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Planetary differentiation

Planetary differentiation is the geophysical process by which a newly formed planetary body separates into distinct layers based on the density and chemical properties of its materials, typically resulting in a dense metallic , a silicate-rich , and a lighter crust. This separation occurs primarily during the early stages of planetary evolution when the body is sufficiently heated to allow molten materials to flow under the influence of , with heavier elements like iron sinking toward the center while lighter silicates rise to the surface. Heat sources driving this process include accretionary impacts during formation, gravitational compression, and radioactive decay of such as and . The process is evident across the terrestrial planets and many asteroids, as seen in meteorites that preserve fragments of differentiated interiors, such as iron meteorites from ancient cores and achondrites from mantles or crusts. For instance, Earth's differentiation around 4.5 billion years ago produced a nickel-iron core with an average density of about 11 g/cm³, a silicate mantle comprising roughly 84% of the planet's volume at an average density of about 4.4 g/cm³, and a crust at about 2.7 g/cm³, further leading to the formation of oceans and an atmosphere from volcanic outgassing. Recent models suggest that core formation may involve percolation of molten iron-sulfide through solid rock via microscopic channels, allowing differentiation without complete planetary melting, as supported by geochemical analysis of meteorites and 3D imaging experiments. Evidence from iron meteorites, such as the IIE group, indicates partial in early planetesimals, where accretion was protracted over more than 1 million years, resulting in bodies with coexisting melted mantles, metallic cores generating dynamo magnetic fields, and unmelted chondritic crusts. This layered structure has profound implications for and dynamics, enabling phenomena like , which protects the atmosphere from erosion, and influencing tectonic activity and . In gas giants like , involves separation of , , ices, and rocks under , though driven more by gradients than extensive .

Definition and Overview

Core Concept

Planetary differentiation is the process by which a planetary body separates into distinct layers—such as a dense metallic , a mantle, and a lighter crust—driven by differences in and , where heavier materials like iron-nickel alloys sink toward the center while lighter rise. This stratification occurs through the segregation of immiscible phases, requiring at least partial liquidity in one or more phases to allow material transport under gravitational forces. The process fundamentally reshapes the internal structure of rocky bodies, establishing a layered that influences long-term geological . For differentiation to proceed, two key prerequisites must be met: sufficient internal heat to induce partial melting, enabling the mobility of dense components, and adequate gravitational acceleration to drive the separation of materials by density. The gravitational potential energy released during this separation, which contributes to heating, is approximated by the change in gravitational self-energy: \delta E_{\text{Grav}} \approx 0.04 \frac{GM^2}{R}, where G is the , M is the , and R is the planetary radius; this energy release can raise temperatures significantly, facilitating further . Without these conditions, homogeneous compositions would persist, preventing . typically unfolds early in a planetary body's , within the first 100 million years following formation, often overlapping with the accretion phase. Evidence for this process is inferred from various observations, including meteorites that preserve remnants of differentiated planetesimals (such as iron meteorites indicative of metallic cores), seismic data revealing distinct internal layers with varying seismic velocities, and planetary generated by in liquid cores. These indicators collectively confirm the ubiquity of among larger planetary bodies.

Historical Development

The idea of planetary differentiation emerged in the late with early geophysical insights into Earth's interior. In 1889, Clarence Edward Dutton proposed the principle of , describing how lighter crustal blocks "float" on a denser underlying layer, implying inherent variations consistent with separation in the planet's . This concept laid groundwork for understanding layered interiors, though mechanisms remained unclear until the discovery of in 1896 provided a heat source for potential melting and segregation. By the 1920s, geophysicist incorporated into models of Earth's thermal evolution, linking it to and proposing to explain crustal movements and internal driven by heat from and decay. Seismic observations in the mid-20th century provided direct evidence for Earth's differentiated layers. During the and , analyses of earthquake-generated waves confirmed the core-mantle boundary at approximately 2,900 km depth, with Francis Birch's 1940 work using velocity discontinuities to argue for a dense iron-nickel , and K.E. Bullen's 1950 studies supporting the inner 's solidity through wave propagation models. These findings shifted theories from uniform composition to a stratified with distinct , , and crust. The Apollo missions of the further extended this to other bodies, as samples from the lunar highlands—particularly anorthositic rocks returned by in 1969—revealed a plagioclase-rich crust formed by flotation during early ocean , marking the Moon's around 4.5 billion years ago. Modern refinements have drawn on analyses and data to trace across the solar system. Studies of achondritic s from the 1980s through the 2000s, such as those on howardites-eucrites-diogenites (HEDs) linked to asteroid 4 Vesta, demonstrated mantle-crust separation via and fractional crystallization, with patterns indicating igneous processes in differentiated parent bodies. NASA's lander (2018–2022) used seismometers to detect Mars' -mantle boundary, revealing a liquid radius of about 1,830 km and, in post-mission analyses, a solid inner of roughly 600 km, affirming rapid shortly after accretion. Similarly, the Juno mission, operational since 2016, employed gravity mapping to uncover Jupiter's interior layers, including a dilute, non-uniform extending to 40–45% of the planet's and compositional gradients from helium rain, illustrating gaseous in gas giants. Isotopic dating techniques have recently clarified timelines, showing Earth's core formation and overall differentiation completed within about 30 million years of solar system formation around 4.567 billion years ago, based on hafnium-tungsten and uranium-lead systematics in rocks and meteorites.

