vary slightly based on inversion techniques.[13] Deeper in the mantle (below 400 km), velocities show a subtle decrease, suggesting a zone of reduced rigidity.[14] Reflected and converted waves in the PSE data provided evidence for a core-mantle boundary at around 1,300–1,400 km depth. A 2023 reanalysis of PSE data combined with lunar laser ranging and gravity observations refined the core model, identifying ScS phases—S-waves reflecting off the core boundary as shear waves—along with PcP and other core phases, implying a fluid outer core with a radius of approximately 362 km that attenuates S-waves, preventing their transmission through a fully liquid layer. These observations, combined with the absence of direct S-wave arrivals through the core, support a two-layer core structure: a liquid outer portion ~100 km thick overlying a solid inner core of radius 258 ± 40 km and density approximately 7,822 kg/m³, consistent with an iron-nickel-sulfur composition.[15] Seismic attenuation, quantified by the quality factor Q (a measure of energy loss per cycle), is notably low in the deep mantle, indicating partial melting. Q values drop to 100–1,000 in the asthenosphere-like zone at 400–1,000 km depth for both P- and S-waves, compared to higher values (4,000–7,000) in the upper mantle, consistent with 1–5% melt fractions that enhance dissipation and explain the long, ringing codas of moonquake signals.[16] This partial melt layer aligns with inferred temperatures of ~1,500°C and is corroborated by density models from gravity data.[16]Gravity and topography mapping
The Gravity Recovery and Interior Laboratory (GRAIL) mission, consisting of two spacecraft launched in 2011 and operating through 2012, employed inter-satellite ranging and Doppler tracking to measure the Moon's gravitational field at high resolution, achieving spherical harmonic degrees up to 900.[17] This dual-spacecraft configuration allowed for the detection of subtle density variations in the lunar interior by tracking orbital perturbations caused by gravitational anomalies. Resulting gravity field models, such as GRGM900C derived from GRAIL primary and extended mission data, reveal prominent positive gravity anomalies known as mascons, particularly in large impact basins like Imbrium, where excess mass concentrations produce free-air gravity highs exceeding 300 mGal.[17] These mascons correlate with topographic depressions from ancient impacts, indicating uncompensated crustal loads that have not fully relaxed isostatically, as evidenced by simulations of impact dynamics and viscous mantle flow. Analysis of the correlation between GRAIL gravity data and Lunar Orbiter Laser Altimeter (LOLA) topography demonstrates that gravity anomalies are positively correlated with topographic features over impact basins, reflecting incomplete isostatic compensation due to a rigid lithosphere. Admittance and coherence functions, which quantify the gravitational response to surface loads, support a global crustal thickness of approximately 34 km on average, varying from 30 to 50 km regionally and thinning to less than 20 km near major basins like Orientale and Imbrium.[18] These spectral analyses favor two-layer loading models, with surface and crustal loads over a denser mantle, to explain observed admittance values; however, incorporating a three-layer structure including a fluid core improves fits for lower-degree harmonics, suggesting minor contributions from deep interior density contrasts.Other geophysical techniques
Other geophysical techniques provide indirect constraints on the Moon's internal structure through measurements of magnetic fields, heat flow, and surface properties that inform thermal evolution and historical dynamics. These methods complement direct structural probes by revealing past dynamo activity, radiogenic heat sources, and bulk density distributions. The Lunar Prospector mission (1998–1999) used its magnetometer to map global crustal remanent magnetic fields, revealing anomalies with strengths up to approximately 300 nT at low orbital altitudes, which indicate magnetization acquired in the presence of an ancient core dynamo during the Moon's early history.[19] These crustal fields, primarily concentrated on the farside and associated with impact basins, suggest that the lunar magnetic field was generated by core convection and persisted long enough to imprint the crust before ceasing.