¹Schmitz, B.,²Farley, K.A.,³Goderis, S.,^4,5Heck, P.R.,⁶Bergström, S.M.,¹Boschi, S.,
⁷Claeys, P.,⁸Debaille, V.,^9,10Dronov, A.,¹¹van Ginneken, M.,¹²Harper, D.A.T.,¹Iqbal, F.,¹ Friberg, J.,^13,14Liao, S.,¹Martin, E.,^15,16 Meier, M.M.M.,¹⁷Peucker-Ehrenbrink, B.,¹⁷Soens, B.,¹⁵Wieler, R.,¹Terfelt, F.
Science Advances 5, eaax4184 Link to Article [DOI: 10.1126/sciadv.aax4184]
¹Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, United States
³Department of Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
⁴Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, United States
⁵Department of the Geophysical Sciences, University of Chicago, Chicago, IL, United States
⁶School of Earth Sciences, Ohio State University, Columbus, OH, United States
⁷Analytical, Environmental, and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
⁸Laboratoire G-Time, Université Libre de Bruxelles, Brussels, Belgium
⁹Geological Institute, Russian Academy of Sciences, Moscow, Russian Federation
¹⁰Institute of Geology and Oil and Gas Technologies, Kazan (Volga Region) Federal University, Kazan, Russian Federation
¹¹Royal Belgian Institute of Natural Sciences, Brussels, Belgium
¹²Department of Earth Sciences, Durham University, Durham, United Kingdom
¹³Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
¹⁴CAS Center for Excellence in Comparative Planetology, Hefei, China
¹⁵Department of Earth Sciences, ETH Zürich, Zürich, Switzerland
¹⁶Naturmuseum St. Gallen, St. Gallen, Switzerland
¹⁷Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, United States

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¹Takafumi Ootsubo,^2,3Hideyo Kawakita,²Yoshiharu Shinnaka,⁴Jun-ichi Watanabe,⁵Mitsuhiko Honda
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113450]
¹Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
²Laboratory of Infrared High-resolution Spectroscopy (LiH), Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan
³Department of Astrophysics and Atmospheric Sciences, Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan
⁴Public Relation Center, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵Faculty of Biosphere-Geosphere Science Department of Biosphere-Geosphere Science, Okayama University of Science, 1-1 Ridaicho, Kita-ku, Okayama-shi 700-0005, Japan
Copyright Elsevier

Comet 21P/Giacobini-Zinner (hereafter, comet 21P/G-Z) is a Jupiter-family comet and a parent comet of the October Draconids meteor shower. If meteoroids originating from a Jupiter-family comet contain complex organic molecules, such as amino acids, they are essential pieces of the puzzle regarding the origin of life on Earth. We observed comet 21P/G-Z in the mid-infrared wavelength region using the Cooled Mid-infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope on UT 2005 July 5. Here, we report the unidentified infrared (UIR) emission features of comet 21P/G-Z, which are likely due to complex organic molecules (both aliphatic and aromatic hydrocarbons), and the thermal emission from amorphous/crystalline silicates and amorphous carbon grains in its mid-infrared low-resolution spectrum. The UIR features at ~8.2 μm, ~8.5 μm, and ~11.2 μm found in the spectrum of comet 21P/G-Z could be attributed to polycyclic aromatic hydrocarbons (or hydrogenated amorphous carbons) contaminated by N- or O-atoms, although part of the feature at ~11.2 μm comes from crystalline olivine. The other feature at ~9.2 μm might originate from aliphatic hydrocarbons. Comet 21P/G-Z is enriched in complex organic molecules. Considering that the derived mass fraction of crystalline silicates in comet 21P/G-Z is typical of comets, we propose that the comet originated from a circumplanetary disk of giant planets (similar to Jupiter and Saturn) where was warmer than the typical comet-forming region (5–30 au from the Sun) and was suitable for the formation of complex organic molecules. Comets from circumplanetary disks might be enriched in complex organic molecules, such as comet 21P/G-Z, and may have provided pre-biotic molecules to ancient Earth by direct impact or meteor showers.

