¹J. Guignard, ²M.J. Toplis,²M. Bystricky, ²M. Monnereau
¹European Synchrotron Radiation Facility, 71 Avenue de martyrs, 38000 Grenoble, France
²IRAP, Université de Toulouse, CNRS, UPS, Toulouse, France

Grain growth experiments in the system forsterite (Fo) + nickel (Ni) have been performed on two analogue mixtures of ordinary chondrites, with volume % of Fo:Ni (95:5) and (80:20). These two mixtures have been studied at temperatures of 1390 °C and 1340 °C, at an oxygen fugacity (fO2) three orders of magnitude below the Ni–NiO buffer, for durations between 2 h and 10 days. Microstructures and grain size distributions show that grain growth is normal and that for durations >10 h the Zener relation is verified (i.e., the ratio of Fo and Ni grain size is independent of time). Comparison with results previously obtained at 1440 °C shows a similar grain growth exponent (n∼5n∼5) for both phases, consistent with growth of forsterite by grain boundary migration, limited by the growth-rate of nickel. The details of size distribution frequencies and the value of grain-growth exponent indicate that the nickel grains, which pin forsterite grain boundaries, grow by diffusion along one-dimensional paths (i.e., along forsterite triple junctions). The derived activation energies for nickel and forsterite are 235±33 kJ/mol235±33 kJ/mol and 400±48 kJ/mol400±48 kJ/mol respectively. Within the framework of the Zener relation, this unexpected difference of activation energy is shown to be related to temperature-dependent variations in the ratio of Ni and Fo grain-size that are consistent with observed variations in Fo–Ni–Fo dihedral angle. These data thus indicate that the presence of all phases should be taken into account when considering the activation energy of growth rate of individual phases. As an application, the experimentally derived growth law for metal has been used in conjunction with temperature–time paths taken from models of the thermal history of the H-chondrite parent body to estimate the grain size evolution of metal in H-chondrites. A remarkably self-consistent picture emerges from experimentally derived grain-growth laws, textural data of metal grains in well characterised H-chondrite samples, and geochemically constrained temperature–time paths, providing the potential to use textural data of metal as a window into the thermal history of chondritic samples.

Reference
Guignard J, Toplis MJ, Bystricky M, Monnereau M (2016) Temperature dependent grain growth of forsterite–nickel mixtures: Implications for grain growth in two-phase systems and applications to the H-chondrite parent body. Earth and Planetary Science Letters 443, 20–31
Link to Article [doi:10.1016/j.epsl.2016.03.007]
Copyright Elsevier

¹Jon Wade, ¹Bernard J. Wood
¹Department of Earth Sciences, South Parks Road, Oxford OX1 3AN, UK

Physical simulations of the origin of the Moon have, until recently, centred on impact, about 100 M.yr after the origin of the solar system, of a Mars-like body (10–20% Earth mass) on a near fully-accreted protoEarth. Although this model provides an explanation of the distribution of mass and moment of inertia of the Earth–Moon system it has recently been found that modification of the initial conditions greatly expands the range of permissible impactor masses. Here we take an alternative approach and consider how the oxidation state and mass of the impactor affect the chemical compositions of the product Earth and Moon. We apply the constraints that silicate Moon is richer in FeO than silicate Earth (9–13% as opposed to 8.05%), that their Hf/W ratios are both ∼25 and that they are virtually identical in isotopes of O, Ti, Si, Ni, Cr and W. We then grow protoEarth using a standard accretionary model which yields the correct mantle abundances of Ni, Co, W, Mo, Nb, V and Cr, and add to this body different masses of impactor. The impactor is assumed to be either highly oxidised (∼18% FeO), highly reduced (∼0.3% FeO) or undifferentiated and chondritic. In order to satisfy the isotopic constraints silicate Moon is assumed to be derived principally from silicate protoEarth.

