^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.