¹Iris Weber,¹Aleksandra N.Stojic,¹Andreas Morlok,¹Maximilian P.Reitze,^1,5Kathrin Markus,¹ Harald Hiesinger,²Sergey G.Pavlov,³Richard Wirth,³ Anja Schreiber,⁴Martin Sohn,²Heinz-Wilhelm Hübers,⁵Jörn Helbert
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2019.115884]
¹Westfälische Wilhelms Universität Münster, Institut für Planetologie, Wilhelm – Klemm Str. 10, 48149 Münster, Germany
²German Aerospace Center (DLR), Institute of Optical Sensor Systems, Rutherfordstr. 2, 12489 Berlin, Germany
³Helmholtz-Zentrum Potsdam, Deutsches Geoforschungszentrum (GFZ), Telegrafenberg, 14473 Potsdam, Germany
⁴Hochschule Emden/Leer, Constantiaplatz 4, 26723 Emden, Germany
⁵German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany
Copyright Elsevier

We studied space-weathering effects caused by micrometeorite bombardment simulated by pulsed intense infrared laser, generating ∼15 mJ per pulse in high vacuum. For our investigation, we selected a natural olivine (San Carlos olivine (Fo₉₁)) and a natural pyroxene (Bamble orthopyroxene (En₈₇)) as important rock forming minerals of the Earth upper mantle as well as key planetary minerals. Irradiated areas of powdered pressed samples were examined by optical reflection spectroscopy in a broad optical and infrared wavelength range (visible-, near-, and mid-infrared) and transmission electron microscopy to identify changes due to micrometeorite impacts. The present study aims to investigate especially the effects of micrometeorite bombardment on reflectance spectra in the mid-IR in preparation for future space missions, as well as for the MERTIS experiment onboard the BepiColombo mission.

For both irradiated samples, we found a reduction in albedo and in the reflectance of characteristic Reststrahlen bands and an increase of the transparency feature. VIS and NIR spectra of both minerals show the typical darkening and reddening as described for other space-weathered samples. TEM studies revealed that space-weathered layers in olivine and pyroxene differ in their respective thickness, ∼450 nm in olivine, 100-250 nm in pyroxene, as well as in developed “nanostratigraphy” of laser-ablated material, like nanophase iron (npFe).

In conclusion, our spectral and structural findings were compared to samples in which space weathering was caused by different processes. A comparison with these data demonstrates that there is no difference in optical reflectance spectroscopy, but a significant difference in the microstructure of minerals due to the weathering source in space, as there are solar wind and solar flares cause other structural and chemical changes as the bombardment with micrometeorites.

¹Jiachao Liu,¹Jie Li
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2019.115834]
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, 48109, USA
Copyright Elsevier

Paleomagnetic records revealed that the early Moon had an Earth-like core dynamo, which was likely driven by thermochemical buoyancy force associated with core solidification. The cause for the cessation of the ancient lunar dynamo at about 3.56 Ga ago remains controversial, partly because the composition and temperature of the Moon are not well constrained and the solidification process of its core remains poorly understood. Here we report experimental data at 5.1 GPa showing that the liquidus temperatures of the Fe–Ni–S system are ∼50–150 K lower than that of the Fe–S system, implying that a Ni-bearing core could remain molten to lower temperatures. Calculating the liquidus temperature gradient using previous data at 3 GPa and the new results at 5.1 GPa, we find that an Fe-S core containing less than ∼4.0 wt.% S would freeze from the center of the Moon. At higher S contents, the core would precipitate solid Fe near the core-mantle boundary. Based on the prevailing lunar core models, a change in core solidification from the bottom-up regime to the top-down regime during the lunar history is possible only if its bulk S content falls between about 2.0 and 4.0 wt.%.

