Axel WITTMANN¹, Randy L. KOROTEV¹, Bradley L. JOLLIFF¹, and Paul K. CARPENTER¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13217]
¹Eyring Materials Center, Arizona State University, 901 S. Palm Walk, PSA 213, Tempe, Arizona 85287–1704, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri63130, USA
Published by arrangement with John Wiley & Sons

Magnesium‐rich spinel assemblages occur in the two lunar vitric breccia meteorites—Dhofar (Dho) 1528 and Graves Nunataks (GRA) 06157. Dho 1528 contains up to ~0.7 mm cumulate Mg‐rich spinel crystals associated with Mg‐rich olivine, Mg‐ and Al‐rich pyroxene, plagioclase, and rare cordierite. Using thermodynamic calculations of these mineral assemblages, we constrain equilibration depths and discuss an origin of these lithologies in the upper mantle of the Moon. In contrast, small, 10 to 20 μm spinel phenocryst assemblages in glassy melt rock clasts in Dho 1528 and GRA 06157 formed from the impact melting of Mg‐rich rocks. Some of these spinel phenocrysts match compositional constraints for spinel associated with “pink spinel anorthosites” inferred from remote sensing data. However, such spinel phenocrysts in meteorites and Apollo samples are typically associated with significant amounts of olivine ± pyroxene that exceed the compositional constraints for pink spinel anorthosites. We conclude that the remotely sensed “pink spinel anorthosites” have not been observed in the collections of lunar rocks. Moreover, we discuss impact‐excavation scenarios for the spinel‐bearing assemblages in Dhofar 1528 and compare the bulk rock composition of Dho 1528 to strikingly similar compositions of Luna 20 samples that contain ejecta from the Crisium impact basin.

Fred J. Ciesla¹, Sebastiaan Krijt¹, Reika Yokochi¹, and Scott Sandford²
Astrophysical Journal 867, 146 Link to Article [DOI: 10.3847/1538-4357/aae1a7]
¹Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL, USA
²NASA Ames Research Center, Moffett Field, CA, USA

Amorphous ice has long been invoked as a means for trapping extreme volatiles into solids, explaining the abundances of these species in comets and planetary atmospheres. Experiments have shown that this trapping is possible and has been used to estimate the abundances of each species in primitive ices after they have formed. However, these experiments have been carried out at deposition rates that exceed those expected in a molecular cloud or solar nebula by many orders of magnitude. Here, we develop a numerical model that reproduces the experimental results and apply it to those conditions expected in molecular clouds and protoplanetary disks. We find that two regimes of ice trapping exist: burial trapping, where the ratio of trapped species to water in the ice reflects that same ratio in the gas; and equilibrium trapping, where the ratio in the ice depends only on the partial pressure of the trapped species in the gas. The boundary between these two regimes is set by both the temperature and rate of ice deposition. These effects must be accounted for when determining the source of trapped volatiles during planet formation.

¹Christopher M.Mauney, ¹Davide Lazzati
Molecular Astrophysics 12, 1-9 Link to Article [https://doi.org/10.1016/j.molap.2018.03.002]
¹Oregon State University, USA

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

MacArthur^a et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.026]
^aInstitute for Space & Earth Observation, Dept. of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK
Copyright Elsevier

Martian meteorite Northwest Africa (NWA) 8114 – a paired stone to NWA 7034 – provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe³⁺/ΣFe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 °C for at least 7 days in an oxidising regolith environment.

We report an approximate ⁴⁰Ar-³⁹Ar maximum age of 1.13 Ga to 1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe³⁺/ΣFe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket.

High temperatures, above 700 °C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days.

Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite.

