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Inefficient volatile loss from the Moon-forming disk: Reconciling the giant impact hypothesis and a wet Moon

Nakajima, Miki and Stevenson, David J. (2018) Inefficient volatile loss from the Moon-forming disk: Reconciling the giant impact hypothesis and a wet Moon. Earth and Planetary Science Letters, 487 . pp. 117-126. ISSN 0012-821X. doi:10.1016/j.epsl.2018.01.026.

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The Earth's Moon is thought to have formed from a circumterrestrial disk generated by a giant impact between the proto-Earth and an impactor approximately 4.5 billion years ago. Since this impact was energetic, the disk would have been hot (4000–6000 K) and partially vaporized (20–100% by mass). This formation process is thought to be responsible for the geochemical observation that the Moon is depleted in volatiles (e.g., K and Na). To explain this volatile depletion, some studies suggest the Moon-forming disk was rich in hydrogen, which was dissociated from water, and it escaped from the disk as a hydrodynamic wind accompanying heavier volatiles (hydrodynamic escape). This model predicts that the Moon should be significantly depleted in water, but this appears to contradict some of the recently measured lunar water abundances and D/H ratios that suggest that the Moon is more water-rich than previously thought. Alternatively, the Moon could have retained its water if the upper parts (low pressure regions) of the disk were dominated by heavier species because hydrogen would have had to diffuse out from the heavy-element rich disk, and therefore the escape rate would have been limited by this slow diffusion process (diffusion-limited escape). To identify which escape the disk would have experienced and to quantify volatiles loss from the disk, we compute the thermal structure of the Moon-forming disk considering various bulk water abundances (100–1000 ppm) and mid-plane disk temperatures (2500–4000 K). Assuming that the disk consists of silicate (SiO_2 or Mg_2SiO_4) and water and that the disk is in the chemical equilibrium, our calculations show that the upper parts of the Moon-forming disk are dominated by heavy atoms or molecules (SiO and O at _(mid) >2500–2800 K and H_2O at _(mid) <2500–2800 K) and hydrogen is a minor species. This indicates that hydrogen escape would have been diffusion-limited, and therefore the amount of lost water and hydrogen would have been small compared to the initial abundance assumed. This result indicates that the giant impact hypothesis can be consistent with the water-rich Moon. Furthermore, since the hydrogen wind would have been weak, the other volatiles would not have escaped either. Thus, the observed volatile depletion of the Moon requires another mechanism.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Stevenson, David J.0000-0001-9432-7159
Additional Information:© 2018 Elsevier B.V. Received 10 March 2017, Revised 22 December 2017, Accepted 22 January 2018, Available online 12 February 2018. This work is supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program Grant NNX14AP26H and the Carnegie DTM Postdoctoral Fellowship. We would like to thank Masahiro Ikoma, Francis Nimmo, Cheng Li, Jay Melosh, Steve Desch, Erik Hauri, Alycia Weinberger, Peng Ni, and anonymous reviewers for helpful discussions.
Group:Astronomy Department
Funding AgencyGrant Number
NASA Earth and Space Science FellowshipNNX14AP26H
Carnegie DTM Postdoctoral FellowshipUNSPECIFIED
Subject Keywords:Moon; giant impact; volatiles; lunar water; volatile loss; hydrodynamics escape
Record Number:CaltechAUTHORS:20180213-093312035
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Official Citation:Miki Nakajima, David J. Stevenson, Inefficient volatile loss from the Moon-forming disk: Reconciling the giant impact hypothesis and a wet Moon, Earth and Planetary Science Letters, Volume 487, 1 April 2018, Pages 117-126, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84807
Deposited By: Tony Diaz
Deposited On:13 Feb 2018 18:23
Last Modified:15 Nov 2021 20:23

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