^1,2Thomas Stephan,^1,2,6Reto Trappitsch,³Peter Hoppe,^1,2,4Andrew M. Davis,^1,2,4,5Michael J. Pellin,^1,2Olivia S. Pardo
The Astrophysical Journal 877,101 Link to Article [https://doi.org/10.3847/1538-4357/ab1c60]
¹Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637, USA
²Chicago Center for Cosmochemistry, Chicago, IL, USA
³Max Planck Institute for Chemistry, 55128 Mainz, Germany
⁴The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
⁵Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
⁶Present address: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.

We have analyzed molybdenum isotopes, together with strontium and barium isotopes, in 18 presolar silicon carbide grains using the Chicago Instrument for Laser Ionization (CHILI), a resonance ionization mass spectrometer. All observed isotope ratios can be explained by mixtures of pure s-process matter with isotopically solar material. Grain residues were subsequently analyzed for carbon, nitrogen, silicon, and sulfur isotopes, as well as a subset for ²⁶Al–²⁶Mg systematics using the NanoSIMS. These analyses showed that all but one grain are mainstream grains, most probably coming from low-mass asymptotic giant branch (AGB) stars. One grain is of the AB type, for which the origin is still a matter of debate. The high precision of molybdenum isotope measurements with CHILI provides the best estimate to date for s-process molybdenum made in low-mass AGB stars. The average molybdenum isotopic abundances produced by the s-process found in the analyzed mainstream SiC grains are 0% ⁹²Mo, 0.73% ⁹⁴Mo, 13.30% ⁹⁵Mo, 36.34% ⁹⁶Mo, 9.78% ⁹⁷Mo, 39.42% ⁹⁸Mo, and 0.43% ¹⁰⁰Mo. Solar molybdenum can be explained as a combination of 45.9% s-process, 30.6% r-process, and 23.5% p-process contributions. Furthermore, the observed variability in the individual grain data provides insights into the variability of conditions (neutron density, temperature, and timescale) during s-process nucleosynthesis in the grains’ parent stars, as they have subtle effects on specific molybdenum isotope ratios. Finally, the results suggest that the ratio between p– and r-process molybdenum in presolar SiC from many different types of parent stars is Mo _p /Mo _r  = 0.767, the value inferred for the solar system and consistent with what has been found in bulk samples and leachates of primitive meteorites.

¹Sota Arakawaand,¹Taishi Nakamoto
The Astrophysical Journal 877, 84 Link to Article [https://doi.org/10.3847/1538-4357/ab1b3e]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

Shock-wave heating within the solar nebula is one of the leading candidates for the source of chondrule-forming events. Here we examine the possibility of compound chondrule formation via optically thin shock waves. Several features of compound chondrules indicate that they are formed via the collisions of supercooled precursors. We evaluate whether compound chondrules can be formed via the collision of supercooled chondrule precursors in the framework of the shock-wave heating model by using semi-analytical methods and discuss whether most of the crystallized chondrules can avoid destruction upon collision in the post-shock region. We find that chondrule precursors immediately turn into supercooled droplets when the shock waves are optically thin, and they can maintain supercooling until the condensation of evaporated fine dust grains. Owing to the large viscosity of supercooled melts, supercooled chondrule precursors can survive high-speed collisions on the order of 1 km s⁻¹ when the temperature is below ~1400 K. From the perspective of the survivability of crystallized chondrules, shock waves with a spatial scale of ~10⁴ km may be potent candidates for the chondrule formation mechanism. Based on our results from one-dimensional calculations, a fraction of compound chondrules can be reproduced when the chondrule-to-gas mass ratio in the pre-shock region is ~2 × 10⁻³, which is approximately half of the solar metallicity.