We find that an oxidised or chondritic impactor of ∼0.15 ME∼0.15 ME can satisfy the isotopic constraints (most importantly ε182W), FeO contents and Nb/Ta of Earth and Moon, but leads to implausibly low Hf/W of ∼12–16∼12–16 in silicate Earth and ∼4–6∼4–6 in silicate Moon. This is because the Moon requires more impactor mantle, with low Hf/W, than Earth to reach its higher FeO content. In contrast, impact of a similar mass (10–20% MEME) of highly reduced, Mercury-like impactor on an oxidised protoEarth (∼10.7% FeO in mantle) satisfies the isotopic constraints, FeO contents, Nb/Ta and Hf/W of silicate Earth and Moon given a small amount of post-impact re-equilibration of terrestrial mantle with impactor core. The presence of a small S-rich lunar core is consistent with this reduced impactor scenario. We conclude that the geochemical properties of Earth and Moon strongly favour a reduced impactor of 10–20% MEME.

Reference
Wade J, Wood BJ (2016) The oxidation state and mass of the Moon-forming impactor. Earth and Planetary Science Letters 442, 186–193
Link to Article [doi:10.1016/j.epsl.2016.02.053]
Copyright Elsevier

¹Gianrico Filacchione et al. (>10)*
¹INAF-IAPS, Istituto di Astrofisica e Planetologia Spaziali, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere, 100, 00133 Rome, Italy
*Find the extensive, full author and affiliation list on the publishers website

From August to November 2014 the Rosetta orbiter has performed an extensive observation campaign aimed at the characterization of 67P/CG nucleus properties and to the selection of the Philae landing site. The campaign led to the production of a global map of the illuminated portion of 67P/CG nucleus. During this prelanding phase the comet’s heliocentric distance decreased from 3.62 to 2.93 AU while Rosetta was orbiting around the nucleus at distances between 100 to 10 km. VIRTIS-M, the Visible and InfraRed Thermal Imaging Spectrometer – Mapping channel (Coradini et al., [2007] Space Sci. Rev., 128, 529–559) onboard the orbiter, has acquired 0.25–5.1 µm hyperspectral data of the entire illuminated surface, e.g. the north hemisphere and the equatorial regions, with spatial resolution between 2.5 and 25 m/pixel. I/F spectra have been corrected for thermal emission removal in the 3.5–5.1 µm range and for surface’s photometric response. The resulting reflectance spectra have been used to compute several Cometary Spectral Indicators (CSI): single scattering albedo at 0.55 µm, 0.5–0.8 µm and 1.0–2.5 µm spectral slopes, 3.2 µm organic material and 2.0 µm water ice band parameters (center, depth) with the aim to map their spatial distribution on the surface and to study their temporal variability as the nucleus moved towards the Sun. Indeed, throughout the investigated period, the nucleus surface shows a significant increase of the single scattering albedo along with a decrease of the 0.5–0.8 and 1.0–2.5 µm spectral slopes, indicating a flattening of the reflectance. We attribute the origin of this effect to the partial removal of the dust layer caused by the increased contribution of water sublimation to the gaseous activity as comet crossed the frost-line. The regions more active at the time of these observations, like Hapi in the neck/north pole area, appear brighter, bluer and richer in organic material than the rest of the large and small lobe of the nucleus. The parallel coordinates method (Inselberg [1985] Vis. Comput., 1, 69–91) has been used to identify associations between average values of the spectral indicators and the properties of the geomorphological units as defined by (Thomas et al., [2015] Science, 347, 6220) and (El-Maarr et al., [2015] Astron. Astrophys., 583, A26). Three classes have been identified (smooth/active areas, dust covered areas and depressions), which can be clustered on the basis of the 3.2 µm organic material’s band depth, while consolidated terrains show a high variability of the spectral properties resulting being distributed across all three classes. These results show how the spectral variability of the nucleus surface is more variegated than the morphological classes and that 67P/CG surface properties are dynamical, changing with the heliocentric distance and with activity processes.

Reference
Filacchione G et al. (2016) The global surface composition of 67P/CG nucleus by Rosetta/VIRTIS. (I) Prelanding mission phase. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.02.055]
Copyright Elsevier

¹Bernard Marty, ¹Guillaume Avice, ²Yuji Sano, ³Kathrin Altwegg, ³Hans Balsiger, ³Myrtha Hässig, ⁴Alessandro Morbidelli, ⁵Olivier Mousis, ³Martin Rubin
¹Centre de Recherches Pétrographiques et Géochimiques, CRPG-CNRS, Université de Lorraine, UMR 7358, 15 rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre lès Nancy, France
²Ocean and Atmosphere Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan
³Physikalisches Institut, University of Bern, Sidlerstr. 5, CH-3012 Bern, Switzerland
⁴Laboratoire Lagrange, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, Boulevard de l’Observatoire, BP 4229, 06304 Nice Cedex 4, France
⁵Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France

