Published May 29, 2025 | Published
Journal Article Open

Thermal asymmetry in the Moon's mantle inferred from monthly tidal response

  • 1. ROR icon Jet Propulsion Lab
  • 2. ROR icon California Institute of Technology
  • 3. ROR icon University of Arizona
  • 4. ROR icon University of California, Santa Cruz
  • 5. ROR icon Delft University of Technology
  • 6. ROR icon Marshall Space Flight Center

Abstract

The Moon undergoes periodic tidal forcing due to its eccentric and oblique orbit around the Earth. The response to this tidal interaction drives temporal changes in the lunar gravity field and is sensitive to the satellite's internal structure. We use data from the NASA GRAIL spacecraft to recover the time-varying lunar gravity field, including a degree-3 gravitational tidal Love number, k. Here, we report our estimated value of k = 0.0163 ± 0.0007, which is about 72% higher than that expected for a spherically symmetric moon. Such a large k can be explained if the elastic shear modulus of the mantle varies by about 2–3% between the nearside and farside, providing an observational demonstration of lateral heterogeneities in the deep lunar interior. This asymmetric structure suggests preservation of a predominantly thermal anomaly of roughly 100–200 K in the nearside mantle that formed surface mare regions 3–4 billion years ago and could influence the spatial distribution of deep moonquakes.

Copyright and License

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Acknowledgement

A portion of this research was carried out at the JPL, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA) (grant no. 80NM0018D0004). R.S.P. and A.S.K. were partially supported by the NASA Advanced Multi-Mission Operations System program. A.B. and M.S. were partially supported by the Future Investigators in NASA Earth and Space Science and Technology (FINESST) Program (grant no. 80NSSC22K1318). M.R.-N. and I.M. were partially supported by the NASA Solar System Workings program (grant no. 80NSSC20K0570). We gratefully acknowledge the use of the Ames Pleiades Supercomputer that was used to generate the GL1800F gravity solutions of this paper.

Data Availability

The GRAIL data used to generate the results of this paper are available at the NASA Planetary Data System Geosciences Node (http://pds-geosciences.wustl.edu). The GL1800F gravity field tied to the principal axis frame and the mean-Earth frame can be downloaded from the GRAIL gravity archival directory (https://pds-geosciences.wustl.edu/grail/grail-l-lgrs-5-rdr-v1/grail_1001/shadr/).

Code Availability

The gravity recovery results presented in this study can be reproduced using the MONTE software available at NASA (https://montepy.jpl.nasa.gov). The LOV3D software is also available at GitHub (https://github.com/mroviranavarro/LOV3D_multi).

Supplemental Material

Extended Data Fig. 1 Conceptual relationship between lunar 3D structure and response to tidal forcing.

Extended Data Fig. 2 Distributions of modeled Love numbers k2m and k3m corresponding to the ensemble of accepted candidate models.

Extended Data Fig. 3 Effective density, gravity spectrums, and global correlations of gravity fields with gravity inferred from topography as a function of harmonic degree.

Extended Data Fig. 4 Description of reference model and low velocity zone viscosity.

Extended Data Fig. 5 Impact of water content on mantle shear modulus.

Extended Data Fig. 6 Post-fit residuals of GRAIL KBRR data using GL0900D and GL1800F.

Extended Data Fig. 7 Global surface radial acceleration of the GL1800F lunar gravity field using spherical harmonics l = 2-600 and excluding  (positive downward).

Extended Data Fig. 8 Bouguer anomaly map of GL1800F lunar gravity field using spherical harmonics l = 2-600 (positive downward).

Extended Data Fig. 9 Estimated k3 per arc (e.g., 2-3 days long) to show the variability in the solution.

Extended Data Table 1 Radial (1D) elastic structure (density, bulk modulus, and shear modulus) assumed for reference models

Extended Data Table 2 Inverted median, standard deviation, 0.3%, and 99.7% of coefficient values describing 3D structure in the lunar crust (superscript C, see Eqn. 7) and the lunar mantle (superscript M)

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Additional details

Created:
August 5, 2025
Modified:
August 5, 2025