¹Takaharu Saito,¹Hiroshi Hidaka,²Seung-Gu Lee
The Astrophysical Journal 877, 73 Link to Article [https://doi.org/10.3847/1538-4357/ab1aa5]
¹Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-8601, Japan
²Geological Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea

Howardite–eucrite–diogenite meteorites are believed to originate in the crust of the asteroid 4 Vesta, whose differentiation processes are still controversial. In this study, the first 176Lu–176Hf isotopic data of nine diogenites are presented together with their 87Rb–87Sr isotopic compositions and rare earth element (REE) abundances to investigate the differentiation process of diogenites. The 176Lu–176Hf data sets of nine diogenites revealed the significantly higher initial 176Hf/177Hf ratio of diogenites than that of eucrites, while there are no resolvable differences between their ages. Based on the high initial ratio and the early formation of diogenites, their source material is estimated to be the Vestan mantle. The 87Rb–87Sr systematics of nine diogenites are entirely disturbed probably due to impact events on Vesta. The significant variation observed in the REE abundances of nine diogenites suggests their crystallization from compositionally diverse melts. Based on the mantle origin and compositional diversity of diogenites, we propose the crystallization of diogenites from partial melts of the Vestan mantle. The variation of the trace element abundances of diogenites can be explained by the variation of the degree of the partial melting. The timescale between the crystallization and partial melting of the Vestan mantle is estimated to be ~100–600 Ma from the 176Lu–176Hf isotopic data of nine diogenites, while a heat source for the partial melting is uncertain.

¹Bernd Helber,^1,2Bruno Dias,^1,3,4Federico Bariselli,¹Luiza F. Zavalan,⁵Lidia Pittarello,⁶Steven Goderis,⁶Bastien Soens,^6,7,8Seann J. McKibbin,⁶Philippe Claeys,¹Thierry E. Magin
The Astrophysical Journal 876, 120 Link to Article [https://doi.org/10.3847/1538-4357/ab16f0]
¹Aeronautics and Aerospace Department, von Karman Institute for Fluid Dynamics, Rhode-Saint-Genèse, Belgium
²Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, Louvain-la-Neuve, Belgium
³Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Brussels, Belgium
⁴Dipartimento di Scienze e Tecnologie Aerospaziali, Politecnico di Milano, Milano, Italy
⁵Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁶Analytical, Environmental, and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
⁷Institute of Earth and Environmental Science, University of Potsdam, Potsdam-Golm, Germany
⁸Geowissenschaftliches Zentrum, Georg-August-Universität Göttingen, Göttingen, Germany

Meteoroids largely disintegrate during their entry into the atmosphere, contributing significantly to the input of cosmic material to Earth. Yet, their atmospheric entry is not well understood. Experimental studies on meteoroid material degradation in high-enthalpy facilities are scarce and when the material is recovered after testing, it rarely provides sufficient quantitative data for the validation of simulation tools. In this work, we investigate the thermo-chemical degradation mechanism of a meteorite in a high-enthalpy ground facility able to reproduce atmospheric entry conditions. A testing methodology involving measurement techniques previously used for the characterization of thermal protection systems for spacecraft is adapted for the investigation of ablation of alkali basalt (employed here as meteorite analog) and ordinary chondrite samples. Both materials are exposed to a cold-wall stagnation point heat flux of 1.2 MW m−2. Numerous local pockets that formed on the surface of the samples by the emergence of gas bubbles reveal the frothing phenomenon characteristic of material degradation. Time-resolved optical emission spectroscopy data of ablated species allow us to identify the main radiating atoms and ions of potassium, calcium, magnesium, and iron. Surface temperature measurements provide maximum values of 2280 K for the basalt and 2360 K for the chondrite samples. We also develop a material response model by solving the heat conduction equation and accounting for evaporation and oxidation reaction processes in a 1D Cartesian domain. The simulation results are in good agreement with the data collected during the experiments, highlighting the importance of iron oxidation to the material degradation.

