Geodetic Evidence for a Compositionally Distinct Lunar Nearside Mantle

Technical Summary: Geodetic Evidence for a Compositionally Distinct Lunar Nearside Mantle

Problem Statement
The Moon exhibits a profound geological and geophysical asymmetry between its nearside and farside. The nearside is characterized by younger, dark basaltic maria overlying the Procellarum KREEP Terrane (enriched in Potassium, Rare-Earth elements, and Phosphorus), while the farside is dominated by older, brighter, and heavily cratered highlands. While previous models have attributed this dichotomy primarily to crustal thickness variations (with the farside crust being nearly twice as thick as the nearside), these models often assume a spherically symmetric mantle or ignore mantle heterogeneity. Recent reanalyses of tidal data suggest interior differences, but interpreting tidal Love numbers ($k_{3m}$) solely in terms of shear modulus variations creates a degeneracy between thermal and compositional effects. Furthermore, static gravity measurements alone cannot uniquely resolve density variations. The origin and depth extent of these hemispheric asymmetries—whether they are confined to the crust or penetrate the deep mantle—remain unresolved.

Methodology
To resolve these ambiguities, the authors perform a joint Bayesian inversion of three distinct observational datasets:

1. Time-variable (Tidal) Gravity: Three-dimensional tidal Love numbers ($k_{2m}$ and $k_{3m}$) associated with the 28-day Earth-Moon tide, derived from GRAIL spacecraft data.
2. Static Gravity: The static gravity field (spherical harmonic coefficients $C_{lm}$ and $S_{lm}$) from the latest GRAIL solution.
3. Center-of-Mass/Center-of-Figure (COM-COF) Offset: Measurements of the offset between the Moon's mass center and its figure center.

The inversion framework utilizes a theory that self-consistently accounts for lateral variations in density ($\rho$), shear modulus ($\mu$), and boundary topography. This approach allows degree-2 tidal forcing to couple with degree-1 structure in density and shear modulus, generating an induced degree-3 tidal response. The model is parameterized as perturbations from a 1D reference model across 30 radial layers, using spherical harmonics up to degree $\ell=3$.

Crucially, the authors jointly invert these datasets to break the trade-off between density and shear modulus. Following the geophysical inversion, they employ a "thermochemical inversion" using experimentally derived scaling relationships to map the recovered $\delta\rho$ and $\delta\mu$ perturbations into variations in temperature ($\Delta T$), iron enrichment ($\Delta Fe$), and water content ($\Delta H_2O$). This second step incorporates "pre-melting" effects on shear modulus, which are significant as temperatures approach the solidus.

Key Results
The joint inversion yields a robust three-dimensional thermochemical model of the lunar mantle with the following findings:

• Hemispheric Asymmetry: The nearside mantle is up to 1.5% denser and 4% less rigid (lower shear modulus) than the farside. This defines a hemispheric-scale compositional asymmetry extending throughout the mantle, not just the crust.
• Thermochemical Interpretation: The geophysical anomalies are most consistent with a nearside mantle that is warmer, iron-rich, and wetter than the farside.
  • Temperature: The temperature anomaly is strongest in the upper mantle (shallow, pronounced anomaly of $\approx \pm 100\text{--}150$ K) and muted in the deep mantle ($\approx \pm 10\text{--}20$ K).
  • Composition: Iron enrichment correlates with the shallower, hotter regions (variations of $\pm 3\%$), while deeper regions show smaller variations ($\pm 1\%$).
  • Water: The model infers a wetter nearside and drier farside, consistent with geochemical constraints from Chang'e-5 and Chang'e-6 samples.
• Surface Heat Flow: The inferred mantle temperature distribution predicts surface heat flow values consistent with measurements from Apollo 15, Apollo 17, and the Blue Ghost LISTER instrument.
• Spectral Signatures: The density field exhibits significant $\ell=1$, $\ell=2$, and $\ell=3$ structure, while the shear modulus field is dominated by $\ell=1$ and $\ell=3$ with negligible $\ell=2$ signal.

Significance and Claims
The paper claims to provide the first jointly constrained 3D thermochemical model of the lunar mantle, offering several critical insights:

1. Deep Interior Origin: The results demonstrate that the lunar nearside-farside dichotomy is not merely a crustal phenomenon but is rooted in a long-lived, hemispheric mantle asymmetry.
2. Mechanism of Formation: The observed $\ell=1$ thermochemical signal (warmer, denser nearside) is inconsistent with a fossil rotational-tidal bulge (which would produce $\ell=2$ structure in both density and rigidity) or purely transient tidal heating. Instead, the data support mechanisms involving asymmetric magma-ocean crystallization and/or mantle overturn (specifically the sequestration of KREEP-rich, Fe/Ti-rich cumulates beneath the nearside).
3. Resolution of Degeneracy: By jointly inverting static and tidal gravity, the study successfully separates thermal from compositional heterogeneities, a task previously impossible with tidal data alone.
4. Consistency with Samples: The inferred properties (wetter, iron-rich nearside; cooler, drier farside) align with recent geochemical measurements from returned lunar samples (Chang'e-5/6) and surface heat flow data, suggesting that relics of early lunar differentiation are preserved in the present-day mantle.

The authors conclude that these results offer a new framework for linking geophysical observations with geochemical data and suggest that similar joint inversion techniques could be applied to future missions (e.g., JUICE, VERITAS, EnVision) to probe the interiors of other planetary bodies.