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Strong tidal heating in an ultralow-viscosity zone at the core–mantle boundary of the Moon

Abstract

Tidal heating of a solid planetary body occurs by viscous dissipation, depending on its internal structure1,2,3,4,5 and thermal5,6,7,8 and orbital6,7,8,9 states. Calculations of the response of the Moon to tidal forces have considered lunar interior structure1,2,3,4,5, but have not reproduced the geodetically observed dependence of dissipation on the lunar tidal period10. The attenuation of seismic waves in the deep lunar interior11,12 is expected to be consistent with a low-viscosity layer at the core–mantle boundary, which may explain the observed frequency dependence13. Here we numerically simulate the viscoelastic tidal response of a Moon that contains a low-viscosity layer at the core–mantle boundary and compare with geodetic observations10,14,15. In our simulations, a layer with a viscosity of about 2 × 1016 Pa s leads to frequency-dependent tidal dissipation that matches tidal dissipation measurements at both monthly and annual periods. Compared with the lunar asthenosphere, the calculated viscosity is extremely low, and suggests partial melting at the lunar core–mantle boundary. We also find that tidal dissipation is not evenly distributed in the lunar interior, but localized within the low-viscosity layer, which implies that this layer may act as a thermal blanket16 on the lunar core and influence the Moon’s thermal evolution.

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Figure 1: Reference interior profiles including the low-viscosity zone.
Figure 2: Dependence of the Love number and quality factor on the low-viscosity zone and tidal period.

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Acknowledgements

We thank Y. Abe, K. Heki, S. Honda, H. Hussmann, S. Kamata, S. Karato, M. Nakada, F. Nimmo, E. Ohtani, G. Schubert, H. Shimizu, T. Spohn, L. Xiao, D. Zhao and S. Zhong for critical discussions and W. Moore for a helpful review. Y.H. was financially supported by China Postdoctoral Science Foundation, Chinese Academy of Sciences and National Natural Science Foundation of China.

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Y.H. designed the study and performed the analysis. Y.H. and S.G. wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Yuji Harada.

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The authors declare no competing financial interests.

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Harada, Y., Goossens, S., Matsumoto, K. et al. Strong tidal heating in an ultralow-viscosity zone at the core–mantle boundary of the Moon. Nature Geosci 7, 569–572 (2014). https://doi.org/10.1038/ngeo2211

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