¹Gert Nolze,²Klaus Heide
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125538]
¹Dept 5.1, Federal Institute for Materials, Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany
²Institut für Geowissenschaften (IGW), Friedrich-Schiller-Universität Jena (FSU), Germany
Copyright Elsevier

Roaldite – Fe4N – has been identified in the São Julião de Moreira iron meteorite using electron backscatter diffraction (EBSD) and simultaneously acquired energy-dispersive x-ray spectroscopy (EDS). Mean-periodic-number images derived from raw EBSD patterns confirm this phase by an even higher spatial resolution compared to EDS.
Roaldite appears in the form of systematically and repetitively aligned plates. Despite the locally heavy plastic deformation, it is shown that the origin of the oriented precipitation of roaldite is linked to the orientation of the kamacite matrix. Roaldite can be considered to be precipitated from kamacite using an inverse Kurdjumov-Sachs (K-S) or Nishiyama-Wassermann (N-W) orientation relationship. A more accurate discrimination is impossible due to the accumulated shock deformation, which blurs the local reference orientation of kamacite. The habit plane of roaldite is found to be {112}R, which is most likely parallel to {120}K of kamacite. Some of the roaldite plates contain two orientation variants which repeatedly alternate. Their misorientation angle is about 12°.

^1,2,3E.S.Steenstra,²J.Berndt, ¹S.Klemme,¹A.Rohrbach,¹E.S.Bullock,³W.van Westrenen
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.10.006]
¹The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., the United States of America
²Institute of Mineralogy, University of Münster, Germany
³Faculty of Science, Vrije Universiteit Amsterdam, The Netherlands
Copyright Elsevier

The geochemistry of asteroidal magmas provides fundamental clues to the processes involved in the origin and early evolution of planetary bodies. Although sulfides are important reservoirs for a diverse suite of major and trace elements, it is currently unclear whether the interiors of asteroid Vesta and the Angrite Parent Body were sulfide liquid saturated during petrogenesis of non-cumulate eucrites and volcanic angrites. To assess the potential of sulfide liquid saturation in the interiors of these bodies, high pressure (P) – temperature (T) experiments were used to quantify the sulfur concentrations at sulfide saturation (SCSS) for volcanic angrites and non-cumulate eucrites. The sulfide-silicate partitioning behavior of various trace elements was simultaneously quantified to study their geochemical behavior at sulfide liquid saturation.

It was found that the measured SCSS values agree well with the SCSS values predicted from a previous thermodynamic model for high-FeO* melts. To assess the possibility of sulfide liquid saturation of the source regions of non-cumulate eucrites and angrites, their S abundances were compared with the calculated SCSS values for their source regions. Results show that if eucritic and angritic source regions were saturated with FeS liquid, significant degassing (> 50–80%) of S must have occurred during or following their magmatic emplacement. Such loss is inconsistent with the S, Cl, Zn and Rb isotopic compositions of non-cumulate eucrites. Sulfide liquid saturation of eucrite and angrite source regions is also excluded from the strongly incompatible behavior of Cu and HSE in non-cumulate eucrites and angrites (Riches et al., 2012).

Additional calculations were performed to further explore the timing and extent of S loss during crystallization of the Vestan magma ocean. The assumption of chondritic bulk S abundances of bulk Vesta would correspond with extremely high S contents of the eucrite source region(s), even after consideration of depletion of S due to core formation. In light of the S, Cl, Zn and Rb stable isotopic compositions of eucrites, the S abundances in eucrites are most consistent with the hypothesis that the Vestan mantle was already strongly depleted in S (>70–80 %) by the time of Vestan magma ocean crystallization, resulting in more realistic S contents of the eucrite source region(s). The depletion of S could have been established during initial accretion of Vesta or it could simply reflect accretion of volatile depleted components that experienced incomplete condensation (Wu et al., 2018). Modeling of the new experimentally determined sulfide-silicate partition coefficients and previously reported Vestan mantle depletions of the various chalcophile and siderophile elements suggests that sulfide liquid segregation during early Vestan magma ocean crystallization is also unlikely. The lack of sulfide liquid saturation in the source regions of non-cumulate eucrites and angrites, as well as during early Vestan magma ocean solidification, shows that current geochemical models of core formation and late accretion remain valid for these bodies.