H. McSween Jr. et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12947]
¹Department of Earth & Planetary Sciences and Planetary Science Institute, University of Tennessee, Knoxville, Tennessee, USA
Published by arrangement with John Wiley & Sons

The mineralogy and geochemistry of Ceres, as constrained by Dawn’s instruments, are broadly consistent with a carbonaceous chondrite (CM/CI) bulk composition. Differences explainable by Ceres’s more advanced alteration include the formation of Mg-rich serpentine and ammoniated clay; a greater proportion of carbonate and lesser organic matter; amounts of magnetite, sulfide, and carbon that could act as spectral darkening agents; and partial fractionation of water ice and silicates in the interior and regolith. Ceres is not spectrally unique, but is similar to a few other C-class asteroids, which may also have suffered extensive alteration. All these bodies are among the largest carbonaceous chondrite asteroids, and they orbit in the same part of the Main Belt. Thus, the degree of alteration is apparently related to the size of the body. Although the ammonia now incorporated into clay likely condensed in the outer nebula, we cannot presently determine whether Ceres itself formed in the outer solar system and migrated inward or was assembled within the Main Belt, along with other carbonaceous chondrite bodies.

¹Masaaki Miyahara, ²Eiji Ohtani, ^3,4Akira Yamaguchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.034]
¹Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan
²Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
³National Institute of Polar Research, Tokyo 190-8518, Japan
⁴Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo 190-8518, Japan
Copyright Elsevier

An impact event recorded in the Northwest Africa (NWA) 8275 LL7 ordinary chondrite was investigated based on high-pressure mineralogy of pervasive shock-melt veins present in the rock. NWA 8275 consists of olivine, low-Ca pyroxene, plagioclase (albite–oligoclase composition), and minor high-Ca pyroxene, K-feldspar, phosphate minerals, metallic Fe–Ni and iron sulfide. Plagioclase and K-feldspar grains near the shock-melt veins have transformed to amorphous, although no high-pressure polymorphs of olivine and pyroxene were identified in or adjacent the shock-melt veins. Raman spectroscopy and focused ion beam (FIB)-assisted transmission electron microscopy (TEM) observations reveal that plagioclase entrained around the center portion of the shock-melt veins has dissociated into a jadeite + coesite assemblage. Alternately stacked jadeite and coesite crystals occurred in the original plagioclase. On approaching the host rock/shock-melt vein, only jadeite is present. Based on the high-pressure polymorph assemblage, the shock pressure and temperature conditions recorded in the shock-melt veins are ∼3–12 GPa and ∼1973–2373 K, respectively. Following a Rankine–Hugoniot relationship, the impact velocity was at least ∼0.45–1.54 km/s. The duration of high-pressure and high-temperature (HPHT) conditions required for the albite dissociation reaction is estimated a maximum of ∼4–5 s using the phase transition rate of albite, implying that a body of up to ∼9–12 km across collided with the parent body of NWA 8275. The coexistence of jadeite and coesite, the latter of which rarely accompanies jadeite in shocked ordinary chondrites, as a dissociation product of albite requires relatively long duration HPHT conditions. Thus, the impact event recorded in NWA 8275 was likely caused by a larger-than-typical projectile.

¹Peter Hoppe, ¹Jan Leitner, ¹János Kodolányi
Nature Astronomy (in Press) Link to Article [doi:10.1038/s41550-017-0215-0]
¹Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128, Mainz, Germany

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

H.B.Franz et al. (>10)
Nature Geoscience (in Press) Link to Article [doi:10.1038/ngeo3002]
¹NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^1,2Haiyang Wang, ^1,2,3Charles H. Lineweaver, ^2,3Trevor R. Ireland
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.08.024]
¹Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
²Planetary Science Institute, The Australian National University, Canberra, ACT 2611, Australia
³Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia
Copyright Elsevier