[20] Paleomagnetic analyses of Apollo-returned lunar rocks further support this, showing that the core dynamo activity ended between approximately 1 and 4 billion years ago, marking the transition to a geologically inactive interior.[21] Heat flow measurements from probes deployed during the Apollo 15 and 17 missions recorded subsurface temperature gradients, yielding endogenic heat flow values of about 21 mW/m² at the Apollo 15 site and 14 mW/m² at Apollo 17, which are indicative of localized variations in radiogenic heating.[22] These data imply a total mantle heat production on the order of 4 × 10¹¹ W, primarily from the decay of thorium, uranium, and potassium, constraining the Moon's internal thermal budget and suggesting a cooling history dominated by radiogenic sources rather than residual accretion heat.[23] Topographic data from the Lunar Orbiter Laser Altimeter (LOLA) on NASA's Lunar Reconnaissance Orbiter and the Terrain Camera on Japan's Kaguya (SELENE) mission have enabled precise calculations of the Moon's moment of inertia factor, determined to be 0.393 ± 0.001, which reflects the radial mass distribution and supports models of a dense core comprising about 1–2% of the Moon's mass. When combined with seismic and gravity data indicating a core radius of 300–400 km, this value helps delineate the mantle-core boundary without relying on density contrasts alone. Remote sensing via electron reflectometry and gamma-ray spectroscopy, as conducted by Lunar Prospector, has mapped surface elemental abundances such as iron, titanium, and thorium, which influence electrical conductivity profiles in the crust and upper mantle. These distributions reveal lateral heterogeneities that affect electromagnetic induction and provide proxies for deeper compositional layering, linking surface geochemistry to internal heat transport mechanisms.[24]Crust
Thickness and global variations
The Moon's crust exhibits a global average thickness of 34–43 km, as derived from high-resolution gravity data collected by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission.[25] A 2025 model constrained by converted seismic phases estimates an average thickness of 29–47 km.[26] This measurement incorporates constraints from seismic experiments and accounts for crustal porosity of approximately 12%, revealing a thinner overall structure than previously estimated from Apollo-era data. Regional variations are pronounced, with the farside crust averaging about 50 km thick—up to 20 km thicker than the nearside, where thicknesses typically range from 20–30 km in highland and mare regions.[27] These asymmetries contribute to the Moon's nearside–farside dichotomy, characterized by thinner crust and more extensive volcanism on the nearside. Impact basins further accentuate crustal thinning, particularly in large structures like the South Pole–Aitken basin, where thicknesses drop below 20 km due to excavation of crustal material during impact and subsequent isostatic rebound.[25] Global maps of crustal thickness, constructed using gravity–topography admittance ratios from GRAIL observations, highlight these variations and link the asymmetric dichotomy to early bombardment events that redistributed material unevenly across the lunar surface.[25] The crust's compensation primarily follows Airy isostasy, where topographic loads are balanced by variations in crustal root depth, with the lithosphere supporting minor flexural contributions at a rigidity of approximately N·m.[28] Over time, the crust evolved from an initial uniform thickness of about 50 km formed during solidification of the lunar magma ocean around 4.4 billion years ago.[29] Subsequent mare volcanism between 3.5 and 1 billion years ago exploited regions of thinned crust, particularly on the nearside, leading to localized infilling of impact basins and further modulation of the global thickness profile.[27]Composition and formation
The lunar crust is primarily composed of two distinct rock types: the anorthositic highlands and the mare basalts. The highlands, which form the majority of the lunar surface, consist predominantly of ferroan anorthosite, characterized by more than 90% plagioclase feldspar, specifically calcic anorthite (An94–98), with minor amounts of pyroxene and olivine.[30] This composition results in low iron oxide (FeO) content, typically 4–6 wt%, and high aluminum (Al-rich), reflecting a highly differentiated, plagioclase-dominated lithology formed early in lunar history.[31] In contrast, the mare basalts, which fill the large impact basins primarily on the nearside, are richer in mafic minerals such as olivine, pyroxene (both clinopyroxene and orthopyroxene), and ilmenite, with plagioclase as a subordinate phase.