Sources of Internal Heating

Accretional and Impact Heating

Accretional heating occurs during the formation of planets as planetesimals collide and merge, converting their into upon impact. This process releases energy as smaller bodies fall toward the growing , with the total heat generated approximated by the formula Q \approx \frac{3}{5} \frac{G M^2}{R}, where G is the , M is the planet's , and R is its . This scaling explains why larger bodies experience disproportionately greater heating, as the energy release grows quadratically with while inversely with , potentially raising temperatures enough to melt significant portions of the interior. Impact heating intensifies during the late stages of accretion, particularly from giant impacts between protoplanets, where the of the colliding body, given by E = \frac{1}{2} m v^2 (with m as the impactor's and v its ), vaporizes and melts large fractions of the target. A prominent example is the Moon-forming giant impact approximately 4.5 billion years ago, involving a Mars-sized body striking the proto-Earth at velocities around 15 km/s, which melted 30–65% of the planet's and partially vaporized , ejecting material to form the lunar disk. Such events dominate the final energy input, often exceeding that from smaller collisions. This combined heating from accretion and impacts creates global magma oceans in the early histories of inner planets, with depths potentially reaching thousands of kilometers and persisting for millions of years as molten layers. For , these oceans formed around 4.5 billion years ago, driven primarily by the cumulative energy of giant impacts that overwhelmed the planet's . Recent advances in () simulations from 2020–2025 demonstrate how these collisions induce rapid in protoplanets by generating localized and mixing, with revised laws showing that outcomes depend strongly on mass ratios and energies, leading to core-mantle segregation even in sub-Moon-sized bodies.

Radioactive Decay and Secular Cooling

Radioactive decay of long-lived isotopes within a planet's interior provides a sustained source of heat that influences differentiation long after formation. The primary contributors are uranium-238 (^238U), thorium-232 (^232Th), and potassium-40 (^40K), which decay through alpha, beta, and gamma emissions, releasing energy that heats the surrounding rock. These isotopes are unevenly distributed during differentiation, with higher concentrations often in the silicate mantle, sustaining partial melting and convection over billions of years. The heat production rate from radioactive decay is given by the formula H = \rho C \lambda E, where H is the heat production per unit volume, \rho is the rock density, C is the concentration of the isotope, \lambda is the decay constant, and E is the energy released per decay event. This process powers mantle dynamics in terrestrial planets, contributing to ongoing chemical and physical separation of materials. Secular cooling refers to the gradual loss of a planet's internal heat through conduction and convection, primarily at the surface via radiative emission from the crust and atmosphere. This cooling drives the solidification of any initial magma oceans, transforming fully molten mantles into layered structures with a solid lithosphere over time. For Earth-like planets, models indicate that deep magma oceans can solidify rapidly, with the mantle becoming significantly more viscous within approximately 20,000 years, though full planetary cooling extends over geological timescales. This heat loss balances radiogenic input, preventing indefinite melting while allowing localized partial melts in the upper mantle. The interplay between radioactive heating and secular cooling maintains dynamic conditions for , such as in that facilitates element segregation. On , radioactive accounts for roughly 50% of the current surface of about 47 terawatts, with the remainder from heat and solidification. Recent advances in hafnium- (Hf-W) dating, including models of isotope evolution during accretion, confirm that early heat budgets were dominated by rapid formation, influencing the distribution of heat-producing elements and supporting prolonged . Following initial heating from accretion and impacts, these internal processes ensure continues for billions of years in rocky planets.