Recent measurements of the volatile composition of the coma of Comet 67P/Churyumov–Gerasimenko (hereafter 67P) allow constraints to be set on the origin of volatile elements (water, carbon, nitrogen, noble gases) in inner planets’ atmospheres. Analyses by the ROSINA mass spectrometry system onboard the Rosetta spacecraft indicate that 67P ice has a D/H ratio three times that of the ocean value (Altwegg et al., 2015) and contains significant amounts of N2, CO, CO2, and importantly, argon (Balsiger et al., 2015). Here we establish a model of cometary composition based on literature data and the ROSINA measurements. From mass balance calculations, and provided that 67P is representative of the cometary ice reservoir, we conclude that the contribution of cometary volatiles to the Earth’s inventory was minor for water (≤1%), carbon (≤1%), and nitrogen species (a few % at most). However, cometary contributions to the terrestrial atmosphere may have been significant for the noble gases. They could have taken place towards the end of the main building stages of the Earth, after the Moon-forming impact and during either a late veneer episode or, more probably, the Terrestrial Late Heavy Bombardment around 4.0–3.8 billion years (Ga) ago. Contributions from the outer solar system via cometary bodies could account for the dichotomy of the noble gas isotope compositions, in particular xenon, between the mantle and the atmosphere. A mass balance based on 36Ar and organics suggests that the amount of prebiotic material delivered by comets could have been quite considerable – equivalent to the present-day mass of the biosphere. On Mars, several of the isotopic signatures of surface volatiles (notably the high D/H ratios) are clearly indicative of atmospheric escape processes. Nevertheless, we suggest that cometary contributions after the major atmospheric escape events, e.g., during a Martian Late Heavy Bombardment towards the end of the Noachian era, could account for the Martian elemental C/N/36Ar ratios, solar-like krypton isotope composition and high 15N/14N ratios. Taken together, these observations are consistent with the volatiles of Earth and Mars being trapped initially from the nebular gas and local accreting material, then progressively added to by contributions from wet bodies from increasing heliocentric distances. Overall, no unified scenario can account for all of the characteristics of the inner planet atmospheres. Advances in this domain will require precise analysis of the elemental and isotopic compositions of comets and therefore await a cometary sample return mission.

Reference
Marty B, Avice G, Sano Y, Altwegg K, Balsiger H, Hässig M, Morbidelli A, Mousis O, Rubin M (2016) Origins of volatile elements (H, C, N, noble gases) on Earth and Mars in light of recent results from the ROSETTA cometary mission. Earth and Planetary Science Letters 441, 91–102
Link to Article [doi:10.1016/j.epsl.2016.02.031]
Copyright Elsevier

John C. Jackson¹, J. Wright Horton Jr.¹, I-Ming Chou² and Harvey E. Belkin^1
1U.S. Geological Survey, Reston, Virginia, USA
²Sanya Institute of Deep-Sea Science and Engineering, Sanya, China

The occurrence of coesite in suevites from the Chesapeake Bay impact structure is confirmed within a variety of textural domains in situ by Raman spectroscopy for the first time and in mechanically separated grains by X-ray diffraction. Microtextures of coesite identified in situ investigated under transmitted light and by scanning electron microscope reveal coesite as micrometer-sized grains (1–3 μm) within amorphous silica of impact-melt clasts and as submicrometer-sized grains and polycrystalline aggregates within shocked quartz grains. Coesite-bearing quartz grains are present both idiomorphically with original grain margins intact and as highly strained grains that underwent shock-produced plastic deformation. Coesite commonly occurs in plastically deformed quartz grains within domains that appear brown (toasted) in transmitted light and rarely within quartz of spheroidal texture. The coesite likely developed by a mechanism of solid-state transformation from precursor quartz. Raman spectroscopy also showed a series of unidentified peaks associated with shocked quartz grains that likely represent unidentified silica phases, possibly including a moganite-like phase that has not previously been associated with coesite.

Reference
Jackson JC, Horton JW Jr., Chou I-M and Belkin HE (2016) Coesite in suevites from the Chesapeake Bay impact structure. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12638]
Published by arrangement with John Wiley & Sons