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Abstract

The Gravity Recovery and Interior Laboratory (GRAIL) mission has sampled lunar gravity with unprecedented accuracy and resolution. The lunar GM, the product of the gravitational constant G and the mass M, is very well determined. However, uncertainties in the mass and mean density, 3345.56 ± 0.40 kg/m³, are limited by the accuracy of G. Values of the spherical harmonic degree-2 gravity coefficients J₂ and C₂₂, as well as the Love number k₂ describing lunar degree-2 elastic response to tidal forces, come from two independent analyses of the 3 month GRAIL Primary Mission data at the Jet Propulsion Laboratory and the Goddard Space Flight Center. The two k₂ determinations, with uncertainties of ~1%, differ by 1%; the average value is 0.02416 ± 0.00022 at a 1 month period with reference radius R = 1738 km. Lunar laser ranging (LLR) data analysis determines (C − A)/B and (B − A)/C, where A < B < C are the principal moments of inertia; the flattening of the fluid outer core; the dissipation at its solid boundaries; and the monthly tidal dissipation Q = 37.5 ± 4. The moment of inertia computation combines the GRAIL-determined J₂ and C₂₂ with LLR-derived (C − A)/B and (B − A)/C. The normalized mean moment of inertia of the solid Moon is Iₛ/MR² = 0.392728 ± 0.000012. Matching the density, moment, and Love number, calculated models have a fluid outer core with radius of 200–380 km, a solid inner core with radius of 0–280 km and mass fraction of 0–1%, and a deep mantle zone of low seismic shear velocity. The mass fraction of the combined inner and outer core is ≤1.5%.

Lunar interior properties from the GRAIL mission

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