¹Alessandra Migliorini,¹M. C. De Sanctis,²D. Lazzaro,³E. Ammannito
Monthly Notices of the Royal Astronomical Society 464, 1718-1726 Link to Article [doi: 10.1093/mnras/stw2441]
¹Institute of Space Astrophysics and Planetology, IAPS-INAF, 1-00133 Rome, Italy
²Observatório Nacional, COAA, 20921-400 Rio de Janeiro, Brazil
³University of California Los Angeles, Earth Planetary and Space Science, Los Angeles, CA90095, USA

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¹C. Jäger, ¹T. Sabri, ²E. Wendler, ³Th. Henning
The Astrophysical Journal 831, 66 Link to Article [http://dx.doi.org/10.3847/0004-637X/831/1/66]
¹Max Planck Institute for Astronomy, Heidelberg, Laboratory Astrophysics and Cluster Physics Group, Institute of Solid State Physics, Friedrich Schiller University Jena, Helmholtzweg 3, D-07743 Jena, Germany
²Institute of Solid State Physics, Friedrich Schiller University Jena, Helmholtzweg 3, D-07743 Jena, Germany
³Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany

Ion-induced processing of dust grains in the interstellar medium and in protoplanetary and planetary disks plays an important role in the entire dust cycle. We have studied the ion-induced processing of amorphous MgFeSiO4 and Mg2SiO4 grains by 10 and 20 keV protons and 90 keV Ar+ ions. The Ar+ ions were used to compare the significance of the light protons with that of heavier, but chemically inert projectiles. The bombardment was performed in a two-beam irradiation chamber for in situ ion-implantation at temperatures of 15 and 300 K and Rutherford Backscattering Spectroscopy to monitor the alteration of the silicate composition under ion irradiation. A depletion of oxygen from the silicate structure by selective sputtering of oxygen from the surface of the grains was observed in both samples. The silicate particles kept their amorphous structure, but the loss of oxygen caused the reduction of ferrous (Fe2+) ions and the formation of iron inclusions in the MgFeSiO4 grains. A few Si inclusions were produced in the iron-free magnesium silicate sample pointing to a much less efficient reduction of Si4+ and formation of metallic Si inclusions. Consequently, ion-induced processing of magnesium-iron silicates can produce grains that are very similar to the glassy grains with embedded metals and sulfides frequently observed in interplanetary dust particles and meteorites. The metallic iron inclusions are strong absorbers in the NIR range and therefore a ubiquitous requirement to increase the temperature of silicate dust grains in IR-dominated astrophysical environments such as circumstellar shells or protoplanetary disks.

¹John M. Brewer, ¹Debra A. Fischer
The Astrophysical Journal 831, 20 Link to Article [http://dx.doi.org/10.3847/0004-637X/831/1/20]
¹Department of Astronomy, Yale University, 260 Whitney Avenue, New Haven, CT 06511, USA

The carbon-to-oxygen ratio in a protoplanetary disk can have a dramatic influence on the compositions of any terrestrial planets formed. In regions of high C/O, planets form primarily from carbonates, and in regions of low C/O, the ratio of magnesium to silicon determines the types of silicates that dominate the compositions. We present C/O and Mg/Si ratios for 852 F, G, and K dwarfs in the solar neighborhood. We find that the frequency of carbon-rich dwarfs in the solar neighborhood is $\lt 0.13 \% $ and that 156 known planet hosts in the sample follow a similar distribution as all of the stars as a whole. The cosmic distribution of Mg/Si for these same stars is broader than the C/O distribution and peaks near 1.0, with $\sim 60 \% $ of systems having $1\,\leqslant $ Mg/Si $\lt \,2$, leading to rocky planet compositions similar to the Earth. This leaves 40% of systems that can have planets that are silicate-rich and that may have very different compositions than our own.

