Letter | Published:

Potassium isotopic evidence for a high-energy giant impact origin of the Moon

Nature volume 538, pages 487490 (27 October 2016) | Download Citation


The Earth–Moon system has unique chemical and isotopic signatures compared with other planetary bodies1,2,3; any successful model for the origin of this system therefore has to satisfy these chemical and isotopic constraints. The Moon is substantially depleted in volatile elements such as potassium compared with the Earth and the bulk solar composition4, and it has long been thought to be the result of a catastrophic Moon-forming giant impact event5. Volatile-element-depleted bodies such as the Moon were expected to be enriched in heavy potassium isotopes during the loss of volatiles; however such enrichment was never found6. Here we report new high-precision potassium isotope data for the Earth, the Moon and chondritic meteorites. We found that the lunar rocks are significantly (>2σ) enriched in the heavy isotopes of potassium compared to the Earth and chondrites (by around 0.4 parts per thousand). The enrichment of the heavy isotope of potassium in lunar rocks compared with those of the Earth and chondrites can be best explained as the result of the incomplete condensation of a bulk silicate Earth vapour at an ambient pressure that is higher than 10 bar. We used these coupled constraints of the chemical loss and isotopic fractionation of K to compare two recent dynamic models that were used to explain the identical non-mass-dependent isotope composition of the Earth and the Moon. Our K isotope result is inconsistent with the low-energy disk equilibration model7, but supports the high-energy, high-angular-momentum giant impact model8 for the origin of the Moon. High-precision potassium isotope data can also be used as a ‘palaeo-barometer’ to reveal the physical conditions during the Moon-forming event.

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This work was funded by the NASA Emerging Worlds program (NNX15AH66G). K.W. acknowledges financial support from the Harvard Origins of Life Initiative Postdoctoral Fellowship during his time at Harvard. We thank NASA JSC and CAPTEM for providing lunar samples and M. Petaev, S. Stewart, S. Lock and B. Jolliff for discussions.

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  1. Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA

    • Kun Wang
    •  & Stein B. Jacobsen
  2. Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63130, USA

    • Kun Wang


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S.B.J. and K.W. () designed the study. K.W. performed the analysis. K.W. and S.B.J. wrote the manuscript.

Corresponding author

Correspondence to Kun Wang.

Reviewer Information Nature thanks A. Treiman and E. Young for their contribution to the peer review of this work.

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