Physical Differentiation Processes

Gravitational Separation

Gravitational separation is a physical process in planetary differentiation where denser materials sink under the influence of gravity within a molten or semi-molten planetary body, resulting in the stratification of layers based on density without any change in chemical composition. This mechanism relies on buoyancy differences, allowing materials like iron-nickel alloys, which have higher densities than surrounding silicate materials, to migrate downward due to negative buoyancy. The driving force for this sinking is the gravitational stress, approximated as \Delta \rho g h, where \Delta \rho is the density contrast between the sinking material and the ambient medium, g is the local gravitational acceleration, and h is the characteristic length scale such as the depth or size of the sinking body. The primary outcome of gravitational separation is the positional sorting of materials, leading to the formation of a dense metallic at the planet's center enveloped by a less dense mantle. This process does not involve chemical reactions or partitioning but simply rearranges existing components according to their densities under . For effective separation to occur, the planetary interior must be sufficiently fluid, typically requiring temperatures above the of , which ranges from approximately 1000°C to 2000°C depending on and . The initiation of gravitational separation is governed by instabilities in the fluid interior, often quantified by the Rayleigh number, \mathrm{Ra} = \frac{\alpha \Delta T g h^3}{\kappa \nu}, where \alpha is the thermal expansivity, \Delta T is the temperature variation, \kappa is the thermal diffusivity, and \nu is the kinematic viscosity; values exceeding a critical threshold (typically around 10^3 to 10^4 for onset) promote convective motions that facilitate the sinking of dense phases. Evidence for such density-driven layering comes from geophysical observations, such as the Earth's normalized moment of inertia factor I / (M R^2) = 0.3307, which is lower than the 0.4 expected for a uniform sphere, indicating a central concentration of dense material consistent with core-mantle separation.

Thermal Convection

Thermal convection in planetary interiors arises from forces generated by -induced variations, where hotter, less dense material rises and cooler, denser material sinks, primarily within . This process is driven by steep gradients established by internal sources, leading to fluid-like motion in the viscous layers of differentiated planets. In terrestrial bodies, such convection begins after initial accretion and , facilitating the transport of from the core-mantle boundary to the surface. The velocity of convective motion scales approximately as v \sim \sqrt{\alpha g \Delta T h}, where \alpha is the thermal expansion coefficient, g is , \Delta T is the difference across the convecting layer, and h is the layer thickness; this relationship derives from balancing and viscous drag in high-viscosity fluids typical of planetary mantles. Such scaling underscores how efficiency depends on planetary size, , and thermal state, with velocities on reaching centimeters per year in the . Initially, this vigorous mixing homogenizes materials, preventing early and allowing for subsequent density-driven separation. As evolves, it transports partial melts upward, concentrating incompatible elements and promoting layered structures through upwellings and plumes that rise from deep mantle sources. Convection styles vary between whole-mantle circulation, where material flows unimpeded from to surface, and layered regimes, where barriers like transitions or compositional contrasts impede exchange; these are influenced by viscosity jumps, often by orders of magnitude, between upper and due to mineral changes. Whole-mantle , inferred for , enhances global mixing and heat loss, while layered , possibly dominant in or ancient Mars, traps heat deeper and slows evolution. Viscosity contrasts, exceeding $10^3 Pa·s differences, can stabilize layers and alter plume , with plumes forming as narrow, buoyant columns that drive localized and influence surface . Seismic tomography studies, including a key 2022 analysis and ongoing interpretations through 2025, have provided evidence of ongoing plumes on and Mars, revealing low-velocity anomalies indicative of hot upwellings. On , high-resolution models confirm broad plume structures beneath hotspots like , extending from the core- boundary with cross-sectional widths of ~1000 km. For Mars, mission data from a 2022 study uncovered an active plume under , spanning ~4,000 km in diameter with a thickness of 200-500 km, suggesting persistent despite a stagnant regime and linking to recent ; later analyses, such as 2024 seismic anisotropy studies under and 2025 insights into heterogeneity, support continued activity. These insights highlight 's role in sustaining planetary activity over billions of years, bridging static models with observed seismic signatures.