To first order, the Earth as well as other rocky planets in the Solar System and rocky exoplanets orbiting other stars, are refractory pieces of the stellar nebula out of which they formed. To estimate the chemical composition of rocky exoplanets based on their stellar hosts’ elemental abundances, we need a better understanding of the devolatilization that produced the Earth. To quantify the chemical relationships between the Earth, the Sun and other bodies in the Solar System, the elemental abundances of the bulk Earth are required. The key to comparing Earth’s composition with those of other objects is to have a determination of the bulk composition with an appropriate estimate of uncertainties. Here we present concordance estimates (with uncertainties) of the elemental abundances of the bulk Earth, which can be used in such studies. First we compile, combine and renormalize a large set of heterogeneous literature values of the primitive mantle (PM) and of the core. We then integrate standard radial density profiles of the Earth and renormalize them to the current best estimate for the mass of the Earth. Using estimates of the uncertainties in i) the density profiles, ii) the core-mantle boundary and iii) the inner core boundary, we employ standard error propagation to obtain a core mass fraction of 32.5 ± 0.3 wt%. Our bulk Earth abundances are the weighted sum of our concordance core abundances and concordance PM abundances. Unlike previous efforts, the uncertainty on the core mass fraction is propagated to the uncertainties on the bulk Earth elemental abundances. Our concordance estimates for the abundances of Mg, Sn, Br, B, Cd and Be are significantly lower than previous estimates of the bulk Earth. Our concordance estimates for the abundances of Na, K, Cl, Zn, Sr, F, Ga, Rb, Nb, Gd, Ta, He, Ar, and Kr are significantly higher. The uncertainties on our elemental abundances usefully calibrate the unresolved discrepancies between standard Earth models under various geochemical and geophysical assumptions.

¹William C. Tucker, ¹Abrar H. Quadery, ¹Alfons Schulte, ^1,3,4Richard G. Blair, ¹William E. Kaden, ^1,2Patrick K. Schelling, ¹Daniel T. Britt
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.08.027]
¹Department of Physics, University of Central Florida, Orlando, FL 32816-2385, USA
²Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32804, USA
³Cluster for the Rational Design of Catalysts for Energy Applications and Propulsion, University of Central Florida, Orlando, FL 32816, USA
⁴Center for Advanced Turbomachinery and Energy Research, University of Central Florida, Orlando, FL 32816, USA
Copyright Elsevier

It is demonstrated that olivine powders heated to subsolidus temperatures in reducing conditions can develop significant concentrations of  10-50 nm diameter Fe nanoparticles on grain surfaces and that these display strong catalytic activity not observed in powders without Fe nanoparticles. Reduced surfaces were exposed to NH₃, CO, and H₂, volatiles that may be present on the surfaces of comet and volatile-rich asteroids. In the case of NH₃ exposure, rapid decomposition was observed. When exposed to a mixture of CO and H₂, significant coking of the mineral surfaces occurred. Analysis of the mineral grains after reaction indicated primarily the presence of graphene or graphitic carbon. The results demonstrate that strong chemical activity can be expected at powders that contain nanophase Fe particles. This suggests space-weathered mineral surfaces may play an important role in the synthesis and processing of organic species. This processing may be part of the weathering processes of volatile-rich but atmosphereless solar-system bodies.

¹Maïa Kuga, ²Guy Cernogora, ¹Yves Marrocchi, ¹Laurent Tissandier,¹ Bernard Marty
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.031]
¹CRPG-CNRS, Université de Lorraine, 15 rue Notre Dame des Pauvres, 54500 Vandoeuvre-les-Nancy, France
²LATMOS, Université Versailles St. Quentin, UPMC Univ. Paris 06, CNRS, 11 Bvd. d’Alembert, 78280 Guyancourt, France
Copyright Elsevier

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. The Q noble gas component shows elemental and isotopic fractionation relative to the Solar, in favor of heavy elements and isotopes. These noble gas characteristics were experimentally simulated using a plasma device called the “Nebulotron”. In this study, we synthesized thirteen solid organic samples by electron-dissociation of CO, in which a noble gas mixture was added. The analysis of their heavy noble gas (Ar, Kr and Xe) contents and isotopic compositions reveals enrichment in the heavy noble gas isotopes and elements relative to the light ones. The isotope fractionation is mass-dependent and is consistent with a mn- type law, where n≥1. Based on a plasma model, we propose that the ambipolar diffusion of ions in the ionized CO gas medium is at the origin of the noble gas isotopic fractionation. In addition, the elemental fractionation of experimental and chondritic samples can be accounted for by the Saha law of plasma equilibrium, which does not depend on the respective noble gas masses but rather on their ionization potentials. Our results suggest that the Q noble gases were trapped into growing organic particles starting from solar gases that were fractionated in an ionized medium by ambipolar diffusion and Saha processes. This would imply that both the formation of chondritic organic matter and the trapping of noble gases took place simultaneously in the ionized areas of the protoplanetary disk.

