¹William Herbst,²James P.Greenwood
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.03.039]
¹Astronomy Department, Wesleyan University, Middletown, CT 06459, United States of America
²Earth & Environmental Sciences Department, Wesleyan University, Middletown, CT 06459, United States of America
Copyright Elsevier

We propose that chondrules and chondrites formed together during a brief radiative heating event caused by the close encounter of a small (m to km-scale), primitive planetesimal (SPP) with incandescent lava on the surface of a large (100 km-scale) differentiated planetesimal (LDP). In our scenario, chondrite lithification occurs by hot isostatic pressing (HIP) simultaneously with chondrule formation, in accordance with the constraints of complementarity and cluster chondrites. Thermal models of LDPs formed near t = 0 predict that there will be a very narrow window of time, coincident with the chondrule formation epoch, during which crusts are thin enough to frequently rupture by impact, volcanism and/or crustal foundering, releasing hot magma to their surfaces. The heating curves we calculate are more gradual and symmetric than the “flash heating” characteristic of nebular models, but in agreement with the constraints of experimental petrology. The SPP itself is a plausible source of the excess O, Na and Si vapor pressure (compared to a solar nebula environment) that is required by chondrule observations. Laboratory experiments demonstrate that FeO-poor porphyritic olivine chondrules, the most voluminous type of chondrule, can be made using heating and cooling curves predicted by the “flyby” model. If chondrules are a by-product of chondrite lithification, then their high volume abundance within well-lithified chondritic material is not evidence that they were once widespread within the Solar System. Relatively rare events, such as the flybys modeled here, could account for their abundance in the meteorite record.

Christian T. Lenz^1,3, Hubert Klahr¹, and Tilman Birnstiel²
Astrophysical Journal 874, 36 Link to Article [DOI: 10.3847/1538-4357/ab05d9 ]
¹Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
²University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, D-81679 Munich, Germany
³Member of the International Max Planck Research School for Astronomy and Cosmic Physics at the Heidelberg University.

We propose an expression for a local planetesimal formation rate proportional to the instantaneous radial pebble flux. The result—a radial planetesimal distribution—can be used as an initial condition to study the formation of planetary embryos. We follow the idea that one needs particle traps to locally enhance the dust-to-gas ratios sufficiently, such that particle gas interactions can no longer prevent planetesimal formation on small scales. The locations of these traps can emerge everywhere in the disk. Their occurrence and lifetime is subject to ongoing research; thus, here they are implemented via free parameters. This enables us to study the influence of the disk properties on the formation of planetesimals, predicting their time-dependent formation rates and the location of primary pebble accretion. We show that large α-values of 0.01 (strong turbulence) prevent the formation of planetesimals in the inner part of the disk, arguing for lower values of around 0.001 (moderate turbulence), at which planetesimals form quickly at all places where they are needed for proto-planets. Planetesimals form as soon as dust has grown to pebbles (mm to dm) and the pebble flux reaches a critical value, which is after a few thousand years at 2–3 au and after a few hundred thousand years at 20–30 au. Planetesimal formation lasts until the pebble supply has decreased below a critical value. The final spatial planetesimal distribution is steeper compared to the initial dust and gas distribution, which helps explain the discrepancy between the minimum mass solar nebula and viscous accretion disks.

^1,2Zanetta, P.-M., ¹Le Guillou, C., ¹Leroux, H., ^2,3,4Zanda, B., ^2,3Hewins, R.H., ⁵Lewin, E., ²Pont, S.
Chemical Geology 514, 27-41 Link to Article [DOI: 10.1016/j.chemgeo.2019.03.025]
¹Univ. Lille, CNRS, INRA, ENSCL, UMR 8207 – UMET – Unité Matériaux et Transformations, Lille, F-59000, France
²IMPMC, Sorbonne Université, MNHN, UPMC Paris 06, UMR CNRS 7590, Paris, 75005, France
³EPS, Rutgers Univ., Piscataway, NJ 08854, United States

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¹A.A.Maksimova,¹M.I.Oshtrakh
Journal of Molecular Structure 1186, 104-117 Link to Article [https://doi.org/10.1016/j.molstruc.2019.02.024]
¹Department of Experimental Physics, Institute of Physics and Technology, Ural Federal University, Ekaterinburg, 620002, Russian Federation

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^1,2,3T. Salge,³H. Stosnach,⁴G. Rosatelli,^2,5L. Hecht,^2,6W. U. Reimold
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13283]
¹Natural History Museum, Imaging and Analysis Centre, Cromwell Road, SW7 5BD London, UK
²Museum für Naturkunde, Leibniz‐Institut für Evolutions‐ und Biodiversitätsforschung, Invalidenstrasse 43, 10115 Berlin, Germany
³Bruker Nano GmbH, Am Studio, 12489 Berlin, Germany
⁴Dipertimento Disputer, Università G. d’Annunzio, 66100 Chieti, Italy
⁵Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74‐100, 12249 Berlin, Germany
⁶ Laboratory of Geochronology, Instituto de Geociências, Universidade de Brasília, 70910 900 Brasília, DF, Brazil
Published by arrangement with John Wiley & Sons