¹S. Fogerty, ¹, W. Forrest, ¹D. M. Watson, ²B. A. Sargent, ³I. Koch
The Astrophysical Journal 830, 71 Link to Article [http://dx.doi.org/10.3847/0004-637X/830/2/71]
¹Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
²Center for Imaging Science and Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA
³Department of Earth & Planetary Sciences, Washington University, St. Louis, MO 63130, USA

The composition of silicate dust in the diffuse interstellar medium and in protoplanetary disks around young stars informs our understanding of the processing and evolution of the dust grains leading up to planet formation. An analysis of the well-known 9.7 μm feature indicates that small amorphous silicate grains represent a significant fraction of interstellar dust and are also major components of protoplanetary disks. However, this feature is typically modeled assuming amorphous silicate dust of olivine and pyroxene stoichiometries. Here, we analyze interstellar dust with models of silicate dust that include non-stoichiometric amorphous silicate grains. Modeling the optical depth along lines of sight toward the extinguished objects Cyg OB2 No. 12 and ζ Ophiuchi, we find evidence for interstellar amorphous silicate dust with stoichiometry intermediate between olivine and pyroxene, which we simply refer to as “polivene.” Finally, we compare these results to models of silicate emission from the Trapezium and protoplanetary disks in Taurus.

^1,2Mathieu Roskosz, ^2,3Boris Laurent, ²Hugues Leroux, ¹Laurent Remusat
The Astrophysical Journal 832, 55 Link to Article [http://dx.doi.org/10.3847/0004-637X/832/1/55]
¹IMPMC, CNRS UMR 7590, Sorbonne Universités, Université Pierre et Marie Curie, IRD, Muséum National d’Histoire Naturelle, CP 52, 57 rue Cuvier, Paris F-75231, France
²Unité Matériaux et Transformations, Université Lille 1, CNRS UMR 8207, Bâtiment C6, F-59655 Villeneuve d’Ascq, France
³Present address: Department of Earth and Environmental Sciences, University of St. Andrews, Irvine Building, KY16 9AL, Fife, Scotland, UK

The origin of hydrogen in chondritic components is poorly understood. Their isotopic composition is heavier than the solar nebula gas. In addition, in most meteorites, hydrous silicates are found to be lighter than the coexisting organic matter. Ionizing irradiation recently emerged as an efficient hydrogen fractionating process in organics, but its effect on H-bearing silicates remains essentially unknown. We report the evolution of the D/H of hydrous silicates experimentally irradiated by electrons. Thin films of amorphous silica, amorphous “serpentine,” and pellets of crystalline muscovite were irradiated at 4 and 30 keV. For all samples, irradiation leads to a large hydrogen loss correlated with a moderate deuterium enrichment of the solid residue. The entire data set can be described by a Rayleigh distillation. The calculated fractionation factor is consistent with a kinetically controlled fractionation during the loss of hydrogen. Furthermore, for a given ionizing condition, the deuteration of the silicate residues is much lower than the deuteration measured on irradiated organic macromolecules. These results provide firm evidence of the limitations of ionizing irradiation as a driving mechanism for D-enrichment of silicate materials. The isotopic composition of the silicate dust cannot rise from a protosolar to a chondritic signature during solar irradiations. More importantly, these results imply that irradiation of the disk naturally induces a strong decoupling of the isotopic signatures of coexisting organics and silicates. This decoupling is consistent with the systematic difference observed between the heavy organic matter and the lighter water typically associated with minerals in the matrix of most carbonaceous chondrites.

¹James M.D. Day, ¹Christopher A. Corder, ²Pierre Cartigny, ³Andrew M. Steele, ²Nelly Assayag, ³Douglas Rumble III, ⁴Lawrence A. Taylor
Geochmica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.11.026]
¹Geosciences Research Division, Scripps Institution of Oceanography, San Diego, La Jolla, CA 92093-0244, USA
²Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris, France
³Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA
⁴Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA
Copyright Elsevier