Chemical Differentiation Processes

Partial Melting

Partial melting occurs when localized regions within a planetary exceed the temperature due to uneven heating, causing select minerals to melt while others remain solid. This process generates low-density melts that are buoyant relative to the surrounding solid matrix, enabling them to rise through porous flow or fractures toward the surface. The degree of melting, denoted as the melt F, is approximated by the F = \frac{T - T_{\text{solidus}}}{T_{\text{liquidus}} - T_{\text{solidus}}} where T is the temperature, T_{\text{solidus}} is the temperature at which melting begins, and T_{\text{liquidus}} is the temperature at which the material is fully molten. This mechanism initiates chemical separation by extracting melt from the residue, concentrating lighter components in the ascending liquid and leaving behind a depleted solid framework. Chemically, partial melts preferentially incorporate incompatible elements—those with low solubility in mantle minerals such as (K) and rare earth elements (REE)—resulting in enrichment relative to the bulk source. For instance, melting of , the dominant mantle rock, produces basaltic compositions rich in these elements at low melt fractions. This selective partitioning arises because incompatible elements partition strongly into the liquid phase during incongruent melting, enhancing their mobility and altering the mantle's overall composition over time. In planetary differentiation, plays a crucial role by extracting materials that form the crust, as the risen melts solidify at shallow depths to build layered structures. This process typically operates at melt fractions of 10-30% in , sufficient to generate voluminous basaltic magmas that contribute to crustal growth without requiring complete . Such depletes the in fusible components while enriching the crust, promoting long-term chemical . Recent laboratory experiments in the 2020s have advanced understanding of high-pressure by incorporating volatile effects, revealing that (H₂O) significantly lowers the temperature and promotes hydrous melt formation even at modest concentrations. For example, water-saturated experiments on at shallow pressures demonstrate enhanced melting and altered relations, with implications for volatile-influenced in water-bearing planetary interiors. These findings update models of melting, showing that volatiles can initiate at lower temperatures than previously assumed under conditions.

Fractional Crystallization

Fractional crystallization represents a fundamental chemical differentiation in planetary bodies, wherein a cooling melt undergoes progressive solidification, with newly formed crystals separating from the evolving residual liquid. This process typically follows events that generate the initial , leading to the extraction and segregation of distinct phases based on their stability fields. In planetary contexts, such as magma oceans on early differentiated worlds, fractional crystallization drives the layering of by preferentially removing minerals from the melt. The process begins with the and growth of early-crystallizing minerals like and , which are compatible with the high-temperature, compositions prevalent in planetary melts. As these crystals form and separate—often sinking due to contrasts—the residual melt becomes progressively depleted in magnesium and iron while enriching in silica, aluminum, and other incompatible elements. This evolution adheres to principles of equilibria, approximated by the , which quantifies the relative proportions of solid and liquid s along a tie-line in a or pseudobinary . For instance, in a simplified system, the fraction of a is determined by the relative distances from the bulk composition to the phase boundaries. The fractionation of elements is further described by the D = \frac{C_{\text{crystal}}}{C_{\text{melt}}}, where C denotes concentration; compatible elements (e.g., Mg in ) exhibit D > 1, concentrating in the early solids, while incompatible elements (D < 1) remain in the melt. The outcomes of fractional crystallization include the accumulation of ultramafic cumulate layers, such as olivine- and pyroxene-dominated assemblages forming the lower mantle in terrestrial planets. On Mars, models of intermediate-depth magma oceans demonstrate how this process builds dense, silica-enriched residuals that can form thermochemical boundary layers at the base of the mantle. In the context of global magma oceans, crystallization often proceeds bottom-up from the base, where the adiabat intersects the solidus, leading to the formation of stratified cumulates; light elements and late-stage melts are trapped toward the top, contributing to the development of a primitive crust. Recent simulations of the lunar magma ocean (2021–2024) incorporate experimental phase equilibria to model this bottom-up solidification, revealing that after 90–95% crystallization, dense ilmenite-bearing cumulates form unstable layers prone to localized overturn, while retaining ~0.5% interstitial melt enriched in heat-producing elements.