[32] These basalts exhibit variable titanium dioxide (TiO2) contents, with high-Ti variants reaching up to 10 wt% in nearside deposits, contributing to their dark, titanium-enriched appearance.[33] The formation of the lunar crust is closely tied to the lunar magma ocean (LMO) hypothesis, which posits that the Moon experienced a global molten layer shortly after its accretion around 4.5 billion years ago. In this model, the LMO crystallized from the top down, with plagioclase crystals floating due to their low density to form a primary anorthositic crust approximately 4.4 billion years ago (Ga).[34] As solidification progressed, denser mafic minerals sank to form the underlying mantle, while the final residual liquid, enriched in potassium (K), rare earth elements (REE), and phosphorus (P)—collectively known as KREEP—concentrated at the base of the crust.[31] Mare basalts erupted later, between 3.9 and 3.2 Ga, as partial melts from the mantle intruded through the crust, flooding impact basins and overprinting the highland terrain.[32] Analyses of samples returned by the Apollo and Luna missions provide key evidence for this compositional framework. Highland anorthosites show depleted trace elements consistent with early plagioclase accumulation, while KREEP-rich materials exhibit highly fractionated REE patterns, with enrichments up to 1000 times chondritic levels.[35] Isotopic studies, including samarium-neodymium (Sm-Nd) ratios such as 143Nd/144Nd and small non-chondritic 142Nd/144Nd anomalies (ε142Nd ≈ 0 ± 0.5), indicate rapid differentiation of the LMO and minimal post-formation disturbance in crustal samples.[36] These data support a model where the initial LMO involved a significant melt fraction, estimated at 10–20% of the Moon's volume, leading to the observed crustal stratification.[37] Differentiation models for the lunar crust contrast the canonical single LMO scenario with serial magmatism, where multiple localized melt bodies contributed to crustal growth after initial ocean solidification. The single LMO model explains the widespread anorthosite and KREEP distribution through global flotation and residual pooling, while serial magmatism accounts for later plutonic intrusions like the Mg-suite rocks, potentially hybridizing with LMO remnants.[34] Although the LMO hypothesis remains dominant, ongoing debates center on the extent of serial processes in refining the final crustal composition, informed by trace element and isotopic signatures from returned samples.[37]Mantle
Structure and mineralogy
The lunar mantle exhibits a stratified structure inferred from electromagnetic sounding, geochemical modeling, and rare sample evidence, with mineral phases reflecting crystallization from a primordial magma ocean followed by dynamic overturn processes. The upper mantle, spanning depths of approximately 100–500 km, is predominantly composed of magnesium-rich olivine (forsterite content Fo80–95) and orthopyroxene, constituting over 80% of the mineral assemblage, alongside minor clinopyroxene.[38][39] This ultramafic harzburgitic composition, with a magnesium number (Mg#) ≈ 80–90, yields an average density of 3.3–3.4 g/cm³, consistent with bulk lunar models constrained by gravity data.[40] The low volatile content, with water levels in the tens of parts per million or lower, underscores the Moon's depletion in hydrous phases relative to Earth.[38] Recent analyses of Chang'e-6 samples (returned in 2024) from the farside South Pole-Aitken basin reveal primitive mantle olivine with high forsterite content (Fo ≈ 90+) and water abundance in the mantle source of 1–1.5 μg/g (1–1.5 ppm), the lowest reported, suggesting a drier farside mantle compared to nearside estimates.[41][42] These findings indicate potential hemispheric variations in mantle composition and hydration.[42] Deeper in the mantle, a transition zone at roughly 500–1000 km depth features evolving mineral stability under increasing pressure, where seismic discontinuities may mark boundaries between olivine-orthopyroxene dominance and denser assemblages. Models of the lower mantle incorporate magnesium silicate perovskite (Mg-perovskite) and ferropericlase ((Mg,Fe)O) as principal phases, potentially forming under the elevated pressures nearing 4–5 GPa at the core-mantle boundary.[38] Additionally, accumulation of ilmenite from late-stage magma ocean crystallization contributes to layering, with dense ilmenite-bearing cumulates sinking to depths around 1000 km and creating chemical stratification.