Ureilite meteorites are partially melted asteroidal-peridotite residues, or more rarely, cumulates that can contain greater than three weight percent carbon. Here we describe an exceptional C-rich lithology, composed of 34 modal% large (up to 0.8 mm long) crystalline graphite grains, in the Antarctic ureilite meteorite Miller Range (MIL) 091004. This C-rich lithology is embedded within a silicate region composed dominantly of granular olivine with lesser quantities of low-Ca pyroxene, and minor FeNi metal, high-Ca pyroxene, spinel, schreibersite and troilite. Petrological evidence indicates that the graphite was added after formation of the silicate region and melt depletion. Associated with graphite is localized reduction of host olivine (Fo88-89) to nearly pure forsterite (Fo99), which is associated with FeNi metal grains containing up to 11 wt.% Si. The main silicate region is typical of ureilite composition, with highly siderophile element (HSE) abundances ∼0.3 × chondrite, 187Os/188Os of 0.1260 to 0.1262 and Δ17O of -0.81 ±0.16‰. Mineral trace-element analyses reveal that the rare earth elements (REE) and the HSE are controlled by pyroxene and FeNi metal phases in the meteorite, respectively. Modelling of bulk-rock REE and HSE abundances indicates that the main silicate region experienced ∼6% silicate and >50% sulfide melt extraction, which is at the lower end of partial melt removal estimated for ureilites. Miller Range 091004 demonstrates heterogeneous distribution of carbon at centimeter scales and a limited range in Mg/(Mg+Fe) compositions of silicate grain cores, despite significant quantities of carbon. These observations demonstrate that silicate rim reduction was a rapid disequilibrium process, and came after silicate and sulfide melt removal in MIL 091004. The petrography and mineral chemistry of MIL 091004 is permissive of the graphite representing late-stage C-rich melt that pervaded silicates, or carbon that acted as a lubricant during anatexis and impact disruption in the parent body. Positive correlation of Pt/Os ratios with olivine core compositions, but a wide range of oxygen isotope compositions, indicates that ureilites formed from a compositionally heterogeneous parent body that experienced variable sulfide and metal melt-loss that is most pronounced in relatively oxidized ureilites with Δ17O between -1.5 and ∼0‰

¹Pilar Redondo, ¹Carmen Barrientos, ¹Antonio Largo
The Astrophysical Journal 828, 45 Link to Article [http://dx.doi.org/10.3847/0004-637X/828/1/45]
¹Departamento de Química Física y Química Inorgánica Facultad de Ciencias, Universidad de Valladolid Campus Miguel Delibes Paseo de Belén 7, E-47011, Valladolid, Spain

Iron is the most abundant transition metal in space. Its abundance is similar to that of magnesium, and until today only, FeO and FeCN have been detected. However, magnesium-bearing compounds such as MgCN, MgNC, and HMgNC are found in IRC+10216. It seems that the hydrides of iron cyanide/isocyanide could be good candidates to be present in space. In the present work we carried out a characterization of the different minima on the quintet and triplet [C, Fe, H, N] potential energy surfaces, employing several theoretical approaches. The most stable isomers are predicted to be hydride of iron cyanide HFeCN, and isocyanide HFeNC, in their 5Δ states. Both isomers are found to be quasi-isoenergetics. The HFeNC isomer is predicted to lie about 0.5 kcal/mol below HFeCN. The barrier for the interconversion process is estimated to be around 6.0 kcal/mol, making this process unfeasible under low temperature conditions, such as those in the interstellar medium. Therefore, both HFeCN and HFeNC could be candidates for their detection. We report geometrical parameters, vibrational frequencies, and rotational constants that could help with their experimental characterization.

¹Edwin S. Kite, ²Bruce Fegley Jr., ³Laura Schaefer, ⁴Eric Gaidos
The Astrophysical Journal 828, 80 Link to Article [http://dx.doi.org/10.3847/0004-637X/828/2/80]
¹University of Chicago, Chicago, IL 60637, USA
²Planetary Chemistry Laboratory, McDonnell Center for the Space Sciences & Department of Earth & Planetary Sciences, Washington University, St Louis MO 63130, USA
³Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁴University of Hawaii at Manoa, Honolulu, HI 96822, USA

We provide estimates of atmospheric pressure and surface composition on short-period, rocky exoplanets with dayside magma pools and silicate-vapor atmospheres. Atmospheric pressure tends toward vapor-pressure equilibrium with surface magma, and magma-surface composition is set by the competing effects of fractional vaporization and surface-interior exchange. We use basic models to show how surface-interior exchange is controlled by the planet’s temperature, mass, and initial composition. We assume that mantle rock undergoes bulk melting to form the magma pool, and that winds flow radially away from the substellar point. With these assumptions, we find that: (1) atmosphere-interior exchange is fast when the planet’s bulk-silicate FeO concentration is low, and slow when the planet’s bulk-silicate FeO concentration is high; (2) magma pools are compositionally well mixed for substellar temperatures lesssim2400 K, but compositionally variegated and rapidly variable for substellar temperatures gsim2400 K; (3) currents within the magma pool tend to cool the top of the solid mantle (“tectonic refrigeration”); (4) contrary to earlier work, many magma planets have time-variable surface compositions.