Core Formation

Siderophile Element Segregation

Siderophile elements, including , , and , exhibit a strong affinity for metallic phases, leading to their preferential segregation into the forming during differentiation. Under the high- and high- conditions prevalent in the early , these elements become increasingly soluble in molten iron, facilitating their extraction from the surrounding material. The metal- (D^{metal/silicate}), defined as the concentration ratio of an element in the metal phase to that in the silicate phase, rises significantly with increasing and ; for instance, experiments up to 20 GPa and 2800°C demonstrate that this coefficient for moderately siderophile elements like Ni and Co can increase by orders of magnitude, promoting efficient coreward transport. This partitioning is governed by , where oxygen and light element content in the metal (e.g., or ) further modulate , though recent models emphasize the dominant role of P-T in achieving the observed depletions. Geochemical evidence for this is evident in the severe depletion of siderophile in planetary s relative to chondritic precursors. In Earth's primitive , concentrations of highly siderophile (HSEs) such as (Os), (Ir), and platinum (Pt) are roughly 0.001 to 0.01 times those in CI chondrites, consistent with near-complete removal into the core during metal-silicate separation. This depletion pattern holds for moderately siderophile as well, with mantle abundances of , , , and indicating that core formation captured over 90% of these after the bulk of the planet had accreted. However, the persistence of trace HSEs in the mantle cannot be fully explained by partitioning alone and supports the late veneer hypothesis, wherein ~0.5-1% of Earth's mass was added as chondritic projectiles after core formation, re-enriching the mantle without significantly perturbing deeper structure. isotope data, showing a slight excess of radiogenic ^{182} in the mantle, corroborate this post-core addition occurring tens of millions of years after initial . The timescale of siderophile element was remarkably rapid, occurring within 10-30 million years of solar system formation around 4.56 Ga, as constrained by the Hf- system. Hafnium-182, a short-lived with a of ~8.9 million years, decays to ^{182}; since Hf is lithophile and remains in the silicate while W is moderately siderophile and partitions into , any delay in segregation produces excess ^{182} in the . Earth's exhibits a ^{182} anomaly of ~20 ppm relative to chondrites, implying core formation concluded by ~30 million years after calcium-aluminum-rich inclusion (CAI) formation, or approximately 4.53 Ga. Recent studies highlight potential deviations from perfect , such as kinetic effects during -driven metal descent through a partially molten , which could lead to incomplete siderophile extraction and subtle isotopic heterogeneities; for example, stress-controlled models suggest that disequilibrium partitioning at low melt fractions preserves some enrichment in moderately siderophile elements like . These insights refine earlier -based models, indicating that core formation involved both rapid global events and localized disequilibrium processes.

Role of Giant Impacts

Giant impacts play a pivotal role in planetary differentiation by disrupting existing structures and accelerating the segregation of metallic from mantles. These collisions, involving of comparable mass, release immense energy that partially or fully melts the target body, creating a global ocean where materials can mix extensively. This mixing facilitates the sinking of dense metallic components, such as iron-nickel alloys from the impactor, toward the planet's center under gravity, thereby promoting efficient formation. In the Earth-Moon system, the collision with —a Mars-sized —exemplifies this process, as it led to widespread re-melting and equilibration of iron-silicon partitioning between the metallic and phases, influencing the final compositions of Earth's and . The effects of such impacts extend to resetting local differentiation states and enhancing overall core growth. By vaporizing and dispersing portions of the mantles, giant impacts can homogenize previously segregated layers, allowing siderophile elements—previously discussed in terms of —to redistribute more effectively through the induced mixing. This not only erases incomplete early but also merges metallic "blobs" from the impactor with the target's , rapidly increasing the core's mass and size. For instance, simulations of Mars-sized impacts indicate that this merger process can significantly increase the core mass, for example by adding several percent of the target's core mass in events like the Moon-forming impact. Numerical models, particularly those using (SPH), demonstrate the rapidity of these post-impact dynamics. In SPH simulations of the impact, the impactor's core material disperses into the proto-Earth's mantle but coalesces with the existing core within hours to days, driven by gravitational instabilities and buoyancy effects in the hot, viscous environment. These models highlight how the collision's and energy influence the efficiency of metal sinking, with higher-impact velocities leading to more complete mixing and faster segregation. Such simulations underscore the stochastic nature of giant impacts, where varying parameters like impact angle and velocity can produce diverse outcomes in core-mantle partitioning. Recent advances in studies, informed by (TESS) data from 2020 onward, provide analogs suggesting giant impacts drive in diverse planetary systems. Observations of high-density super-Earths and mini-Neptunes, with inferred metal-rich cores, align with models where late-stage collisions strip volatile envelopes and enhance metallic segregation, mirroring solar system processes. For example, analyses of TESS-discovered worlds indicate that inefficient accretion punctuated by giant impacts can explain the observed compositional diversity, with metal fractions up to 50% in some cases attributable to core-merging events. These findings emphasize the universality of impact-driven differentiation beyond our solar system.