[43] This overturn process, driven by Rayleigh-Taylor instabilities, relocates 50–70% of these cumulates to the deep interior while mixing shallower portions with overlying material.[43] Evidence for these mantle phases derives from rare xenolithic inclusions in lunar meteorites, such as the dunite fragment in Northwest Africa (NWA) 11421, which displays cumulate textures with ~95% olivine (Fo83), minor pyroxenes, plagioclase, and chrome spinel, equilibrated at upper mantle conditions (~0.4 GPa, 980°C).[44] This sample exhibits granoblastic-polygonal fabrics indicative of metamorphic re-equilibration and confirms low volatile signatures, with no significant hydrous phases detected beyond minor terrestrial alteration.[44] Surface exposures of olivine- and low-calcium pyroxene-rich rocks in impact basins further support heterogeneous mantle upwelling, aligning with overturn-driven redistribution.[45]Thermal and seismic properties
The thermal structure of the lunar mantle is characterized by a temperature profile that increases from near-surface values of approximately 250 K to around 1500–1700 K at the core-mantle boundary, reflecting conductive heat transport in the upper regions and potentially adiabatic conditions in the deeper mantle.[46] Chang'e-6 samples indicate a hemispheric dichotomy, with farside mantle potential temperatures ~100–180°C lower than nearside (~1400 K vs. ~1500–1600 K), as of analyses in 2025.[47][48] This gradient is influenced by the Moon's limited internal heat sources and lack of active convection, with the lower mantle following an adiabat due to higher pressures and temperatures approaching partial melting thresholds.[49] Internal heat production in the lunar mantle arises primarily from the radioactive decay of uranium (U), thorium (Th), and potassium (K), with concentrations decreasing with depth as a result of magmatic differentiation that concentrated these heat-producing elements (HPEs) toward the crust and upper mantle during the Moon's early evolution.[50] The total lunar heat budget, inferred from surface heat flow measurements, corresponds to an average flux of about 17 mW/m², dominated by radiogenic contributions estimated at roughly 0.65 TW globally, though recent models suggest lower values around 8 mW/m² due to revised HPE distributions.[51][52][53] Seismic properties of the lunar mantle reveal shear-wave velocities (Vs) of approximately 4.4 km/s in the upper mantle, with gradual increases to around 4.5–4.6 km/s at greater depths, indicating a relatively homogeneous structure punctuated by a low-velocity zone possibly linked to anelastic effects or minor compositional variations.[46] Attenuation is quantified by a quality factor (Q) that is generally high in the upper mantle (around 4000–7000), reflecting low intrinsic dissipation, but decreases to values near 1000 in the lower mantle, suggesting enhanced anelasticity that contributes to tidal energy loss.[14][54] Viscosity in the lunar mantle varies significantly with depth, estimated at about 10²¹ Pa·s in the upper regions where cooler temperatures and higher rigidity dominate, dropping to around 10¹⁹ Pa·s in the lower mantle due to elevated temperatures and potential weakening mechanisms, which collectively inhibit large-scale convection and promote a stagnant lid regime.[43] These high viscosities limit mantle overturn and explain the Moon's lack of plate tectonics, with models showing that even modest reductions in viscosity are insufficient to drive vigorous flow.[55] Evidence for partial melting in the lunar asthenosphere, at depths of roughly 200–400 km, comes from analyses of tidal quality factor (Q) measurements, which indicate 1–5% melt fractions sufficient to explain observed seismic low-velocity zones and enhanced dissipation without requiring widespread liquidity.[16] These melt pockets likely arise near the solidus in regions of elevated temperature or HPE concentration, influencing local rheology but not global dynamics. Mineral phase transitions, such as those involving olivine to higher-pressure forms, may contribute to velocity gradients but are secondary to thermal controls in the overall profile.[46]Core
Size and physical state
