¹Miki Nakajima, ¹David J. Stevenson
¹Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd., MC 150-21, Pasadena, CA 91125, USA

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 (MgSiO3 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.

Reference
Nakajima M, Stevenson DJ (2015) Melting and mixing states of the Earth’s mantle after the Moon-forming impact. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.06.023]
Copyright Elsevier

^1,2Yang Chen, ²Youxue Zhang, ²Yang Liu, ³Yunbin Guan, ³John Eiler, ³Edward M. Stolper
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

The concentrations of volatile elements in the moon have important implications for the formation of the earth–moon system. There is currently a debate regarding the water content of the lunar mantle: Authors studying H2O in lunar pyroclastic glass beads and in olivine-hosted melt inclusions in such pyroclastic samples and in plagioclase crystals in lunar highland anorthosites infer hundreds of ppm H2O in the lunar mantle. In contrast, authors studying Zn/Fe ratios infer that the H2O concentration in the lunar mantle is ≤1 ppm≤1 ppm, and they argue that the glassy lunar basalts are a local anomaly. We contribute to a resolution of the debate by a broader examination of the concentrations of H2O and other volatile components in olivine-hosted melt inclusions in a wider range of lunar mare basalts, including crystalline melt inclusions that are homogenized by melting in the laboratory. We find that F, Cl, and S concentrations in various lunar melt inclusions (including those in glassy lunar basalts) are similar to one another, and previously studied glassy lunar basalts are not a local anomaly in terms of these volatile concentrations. Furthermore, we estimate the pre-degassing H2O/Ce, F/Nd, and S/Dy ratios of mare basaltic magmas to be at least 64, 4.0 and 100 respectively. These ratios are lower than those of primitive earth mantle by a factor of 3, 5, and 4 respectively. The depletion factors of these volatile elements relative to the earth’s primitive mantle do not correlate strongly with volatility or bonding energy, and indeed they are roughly constant and similar to those of other volatile elements such as Li, Cs, Rb and K. This approximate constancy of volatile depletion in the moon relative to the earth can be explained by assuming that both the earth and the moon acquired volatiles from a similar source or by a similar mechanism but the earth was more efficient in acquiring the volatiles. We estimate the H2O, F and S concentrations in the primitive lunar mantle source to be at least 110, 5.3, and 70 ppm, respectively – similar to or slightly lower than those in terrestrial MORB mantle.

Reference
Chena Y, Zhang Y, Liu Y, Guan Y, Eiler J, Stolper EM (2015) Water, fluorine, and sulfur concentrations in the lunar mantle. Earth and Planetary Science Letters 427, 37–46
Link to Article [doi:10.1016/j.epsl.2015.06.046]
Copyright Elsevier

¹P. Beck et al. (>10)
¹Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France
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The Martian surface is covered by a fine-layer of oxidized dust responsible for its red color in the visible spectral range (Bibring et al., 2006 and Morris et al., 2006). In the near infrared, the strongest spectral feature is located between 2.6 and 3.6 μm and is ubiquitously observed on the planet (Jouglet et al., 2007 and Milliken et al., 2007). Although this absorption has been studied for many decades, its exact attribution and its geological and climatic implications remain debated. We present new lines of evidence from laboratory experiments, orbital and landed missions data, and characterization of the unique Martian meteorite NWA 7533, all converging toward the prominent role of hydroxylated ferric minerals. Martian breccias (so-called “Black Beauty” meteorite NWA7034 and its paired stones NWA7533 and NWA 7455) are unique pieces of the Martian surface that display abundant evidence of aqueous alteration that occurred on their parent planet (Agee et al., 2013). These dark stones are also unique in the fact that they arose from a near surface level in the Noachian southern hemisphere (Humayun et al., 2013). We used IR spectroscopy, Fe-XANES and petrography to identify the mineral hosts of hydrogen in NWA 7533 and compare them with observations of the Martian surface and results of laboratory experiments. The spectrum of NWA 7533 does not show mafic mineral absorptions, making its definite identification difficult through NIR remote sensing mapping. However, its spectra are virtually consistent with a large fraction of the Martian highlands. Abundant NWA 7034/7533 (and paired samples) lithologies might abound on Mars and might play a role in the dust production mechanism.

Reference
Beck P et al. (2015) A Noachian source region for the “Black Beauty” meteorite, and a source lithology for Mars surface hydrated dust? Earth and Planetary Science Letters 427, 104–111
Link to Article [doi:10.1016/j.epsl.2015.06.033]

Copyright Elsevier