Letter | Published:

The tidal–rotational shape of the Moon and evidence for polar wander

Nature volume 512, pages 181184 (14 August 2014) | Download Citation

Abstract

The origin of the Moon’s large-scale topography is important for understanding lunar geology1, lunar orbital evolution2 and the Moon’s orientation in the sky3. Previous hypotheses for its origin have included late accretion events4, large impacts5, tidal effects6 and convection processes7. However, testing these hypotheses and quantifying the Moon’s topography is complicated by the large basins that have formed since the crust crystallized. Here we estimate the large-scale lunar topography and gravity spherical harmonics outside these basins and show that the bulk of the spherical harmonic degree-2 topography is consistent with a crust-building process controlled by early tidal heating throughout the Moon. The remainder of the degree-2 topography is consistent with a frozen tidal–rotational bulge that formed later, at a semi-major axis of about 32 Earth radii. The probability of the degree-2 shape having both tidal-heating and frozen shape characteristics by chance is less than 1%. We also infer that internal density contrasts eventually reoriented the Moon’s polar axis by 36 ± 4°, to the configuration we observe today. Together, these results link the geology of the near and far sides, and resolve long-standing questions about the Moon’s large-scale shape, gravity and history of polar wander.

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Acknowledgements

This work was supported by the BK21-plus programme through the National Research Foundation (NRF), funded by the Ministry of Education of Korea. Conversations with E. Mazarico are appreciated. We acknowledge the GRAIL and LRO teams for the gravity and topography data used in the analysis.

Author information

Author notes

    • Viranga Perera

    Present address: School of Earth and Space Exploration, Arizona State University, PO Box 876004, Tempe, Arizona 85287-6004, USA.

Affiliations

  1. Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA

    • Ian Garrick-Bethell
    • , Viranga Perera
    •  & Francis Nimmo
  2. School of Space Research, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Korea

    • Ian Garrick-Bethell
  3. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Maria T. Zuber

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Contributions

I.G.-B. and M.T.Z. planned the research. V.P. performed the power-law calculations and helped develop the spherical harmonic fitting procedures. F.N. performed the tidal heating calculations. I.G.-B. performed the remainder of the research and wrote the paper, with contributions from all authors.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Ian Garrick-Bethell.

Supplementary information

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    Supplementary Information

    This file contains Supplementary Text, Supplementary References, Supplementary Tables 1-13 and Supplementary Figures 1-14.

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DOI

https://doi.org/10.1038/nature13639

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