The Moon's core is composed of a solid inner core surrounded by a fluid outer core, as inferred from integrated geophysical models combining seismic, gravity, and moment-of-inertia data.[56] Estimates for the inner core radius range from approximately 240 km, based on reanalysis of Apollo seismic data, to 258 ± 40 km from recent modeling incorporating tidal dissipation and mantle properties.[57][58] The outer liquid core extends to a radius of 330–420 km, constrained by the Moon's normalized moment of inertia (I_s/MR^2 = 0.3927 ± 0.000012) derived from GRAIL gravity measurements and Lunar Laser Ranging, which indicate a core mass fraction of less than 1.5–2% of the total lunar mass.[56] The density of the inner core is estimated at 7.5–8 g/cm³, consistent with an iron-rich composition alloyed with 6–10% light elements such as sulfur or oxygen to match the overall lunar bulk density and seismic velocities.[58][59] Evidence for the liquid state of the outer core comes from Apollo-era seismic recordings of deep moonquakes, which show core-reflected shear waves (ScS phases) and free oscillations with periods of 5–10 minutes that require a fluid layer to explain the observed dissipation and tidal response. The solid inner core is supported by velocity models analogous to Earth's Preliminary Reference Earth Model (PREM), featuring a sharp increase in shear-wave velocity at the inner-outer core boundary due to solidification under pressures around 4–5 GPa.[57] Current models suggest the lunar core lacks active convection today, attributable to a low Rossby number arising from the Moon's slow rotation rate, which limits the vigor of fluid motions necessary for dynamo generation.[60] This stagnant state aligns with the absence of a present-day magnetic field and is briefly referenced in mantle seismic attenuation patterns near the core-mantle boundary, indicating minimal heat transfer from below.[12]Composition and magnetic implications
The lunar core is primarily composed of an Fe-Ni-S alloy, with models indicating approximately 85 wt% iron, 5 wt% nickel, and 5-10 wt% sulfur, a composition that aligns with density estimates derived from cosmochemical partitioning during planetary differentiation.[61] This elemental makeup is constrained by siderophile element depletions in the lunar mantle and seismic models requiring a dense metallic phase to match the Moon's overall moment of inertia.[62] The presence of sulfur as a key light element lowers the core's density to values around 7000-7800 kg/m³, consistent with geophysical inversions.[63] Sulfur plays a critical role in the light element budget of the core, with its solubility in liquid iron reaching up to 20 wt% under lunar inner core pressures of approximately 4-5 GPa and temperatures of 1500-2000 K.[63] This high solubility facilitates the incorporation of sulfur during core formation, potentially driving compositional convection through inner core crystallization, which releases buoyant light-element-rich fluids into the outer core. Such processes are essential for sustaining dynamo activity, as thermal convection alone is insufficient given the Moon's small core size of roughly 300-400 km radius.[61] Paleomagnetic records from crustal remanence in Apollo samples indicate that a core dynamo was active between approximately 4.2 and 3.5 billion years ago (Ga), powered primarily by the growth of a solid inner core that induced compositional buoyancy, with surface magnetic fields estimated at 40-110 μT in the maria basalts.[64][65] Recent analyses of Chang'e-6 farside basalt samples dated to ~2.8 Ga reveal a rebound in dynamo activity at that time, with paleointensities of 5–21 μT (median ~13 μT), suggesting the dynamo persisted or revived later than previously indicated by Apollo data alone and possibly driven by mechanisms such as a basal magma ocean or precession in addition to core crystallization.[66] Subsequent studies suggest a weak but persistent magnetic field during the Moon's midstage history (~2–3 Ga) at intensities of 2–4 μT.[67] In contrast to Earth's core, the Moon's smaller dimensions restrict the available power for compositional convection to around 10^{12} W, limiting the dynamo's longevity and intensity.[68]Evolutionary context
Formation and differentiation