Solar System Examples

Earth and Terrestrial Planets

Earth's planetary differentiation is well-characterized through seismic, geochemical, and cosmochemical evidence, revealing a layered structure consisting of a granitic , a peridotitic , and an iron-rich containing 5-10% light elements such as . The is dominated by , a rock type rich in and pyroxenes, while the continental crust exhibits a granitic composition with higher silica content derived from repeated and fractional processes. Seismic studies identify key discontinuities, including the 410 km depth marking the transition from to wadsleyite in the and the core- (CMB) at approximately 2900 km, where P-wave velocity increases sharply due to the density contrast between the silicate and metallic . This differentiation occurred rapidly shortly after Earth's formation, around 4.53 billion years ago (), as evidenced by hafnium-tungsten systematics indicating core segregation within the first 30 million years of solar system history. Mars exhibits a differentiated interior with a thicker crust averaging about 50 km compared to Earth's , a silicate , and a smaller relative to its planetary size, reflecting less efficient segregation possibly due to its smaller mass and lower internal temperatures. Data from NASA's mission (2018-2022) revealed a consisting of a solid inner with radius approximately 610 km and a total radius of approximately 1830 km, including a liquid outer , surrounded by a indicative of in the upper , suggesting incomplete or ongoing differentiation processes. Unlike Earth, Mars lacks active , leading to a more static crustal structure dominated by basaltic from ancient plumes. Venus shares compositional similarities with , featuring a differentiated structure of a basaltic crust, mantle, and iron core, but its surface has been extensively resurfaced by catastrophic around 500-750 million years ago, erasing much of the early tectonic record. Seismic data are limited, but radar mapping from missions like Magellan indicates a crust thickness of 20-50 km, with evidence of driving widespread basaltic flows. Mercury's extreme is highlighted by its disproportionately large core, comprising about 85% of the planet's radius and roughly 57% of its volume, attributed to volatile loss during a giant that stripped away much of the original mantle. This event left a thin crust (20-35 km) and a reduced, sulfur-poor mantle, with the core's high iron content explaining the planet's elevated . Among the terrestrial planets, a key commonality is the formation of basaltic crust through of , as seen in Earth's mid-ocean ridge basalts, Mars' volcanics, ' tesserae plains, and Mercury's northern smooth plains, underscoring the universal role of thermal convection in driving and surface renewal.

Moon and KREEP Distribution

The Moon's differentiation is closely tied to its formation via a giant impact between proto-Earth and a Mars-sized body known as , which ejected debris that coalesced into the , resulting in a hybrid composition blending materials from both parent bodies. This event, occurring approximately 4.5 billion years ago, left the nascent in a molten state, forming a global (LMO) that underwent extensive . As the LMO cooled, plagioclase-rich floated to form a primary crust, while denser minerals sank to create an olivine-orthopyroxene-dominated mantle; the incompatible-element-enriched residual liquid, known as (rich in potassium, rare earth elements, and phosphorus), became concentrated in the . This process exemplifies fractional , where sequential mineral removal from the melt progressively enriches the remaining liquid in incompatible components. KREEP serves as a key tracer of the Moon's late-stage differentiation, marking the final residues of LMO solidification and subsequent magmatic events. Its distribution is highly asymmetric, with the majority concentrated in the nearside Procellarum KREEP Terrain (PKT), a vast region encompassing Oceanus Procellarum and surrounding highlands, where thorium concentrations—a proxy for KREEP—reach up to 10-15 ppm, far exceeding the lunar average of ~1 ppm. Apollo missions, landing primarily within or near the PKT (e.g., Apollo 12, 14, 15, and 16), recovered samples exhibiting pronounced thorium anomalies, such as breccias and basalts with thorium levels of 5-50 ppm, confirming localized enrichment from post-LMO volcanism and impact mixing. In contrast, the farside and non-PKT nearside regions show minimal KREEP, reflecting incomplete overturn of the cumulate mantle or asymmetric redistribution during early bombardment. The giant impact not only initiated the LMO but also drove further differentiation through post-impact re-melting, as retained and accretional remobilized the proto-lunar disk material, enhancing chemical . This re-melting facilitated the extraction and concentration of , which later influenced localized , such as the formation of Mg-suite intrusions and mare basalts in KREEP-rich areas. Unlike , the lacks ongoing internal today, having cooled rapidly due to its smaller size and lack of a significant atmosphere, resulting in a rigid and cessation of global-scale mixing around 3-4 billion years ago. Recent missions have refined our understanding of these processes. The Chang'e-5 mission (2020) sampled young basalts in the PKT with low signatures, indicating derivation from depleted mantle sources, while Chang'e-6 (2024) returned farside samples from the Apollo basin, including ferroan clasts with compositions matching nearside highlands, providing direct evidence for a global anorthositic crust and affirming the LMO's uniformity across the . These findings resolve prior debates on whether anorthosite formation was localized, highlighting 's role as a nearside-specific residue rather than a global feature.