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

Martin Schmieder^1,2 et al. (>10)*
¹Lunar and Planetary Institute, Houston, Texas, USA
²NASA Solar System Exploration Research Virtual Institute (SSERVI)
*Find the extensive, full author and affiliation list on the publishers website

The two neighboring Suvasvesi North and South impact structures in central-east Finland have been discussed as a possible impact crater doublet produced by the impact of a binary asteroid. This study presents ⁴⁰Ar/³⁹Ar geochronologic data for impact melt rocks recovered from the drilling into the center of the Suvasvesi North impact structure and melt rock from glacially transported boulders linked to Suvasvesi South. ⁴⁰Ar/³⁹Ar step-heating analysis yielded two essentially flat age spectra indicating a Late Cretaceous age of ~85 Ma for the Suvasvesi North melt rock, whereas the Suvasvesi South melt sample gave a Neoproterozoic minimum (alteration) age of ~710 Ma. Although the statistical likelihood for two independent meteorite strikes in close proximity to each other is rather low, the remarkable difference in ⁴⁰Ar/³⁹Ar ages of >600 Myr for the two Suvasvesi impact melt samples is interpreted as evidence for two temporally separate, but geographically closely spaced, impacts into the Fennoscandian Shield. The Suvasvesi North and South impact structures are, thus, interpreted as a “false” crater doublet, similar to the larger East and West Clearwater Lake impact structures in Québec, Canada, recently shown to be unrelated. Our findings have implications for the reliable recognition of impact crater doublets and the apparent rate of binary asteroid impacts on Earth and other planetary bodies in the inner solar system.

Reference
Schmieder M (2016) The two Suvasvesi impact structures, Finland: Argon isotopic evidence for a “false” impact crater doublet. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12636]
Published by arrangement with John Wiley & Sons

Isabella Pignatelli^1,2, Yves Marrocchi^1,2, Lionel. G. Vacher^1,2, Rémi Delon^1,2 andMatthieu Gounelle^3,4
1Université de Lorraine, CRPG, UMR 7358, Vandoeuvre-lès-Nancy F-54501, France
²CNRS, CRPG UMR 7358, Vandoeuvre-lès-Nancy, France
³IMPMC, MNHM, UPMC, UMR CNRS 7590, 75005 Paris, France
⁴Institut Universitaire de France, Maison des Universités, 75005 Paris, France

We report a petrographic and mineralogical survey of tochilinite/cronstedtite intergrowths (TCIs) in Paris, a new CM chondrite considered to be the least altered CM identified to date. Our results indicate that type-I TCIs consist of compact tochilinite/cronstedtite rims surrounding Fe-Ni metal beads, thus confirming kamacite as the precursor of type-I TCIs. In contrast, type-II TCIs are characterized by complex compositional zoning composed of three different Fe-bearing secondary minerals: from the outside inwards, tochilinite, cronstedtite, and amakinite. Type-II TCIs present well-developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors. The results demonstrate that type-II TCIs formed by pseudomorphism of the anhydrous silicates, olivine, and pyroxene. Hence, there is no apparent genetic relationship between type-I and type-II TCIs. In addition, the complex chemical zoning observed within type-II TCIs suggests that the alteration conditions evolved dramatically over time. At least three stages of alteration can be proposed, characterized by alteration fluids with varying compositions (1) Fe- and S-rich fluids; (2) S-poor and Fe- and Si-rich fluids; and (3) S- and Si-poor, Fe-rich fluids. The presence of unaltered silicates in close association with euhedral type-II TCIs suggests the existence of microenvironments during the first alteration stages of CM chondrites. In addition, the absence of Mg-bearing secondary minerals in Paris TCIs suggests that the Mg content increases during the course of alteration.

Reference
Pignatelli I, Marrocchi Y, Vacher LG, Delon R and Gounelle M (2016) Multiple precursors of secondary mineralogical assemblages in CM chondrites. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12625]
Published by arrangement with John Wiley & Sons

Sean S. Lindsay^1,2, Tasha L. Dunn³, Joshua P. Emery² and Neil E. Bowles^1
1Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK
²Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, USA
³Department of Geology, Colby College, Waterville, Maine, USA

Near-infrared reflectance spectra of S-type asteroids contain two absorptions at 1 and 2 μm (band I and II) that are diagnostic of mineralogy. A parameterization of these two bands is frequently employed to determine the mineralogy of S(IV) asteroids through the use of ordinary chondrite calibration equations that link the mineralogy to band parameters. The most widely used calibration study uses a Band II terminal wavelength point (red edge) at 2.50 μm. However, due to the limitations of the NIR detectors on prominent telescopes used in asteroid research, spectral data for asteroids are typically only reliable out to 2.45 μm. We refer to this discrepancy as “The Red Edge Problem.” In this report, we evaluate the associated errors for measured band area ratios (BAR = Area BII/BI) and calculated relative abundance measurements. We find that the Red Edge Problem is often not the dominant source of error for the observationally limited red edge set at 2.45 μm, but it frequently is for a red edge set at 2.40 μm. The error, however, is one sided and therefore systematic. As such, we provide equations to adjust measured BARs to values with a different red edge definition. We also provide new ol/(ol+px) calibration equations for red edges set at 2.40 and 2.45 μm.