¹Kaori Jogo, ²Tomoki Nakamura, ³Motoo Ito, ⁴Shigeru Wakita, ⁵Mikhail Yu. Zolotov, ⁶Scott R. Messenger
Geochmica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.11.027]
¹Division of Earth-System Sciences, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon 406-840, South Korea
²Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
³Kochi Institute for Core Sample Research, JAMSTEC B200 Monobe, Nankoku, Kochi 783-8502, Japan
⁴Center for Computational Astrophysics, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287–1404, USA
⁶Robert M. Walker Laboratory for Space Science, NASA Johnson Space Center, ARES, Mail Code KR, 2101 NASA Parkway, Houston, Texas 77058, USA
Copyright Elsevier

Chondritic planetesimals are among the first planetary bodies that accreted inside and outside water snow line in the protoplanetary disk. CV3 carbonaceous chondrite parent body accreted relatively small amount of water ice, probably near the snow line, and experienced water-assisted metasomatic alteration that resulted in formation of diverse secondary minerals, including fayalite (Fa80–100). Chemical compositions of the CV fayalite and its Mn-Cr isotope systematics indicate that it formed at different temperature (10–300°C) and fluid pressure (3–300 bars) but within a relatively short period of time. Thermal modeling of the CV parent body suggests that it accreted ∼3.2–3.3 Ma after CV CAIs formation and had a radius of >110–150 km. The inferred formation age of the CV parent body is similar to that of the CM chondrite parent body that probably accreted beyond the snow line, but appears to have postdated accretion of the CO and ordinary chondrite parent bodies that most likely formed inside the snow line. The inferred differences in the accretion ages of chondrite parent bodies that formed inside and outside snow line are consistent with planetesimal formation by gravitational/streaming instability.

¹Lars E. Borg, ²James N. Connelly, ¹William Cassata, ¹Amy M. Gaffney, ²Martin Bizzarro
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.11.021]
¹Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore CA 94550, USA
²Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7 Copenhagen, Denmark
Copyright Elsevier

Ages have been obtained using the 87Rb-87Sr, 147Sm-143Nd, and 146Sm-142Nd isotopic systems for one of the most slowly cooled lunar rocks, Apollo 17 Mg-suite troctolite 76535. The 147Sm-143Nd, 146Sm-142Nd, and Rb-Sr ages derived from plagioclase, olivine, and pyroxene mineral isochrons yield concordant ages of 4307 ± 11 Ma, 4299 +29/-35 Ma, and 4279 ± 52 Ma, respectively. These ages are slightly younger than the age determined on ferroan anorthosite suite (FAS) rock 60025 and are therefore consistent with the traditional magma ocean model of lunar differentiation in which the Mg-suite is intruded into the anorthositic crust. However, the Sm-Nd ages record when the rock passed below the closing temperature of the Sm-Nd system in this rock at ∼825 ⁰C, whereas the Rb-Sr age likely records the closure temperature of ∼650 ⁰C. A cooling rate of 3.9 ⁰C/Ma is determined using the ages reported here and in the literature and calculated closure temperatures for the Ar-Ar, Pb-Pb, Rb-Sr, and Sm-Nd systems. This cooling rate is in good agreement with cooling rates estimated from petrographic observations. Slow cooling can lower apparent Sm-Nd crystallization ages by up to ∼80 Ma in the slowest cooled rocks like 76535, and likely accounts for some of the variation of ages reported for lunar crustal rocks. Nevertheless, slow cooling cannot account for the overlap in FAS and Mg-suite rock ages. Instead, this overlap appears to reflect the concordance of Mg-suite and FAS magmatism in the lunar crust as indicated by ages calculated for the solidus temperature of 76535 and 60025 of 4384 ± 24 Ma and 4383 ± 17, respectively. Not only are the solidus ages of 76535 and 60025 nearly concordant, but the Sm-Nd isotopic systematics suggest they are derived from reservoirs that were minimally differentiated prior to ∼4.38 Ga. Although the Sr isotopic composition of 60025 indicates its source was minimally differentiated, the Sr isotopic composition of 76535 indicates it underwent fractionation just prior to solidification of the 76535. These observations are consistent with both a magma ocean or a serial magmatism model of lunar differentiation. In either model, differentiation of lunar source regions must occur near the solidification age of thee samples. Perhaps the best estimate for the formation age of lunar source regions is the Rb-Sr model age of the 76535 source region age of 4401 ± 32 Ma. This is in good agreement with Sm-Nd model ages for the formation of ur-KREEP and suggests that differentiation of a least part of the Moon could not have occurred prior to ∼4.43 Ga.