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Melting and mixing states of the Earth’s mantle after the Moon-forming impact

Nakajima, Miki and Stevenson, David J. (2015) Melting and mixing states of the Earth’s mantle after the Moon-forming impact. Earth and Planetary Science Letters, 427 . pp. 286-295. ISSN 0012-821X.

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The Earth's Moon is thought to have formed by an impact between the Earth and an impactor around 4.5 billion years ago. This impact could have been so energetic that it could have mixed and homogenized the Earth's mantle. However, this view appears to be inconsistent with geochemical studies that suggest that the Earth's mantle was not mixed by the impact. Another outcome of the impact is that this energetic impact melted the whole mantle, but the extent of mantle melting is not well understood even though it must have had a significant effect on the subsequent evolution of the Earth's interior and atmosphere. To understand the initial state of the Earth's mantle, we perform giant impact simulations using smoothed particle hydrodynamics (SPH) for three different models: (a) standard: a Mars-sized impactor hits the proto-Earth, (b) fast-spinning Earth: a small impactor hits a rapidly rotating proto-Earth, and (c) sub-Earths: two half Earth-sized planets collide. We use two types of equations of state (MgSiO_3 liquid and forsterite) to describe the Earth's mantle. We find that the mantle remains unmixed in (a), but it may be mixed in (b) and (c). The extent of mixing is most extensive in (c). Therefore, (a) is most consistent and (c) may be least consistent with the preservation of the mantle heterogeneity, while (b) may fall between. We determine that the Earth's mantle becomes mostly molten by the impact in all of the models. The choice of the equation of state does not affect these outcomes. Additionally, our results indicate that entropy gains of the mantle materials by a giant impact cannot be predicted well by the Rankine–Hugoniot equations. Moreover, we show that the mantle can remain unmixed on a Moon-forming timescale if it does not become mixed by the impact.

Item Type:Article
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URLURL TypeDescription Paper
Stevenson, David J.0000-0001-9432-7159
Additional Information:© 2015 Elsevier B.V. Received 10 October 2014, Revised 30 May 2015, Accepted 13 June 2015, Available online 17 July 2015. Editor: C. Sotin. This work is supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program – Grant NNX14AP26H. We would like to thank David Rubie and an anonymous reviewer for insightful comments, Kevin Walsh for providing the Grand Tack simulations, Paul Asimow, Kaveh Pahlevan, Aaron Wolf, Sarah Stewart and Tobias Bischoff for helpful discussions, Takaaki Takeda for providing a visualization software, Zindaiji 3. Numerical computations were partly carried out on GRAPE system at Center for Computational Astrophysics, National Astronomical Observatory of Japan.
Funding AgencyGrant Number
NASA Earth and Space Science FellowshipNNX14AP26H
Subject Keywords:mantle heterogeneity; deep Earth; Moon; satellite formation; thermodynamics
Record Number:CaltechAUTHORS:20150904-071135923
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Official Citation:Miki Nakajima, David J. Stevenson, Melting and mixing states of the Earth's mantle after the Moon-forming impact, Earth and Planetary Science Letters, Volume 427, 1 October 2015, Pages 286-295, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:60066
Deposited By: Ruth Sustaita
Deposited On:04 Sep 2015 19:42
Last Modified:03 Oct 2019 08:53

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