Reference
Lindsay SS, Dunn TL, Emery JP and Bowles NE (2016) The Red Edge Problem in asteroid band parameter analysis. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12611]
Published by arrangement with John Wiley & Sons

John T. Wasson^1,2
1Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California, USA
²Department of Chemistry and Biochemistry, University of California, Los Angeles, California, USA

Treysa and Delegate have compositions closely similar to those of IIIAB irons but plot above the IIIAB field on Ir-Au diagrams; for this reason they are designated anomalous members of IIIAB. All refractory siderophiles share this anomaly. Wasson (1999) interpreted the large spread on IIIAB Ir-Au diagrams to result from melt-trapping and generated solid and liquid fractional crystallization tracks; almost all IIIAB irons fall between the tracks. In contrast, Treysa, Delegate, and three other irons (the Treysa quintet) plot beyond the liquid track. Ideal fractional crystallization cannot account for compositions that plot outside the region between the tracks. Possible explanations for the anomalous compositions of the Treysa quintet are that (1) these meteorites did not form in the IIIAB magma or (2) they formed by the mixing of early crystallized solids with a late liquid. The weight of the evidence including cosmic-ray ages favor the second explanation. Although this explanation can account for positions plotting above the liquid track, it requires special circumstances. The infalling blocks must be assimilated and the resulting melt must crystallize quickly into pockets small enough (<1 m) to allow igneous gradients to be leveled by subsequent diffusion. The Treysa quintet shares the region beyond the liquid track with most main-group pallasites (PMG), which may have also originated in the IIIAB body. It appears that Treysa, its relatives, and the PMG were formed in one or more impact events that mixed olivine and solid metal formed near the core-mantle boundary with nearby magma. It is then necessary to cool the melt rapidly; the best way to achieve rapid cooling is by heat exchange with cooler solids. That the Treysa quintet and the PMG can be explained by the same processes operating on late IIIAB magma supports the conclusion that PMG formed on the IIIAB parent asteroid.

Reference
Wasson JT (2016) Formation of the Treysa quintet and the main-group pallasites by impact-generated processes in the IIIAB asteroid. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12635]
Published by arrangement with John Wiley & Sons

E. Dobrică and A. J. Brearley
Department of Earth and Planetary Sciences, University of New Mexico, MSC03-2040, Albuquerque, New Mexico, USA

The diversity of silicate, glassy spherules analogous to chondrules, called microchondrules, and the implications for their presence in unequilibrated ordinary chondrites (UOCs) were investigated using different electron microscope techniques. Our observations show that the abundance of microchondrules in UOCs is much larger than the values proposed by previous studies. We identified two different types of microchondrules, porous and nonporous, embedded within fine-grained matrices and type I chondrule rims. The porous microchondrules are characterized by distinctive textures and chemical compositions that have not been recognized previously. Additionally, we show detailed textures and chemical compositions of protuberances of silicate materials, connected to the chondrules and ending with microchondrules. We suggest that microchondrules and protuberances formed from materials splattered from the chondrules during stochastic collisions when they were still either completely or partially molten. The occurrence and distinct morphologies of microchondrules and protuberances suggest that rather than just a passive flash melting of chondrules, an additional event perturbed the molten chondrules before they underwent cooling. The bulk chemical compositions suggest that (1) nonporous microchondrules and protuberances were formed by splattering of materials that are compositionally similar to the bulk silicate composition of type I chondrules, and (2) the porous microchondrules could represent the splattered melt products of a less evolved, fine-grained dust composition. The preservation of protuberances and microchondrules in the rims suggests that the cooling and accretion rates were exceptionally fast and that they represent the last objects that were formed before the accretion of the parent bodies of OCs.

Reference
Dobrică E and Brearley AJ (2016) Microchondrules in two unequilibrated ordinary chondrites: Evidence for formation by splattering from chondrules during stochastic collisions in the solar nebula. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12633]
Published by arrangement with John Wiley & Sons

N. Meftah^1,2,3, S. Mostefaoui⁴, A. Jambon², E. H. Guedda³ and S. Pont^4
1Faculté des Mathématiques et des Sciences de la Matière, Université Kasdi Merbah, Ouargla, Algeria
²Sorbonne Universités ISTEP UMR 7193, UPMC, Paris Cedex 05, France
³Laboratoire LEVRES et Faculté des Sciences et technologie, Université d’El-Oued, El-Oued, Algeria
⁴IMPMC, Muséum National d’Histoire Naturelle, Paris, France

We report in situ NanoSIMS siderophile minor and trace element abundances in individual Fe-Ni metal grains in the unequilibrated chondrite Krymka (LL3.2). Associated kamacite and taenite of 10 metal grains in four chondrules and one matrix metal were analyzed for elemental concentrations of Fe, Ni, Co, Cu, Rh, Ir, and Pt. The results show large elemental variations among the metal grains. However, complementary and correlative variations exist between adjacent kamacite-taenite. This is consistent with the unequilibrated character of the chondrite and corroborates an attainment of chemical equilibrium between the metal phases. The calculated equilibrium temperature is 446 ± 9 °C. This is concordant with the range given by Kimura et al. (2008) for the Krymka postaccretion thermal metamorphism. Based on Ni diffusivity in taenite, a slow cooling rate is estimated of the Krymka parent body that does not exceed ~1K Myr⁻¹, which is consistent with cooling rates inferred by other workers for unequilibrated ordinary chondrites. Elemental ionic radii might have played a role in controlling elemental partitioning between kamacite and taenite. The bulk compositions of the Krymka metal grains have nonsolar (mostly subsolar) element/Ni ratios suggesting the Fe-Ni grains could have formed from distinct precursors of nonsolar compositions or had their compositions modified subsequent to chondrule formation events.

Reference
Meftah N, Mostefaoui S, Jambon A, Guedda EH and Pont S (2016) Minor and trace element concentrations in adjacent kamacite and taenite in the Krymka chondrite. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12617]
Published by arrangement with John Wiley & Sons