^1,2Alberto G. Fairén,¹Carolina Gil-Lozano,³Esther R. Uceda,⁴Elisabeth Losa-Adams,⁵Alfonso F. Davila,⁴Luis Gago-Duport
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2016JE005229]
¹Centro de Astrobiología (CSIC-INTA), Madrid, Spain
²Department of Astronomy, Cornell University, Ithaca, NY, USA
³Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco Madrid, Spain
⁴Departamento de Geociencias Marinas, Universidad de Vigo, Lagoas Marcosende, Vigo, Spain
⁵NASA Ames Research Center, Moffett Field, CA, USA
Published by arrangement with John Wiley & Sons

Geochemical models of secondary mineral precipitation on Mars generally assume semi-open systems (open to the atmosphere but closed at the water-sediment interface) and equilibrium conditions. However, in natural multicomponent systems, the reactive surface area of primary minerals controls the dissolution rate and affects the precipitation sequences of secondary phases; and simultaneously the transport of dissolved species may occur through the atmosphere-water and water-sediment interfaces. Here we present a suite of geochemical models designed to analyze the formation of secondary minerals in basaltic sediments on Mars, evaluating the role of (i) reactive surface areas and (ii) the transport of ions through a basalt sediment column. We consider fully open conditions, both to the atmosphere and to the sediment, and a kinetic approach for mineral dissolution and precipitation. Our models consider a geochemical scenario constituted by a basin (i.e., a shallow lake) where supersaturation is generated by evaporation/cooling, and the starting point is a solution in equilibrium with basaltic sediments. Our results show that cation removal by diffusion, along with the input of atmospheric volatiles and the influence of the reactive surface area of primary minerals, play a central role in the evolution of the secondary mineral sequences formed. We conclude that precipitation of evaporites finds more restrictions in basaltic sediments of small grain size than in basaltic sediments of greater grain size.

¹Lu Pan, ^1,2Bethany L. Ehlmann, ³John Carter, ⁴Carolyn M. Ernst
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005276]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
³Institut d’Astrophysique Spatiale, Orsay, France
⁴The John Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
Published by arrangement with John Wiley & Sons

The basin-filling materials of the northern lowlands, which cover ~1/3 of Mars’ surface, record the long-term evolution of Mars’ geology and climate. The buried stratigraphy was inferred through analyses of impact crater mineralogy, detected using data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Examining 1045 impact craters across the northern lowlands, we find widespread olivine and pyroxene and diverse hydrated/hydroxylated minerals, including Fe/Mg smectite, chlorite, prehnite, and hydrated silica. The distribution of mafic minerals is consistent with infilling volcanic materials across the entire lowlands (~1–4⋅107 km3), indicating a significant volume of volatile release by volcanic outgassing. Hydrated/hydroxylated minerals are detected more frequently in large craters, consistent with the scenario that the hydrated minerals are being excavated from deep basement rocks, beneath 1-2 km thick mafic lava flows or volcaniclastic materials. The prevalences of different types of hydrated minerals are similar to statistics from the southern highlands. No evidence of concentrated salt deposits has been found, which would indicate a long-lived global ocean. We also find significant geographical variations of local mineralogy and stratigraphy in different basins (geological provinces), independent of dust cover. For example, many hydrated and mafic minerals are newly discovered within the polar Scandia region (> 60°N), and Chryse Planitia has more mafic mineral detections than other basins, possibly due to a previously unrecognized volcanic source.

^1,2P.M. Ranjith, ^1,2Huaiyu He, ³Bingkui Miao, ¹Fei Su, ^3,4Chuantong Zhang, ³Zhipeng Xia, ³Lanfang Xie, ¹Rixiang Zhu
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.08.009]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China
²University of Chinese Academy of Sciences, Beijing 100049, PR China
³Institute of Meteorites and Planetary Research, Guilin University of Technology, Guilin 541004, PR China
⁴Key Laboratory of Planetray Geological Evolution, Guilin University of Technology, Guilin 541004, PR China

We currently do not have a copyright agreement with this publisher and cannot display the abstract here