Drill core UNAM‐7, obtained 126 km from the center of the Chicxulub impact structure, outside the crater rim, contains a sequence of 126.2 m suevitic, silicate melt‐rich breccia on top of a silicate melt‐poor breccia with anhydrite megablocks. Total reflection X‐ray fluorescence analysis of altered silicate melt particles of the suevitic breccia shows high concentrations of Br, Sr, Cl, and Cu, which may indicate hydrothermal reaction with sea water. Scanning electron microscopy and energy‐dispersive spectrometry reveal recrystallization of silicate components during annealing by superheated impact melt. At anhydrite clasts, recrystallization is represented by a sequence of comparatively large columnar, euhedral to subhedral anhydrite grains and smaller, polygonal to interlobate grains that progressively annealed deformation features. The presence of voids in anhydrite grains indicates SO_x gas release during anhydrite decomposition. The silicate melt‐poor breccia contains carbonate and sulfate particles cemented in a microcrystalline matrix. The matrix is dominated by anhydrite, dolomite, and calcite, with minor celestine and feldspars. Calcite‐dominated inclusions in silicate melt with flow textures between recrystallized anhydrite and silicate melt suggest a former liquid state of these components. Vesicular and spherulitic calcite particles may indicate quenching of carbonate melts in the atmosphere at high cooling rates, and partial decomposition during decompression at postshock conditions. Dolomite particles with a recrystallization sequence of interlobate, polygonal, subhedral to euhedral microstructures may have been formed at a low cooling rate. We conclude that UNAM‐7 provides evidence for solid‐state recrystallization or melting and dissociation of sulfates during the Chicxulub impact event. The lack of anhydrite in the K‐Pg ejecta deposits and rare presence of anhydrite in crater suevites may indicate that sulfates were completely dissociated at high temperature (T > 1465 °C)—whereas ejecta deposited near the outer crater rim experienced postshock conditions that were less effective at dissociation.

^1,3Gyollai, I.,¹Kereszturi, Á.,²Chatzitheodoridis, E.
Central European Geology 62, 56-82 Link to Article [DOI: 10.1556/24.61.2018.12]
¹Konkoly Thege Miklós Astronomical Institute, HAS Research Centre for Astronomy and Earth Sciences, Budapest, Hungary
²Department of Geological Sciences, School of Mining and Metallurgical Engineering, National Technical University of Athens, Athens, Greece
³Geobiomineralization, Astrobiology Research Group, HAS Research Centre for Astronomy and Earth Sciences, Institute for Geological and Geochemical Research, Budaörsi út 45., Budapest, H-1112, Hungary

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Klaus M. Pontoppidan¹, Colette Salyk², Andrea Banzatti³, Geoffrey A. Blake⁴, Catherine Walsh⁵, John H. Lacy⁶, and Matthew J. Richter⁷
Astrophysical Journal 874, 92 Link to Article [DOI: 10.3847/1538-4357/ab05d8 ]
¹Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²Vassar College Physics and Astronomy Department, 124 Raymond Avenue, Poughkeepsie, NY 12604, USA
³Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, USA
⁴Division of Geological and Planetary Sciences, California Institute of Technology, MC 150-21, 1200 E California Boulevard, Pasadena, CA 91125, USA
⁵School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
⁶Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
⁷Department of Physics, University of California Davis, 1 Shields Avenue, Davis, CA 95616, USA

The dominant reservoirs of elemental nitrogen in protoplanetary disks have not yet been observationally identified. Likely candidates are HCN, NH₃, and N₂. The relative abundances of these carriers determine the composition of planetesimals as a function of disk radius due to strong differences in their volatility. A significant sequestration of nitrogen in carriers less volatile than N₂ is likely required to deliver even small amounts of nitrogen to the Earth and potentially habitable exoplanets. While HCN has been detected in small amounts in inner disks (<10 au), so far only relatively insensitive upper limits on inner disk NH₃ have been obtained. We present new Gemini-TEXES high-resolution spectroscopy of the 10.75 μm band of warm NH₃, and use two-dimensional radiative transfer modeling to improve previous upper limits by an order of magnitude to at 1 au. These NH₃ abundances are significantly lower than those typical for ices in circumstellar envelopes (). We also consistently retrieve the inner disk HCN gas abundances using archival Spitzer spectra, and derive upper limits on the HCN ice abundance in protostellar envelopes using archival ground-based 4.7 μm spectroscopy ([HCN_ice]/[H₂O_ice] < 1.5%–9%). We identify the NH₃/HCN ratio as an indicator of chemical evolution in the disk, and we use this ratio to suggest that inner disk nitrogen is efficiently converted from NH₃ to N₂, significantly increasing the volatility of nitrogen in planet-forming regions.

¹Y.Kebukawa et al. (>10)
Scientific Reports 9, 3169 Link to Article [DOI: 10.1038/s41598-019-39357-1]
¹Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan

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¹M.A.Fitzgerald,²K.B.Knight,²J.E.Matzel,¹K.R.Czerwinski
Chemical Geology 507,96-119 Link to Article [https://doi.org/10.1016/j.chemgeo.2018.12.018]
¹University of Nevada Las Vegas, 4505 S. Maryland Pkwy, Las Vegas, NV 89154, USA
²Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA

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¹Trey A.Lobpries,¹Thomas J.Lapen
Journal of African Earth Sciences 150,673-684 Link to Article [https://doi.org/10.1016/j.jafrearsci.2018.09.020]
¹Department of Earth and Atmospheric Sciences, University of Houston, 312 Science and Research 1, 3507 Cullen Blvd, Rm. 312, Houston, TX, 77204, USA

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