Letter | Published:

Lunar volatile depletion due to incomplete accretion within an impact-generated disk

Nature Geoscience volume 8, pages 918921 (2015) | Download Citation

Abstract

The Moon may have formed from an Earth-orbiting disk of vapour and melt produced by a giant impact1. The mantles of the Moon and Earth have similar compositions. However, it is unclear why lunar samples are more depleted in volatile elements than terrestrial mantle rocks2,3, given that an evaporative escape mechanism4 seems inconsistent with expected disk conditions5. Dynamical models6,7 suggest that the Moon initially accreted from the outermost disk, but later acquired up to 60% of its mass from melt originating from the inner disk. Here we combine dynamical, thermal and chemical models to show that volatile depletion in the Moon can be explained by preferential accretion of volatile-rich melt in the inner disk to the Earth, rather than to the growing Moon. Melt in the inner disk is initially hot and volatile poor, but volatiles condense as the disk cools. In our simulations, the delivery of inner disk melt to the Moon effectively ceases when gravitational interactions cause the Moon’s orbit to expand away from the disk, and this termination of lunar accretion occurs before condensation of potassium and more volatile elements. Thus, the portion of the Moon derived from the inner disk is expected to be volatile depleted. We suggest that this mechanism may explain part or all of the Moon’s volatile depletion, depending on the degree of mixing within the lunar interior.

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Acknowledgements

We thank D. Stevenson for comments and suggestions. Funding from NASA’s Solar System Exploration Research Virtual Institute (SSERVI, for R.M.C., C.V. and J.S.), NASA’s Lunar Advanced Science and Exploration Research Program (LASER, for R.M.C.), and from NSF’s Planetary Astronomy Program (grant 1412175, for B.F.Jr) is gratefully acknowledged.

Author information

Affiliations

  1. Planetary Sciences Directorate, Southwest Research Institute, Boulder, Colorado 80302, USA

    • Robin M. Canup
    • , Channon Visscher
    •  & Julien Salmon
  2. Chemistry and Planetary Sciences, Dordt College, Sioux Center, Iowa 51250, USA

    • Channon Visscher
  3. Department of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St Louis, Missouri 63130, USA

    • Bruce Fegley Jr

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Contributions

R.M.C. conceived the idea, integrated the component models, and wrote the paper. C.V. performed the chemical modelling and related analysis, J.S. performed the data analysis for the accretion models, and B.F.Jr provided the MAGMA model and relevant thermodynamic calculations for K, Na and Zn.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Robin M. Canup.

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DOI

https://doi.org/10.1038/ngeo2574

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