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

Nature volume 538, pages 487–490 (2016)Cite this article

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Abstract

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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Figure 1: K isotope compositions of the Earth, the Moon and meteorites.
Figure 2: K isotope compositions versus the K/U ratio for the Earth and the Moon.
Figure 3: Proposed models for the post-giant-impact state to explain the identical non-mass-dependent isotope compositions of the Earth and the Moon.

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Acknowledgements

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.

Author information

Authors and Affiliations

  1. Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, 02138, Massachusetts, 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, 63130, Missouri, USA

    Kun Wang

Authors
  1. Kun Wang
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Contributions

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.

Additional information

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

Extended data figures and tables

Extended Data Figure 1 The estimation of the pressure effect on α.

The experiments on K isotope fractionation during evaporation and condensation are very limited. One study22 conducted evaporation experiments on chondritic analogue materials in a high vacuum (around 10−9 atm) and at various higher pressures (1 × 10−5, 9 × 10−5, 7.2 × 10−5, 1.8 × 10−5, 1.8 × 10−4, 1.4 × 10−3, 1.3 × 10−2 and 1 atm). One of their major observations is that the degree of K isotope fractionation during evaporation decreases with higher pressures. Another study23 indicates that not only the pressure but also the composition (that is, H2, He, CO2) of the surrounding gas would affect the degree of K isotope fractionation. Here the red dots show the calculated effective Rayleigh fractionation factors based on the experimental data22,23 versus pressures. The effective Rayleigh fractionation factor is correlated with pressures and the observed value from this study suggests a >10 bar pressure during the loss of K from the Moon. Note that because not all the experiments are conducted at the same surrounding gas environment and the composition of the surrounding gas would also affect the effective fractionation factor (downward arrows in the figure), the correlation between the effective fractionation factor and pressure is not perfect. This >10 bar pressure from this study is at best a first-order estimation.

Source data

Extended Data Figure 2

An artist’s rendering of the two recent models of the origin of the Moon and their implications for K isotopes. Scenario 1 is from ref. 7; scenario 2 is from ref. 8.

Extended Data Table 1 K isotope compositions of terrestrial igneous rocks
Full size table
Extended Data Table 2 K isotope compositions of carbonaceous chondrites
Full size table
Extended Data Table 3 K isotope compositions of Apollo lunar samples
Full size table

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Wang, K., Jacobsen, S. Potassium isotopic evidence for a high-energy giant impact origin of the Moon. Nature 538, 487–490 (2016). https://doi.org/10.1038/nature19341

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