The prevailing model for the Moon's origin is the giant impact hypothesis, which posits that approximately 4.5 billion years ago (Ga), a Mars-sized protoplanet called Theia collided with the proto-Earth, vaporizing portions of both bodies and ejecting debris into orbit.[69] This event likely generated a synestia—a rapidly rotating, vapor-rich disk of rock—surrounding Earth, from which the Moon accreted through the coalescence of molten and vaporized material predominantly derived from Earth's mantle. Following accretion, the Moon existed as a largely molten body with a global lunar magma ocean (LMO) extending to depths of around 1000 km, encompassing much of its mantle. Crystallization of this magma ocean proceeded from the bottom up and top down over approximately 100–200 million years (Ma), beginning with the settling of dense mafic minerals like olivine and pyroxene to form a cumulate mantle, while buoyant plagioclase floated to create a primary anorthositic crust.[70] This sequential solidification process established the Moon's initial layered internal structure, with incompatible elements concentrating in a residual melt that later contributed to late-stage magmatic activity. Amid LMO crystallization, metal-silicate separation occurred as denser iron-rich metallic liquids sank toward the center, forming the lunar core around 4.4 Ga and comprising roughly 1.6–2% of the Moon's total mass. The core's small size reflects the limited metallic content in the post-impact debris, consistent with geochemical models of siderophile element partitioning during this differentiation phase.[72] The resulting crust exhibited hemispheric asymmetry, with the nearside thinner and more prone to basaltic flooding due to enhanced tidal heating from the nearby Earth, which sustained elevated temperatures and facilitated mare volcanism on that hemisphere.[73] Overall differentiation, including LMO solidification and major layering, was effectively complete by approximately 4.35 Ga, as evidenced by Lu-Hf isotope systematics in ancient lunar zircons and anorthosites from Apollo samples.[74]Current models and uncertainties
Current integrated models of the Moon's interior rely on joint inversions of gravity data from the GRAIL mission, Apollo-era seismic observations, and constraints from the Moon's moment of inertia (MOI) to derive three-layer structures comprising a crust, mantle, and core.[12] These models typically indicate a core radius of approximately 300–400 km, a mantle extending to about 1,700 km depth, and a crust varying from 30–50 km thick, with densities increasing from ~3.0 g/cm³ in the upper mantle to ~3.5 g/cm³ near the core-mantle boundary.[75] Such inversions reduce ambiguities by combining complementary datasets, though they assume simplified radial variations and face challenges in resolving lateral heterogeneities.[12] Recent analyses of tidal deformation data as of 2025 reveal thermal asymmetries in the mantle, with the nearside warmer by 100–200 K, potentially linked to early tidal heating and mantle evolution.[73] Geodynamo simulations incorporating thermal and compositional convection in the lunar core suggest that dynamo activity, responsible for early magnetic fields, likely ceased around 3.65 Ga due to insufficient convective vigor in post-magma-ocean regimes.[76] These models indicate that inner core solidification could briefly sustain convection, but long-lived fields beyond 3 Ga are difficult to maintain without additional heat sources or enhanced light element abundances.[77] Key uncertainties persist regarding the core's light element composition, with sulfur (S) as the leading candidate at 6–10 wt% to explain density deficits, though oxygen (O) or carbon (C) alternatives remain debated due to partitioning behavior during core formation.[76] The extent of mantle overturn, involving the sinking of dense ilmenite-bearing cumulates from the magma ocean, is also unresolved, with models suggesting partial (up to 50%) rather than global participation to match trace element distributions in basalts.[55] Additionally, the fraction of deep partial melt in the lower mantle, inferred from seismic attenuation, varies widely between 1–5% in models, influencing viscosity and heat transport but lacking direct constraints.[16] Future missions aim to address these gaps. Artemis program sample returns from the lunar south pole will enable improved Hf-W isotope dating, potentially resolving core formation timelines within 10–20 Ma precision.[78] Discrepancies between seismic velocities and gravity-derived densities in the deep interior hint at hidden layers, such as a partially molten boundary zone at ~1,000–1,200 km depth, which could reconcile low shear-wave speeds with higher-than-expected densities from GRAIL.[16] These inconsistencies suggest unmodeled heterogeneities, possibly from relic overturn or volatile enrichment, underscoring the need for seismic network deployments in upcoming missions.[79]References
- https://science.[nasa](/page/NASA).gov/moon/formation/