Haonan Qi¹, Sylvain Picaud², Michel Devel³, Enwei Liang¹, and Zhao Wang¹
Astrophysical Journal 867, 133 Link to Article [DOI: 10.3847/1538-4357/aae4e4]
¹Guangxi Key Laboratory for Relativistic Astrophysics, Department of Physics, Guangxi University, Nanning 530004, People’s Republic of China
²Institut UTINAM, CNRS UMR 6213, Université Bourgogne Franche-Comté, F-25030 Besançon, France
³FEMTO-ST institute, UBFC, CNRS, ENSMM, 15B avenue des Montboucons, F-25030 Besançon, France

Using atomistic simulations, we characterize the adsorption process of organic molecules on carbon nanoparticles, both of which have been reported to be abundant in the interstellar medium (ISM). The aromatic organics are found to adsorb more readily than the aliphatic ones. This selectivity would favor the formation of polycyclic aromatic hydrocarbons (PAHs) or fullerene-like structures in the ISM due to a structural similarity. In our simulations, we also observed that the molecules form a monolayer over the nanoparticle surface before stacking up in aggregates. This suggests a possible layer-by-layer formation process of onion-like nanostructures in the ISM. These findings reveal the possible role of carbon nanoparticles as selective catalysts that could provide reaction substrates for the formation of interstellar PAHs, high fullerenes, and soots from gas-phase molecules.

¹Li, Q., ¹Dai, W., ^1,2,3Liu, B.S.,⁴Sarre, P.J.d, ⁵Xie, M.H., ¹Cheung, A.S.-C.
Molecular Astrophysics 13, 22-29 Link to Article [DOI: 10.1016/j.molap.2018.09.002]
¹Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong
²Department of Chemistry, Tianjin University, Tianjin, 300072, China
³The National Collaborative Innovative Center of Chem. Sci. Eng. Tianjin, Tianjin, 300072, China
³School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom
⁴Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong

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

Gavin G. Kenny^a, Martin Schmieder^b,c, Martin J. Whitehouse^a, Alexander A.Nemchin^a,d, Luiz F. G. Morales^e Elmar Buchner^f,g, Jeremy J.Bellucci^a, Joshua F. Snape^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.012]
^aDepartment of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
^bLunar and Planetary Institute – USRA, 3600 Bay Area Boulevard, Houston TX 77058, USA
^cNASA – Solar System Exploration Research Virtual Institute (SSERVI)
^dDepartment of Applied Geology, Curtin University, Perth, WA 6845, Australia
^eScientific Center for Optical and Electron Microscopy (ScopeM), HPT D 9, Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland
^fHNU Neu-Ulm University of Applied Sciences, Wileystraße 1, 89231 Neu-Ulm, Germany
^gInstitut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstraße 18, 70174 Stuttgart, Germany
Copyright Elsevier

Accurate and precise dating of terrestrial impact craters is a critical requirement for correlating impacts with events such as mass extinctions. A number of isotopic systems have been applied to impact chronology but it is important to understand what an age actually represents and, thus, if it accurately represents the ‘true’ impact age and is suitable for use in correlation. Here we report imaging, microstructural characterisation and high spatial resolution ion microprobe U-Pb analysis of shocked zircon from the approximately 23 km-in-diameter Lappajärvi impact structure, Finland, for which a well-established ⁴⁰Ar/³⁹Ar framework exists. Microstructural analysis identified two distinct styles of shock recrystallisation: (i) granular zircon that displays multiple domains of similarly oriented neoblasts, some of which are interpreted to be the product of reversion from the high-pressure ZrSiO₄ polymorph, reidite, and (ii) granular zircon composed entirely of similarly oriented neoblasts. Only the former gave concordant U-Pb data interpreted to record the age of the impact. The U-Pb ‘concordia age’ reported here, 77.85 ± 0.78 Ma (1.0 %; MSWD = 0.60; probability = 0.87; n = 8; 2σ; full external uncertainty), is resolvable from the previously published ‘best estimate’ ⁴⁰Ar/³⁹Ar age for impact melt rock (76.20 ± 0.29 Ma) and ⁴⁰Ar/³⁹Ar K-feldspar ages as young as 75.11 ± 0.36 Ma, and is therefore interpreted to more accurately reflect the age of the impact event. The resolvable disparity between the zircon U-Pb and the ⁴⁰Ar/³⁹Ar data indicates that even the oldest statistically robust ⁴⁰Ar/³⁹Ar ages obtained at medium- and large-sized impact craters may not accurately record the timing of an impact event at a kyr level. The offset between the U-Pb and ⁴⁰Ar/³⁹Ar data is interpreted to be, at least in part, a result of the zircon data recording a higher isotopic closure temperature, and the younger ⁴⁰Ar/³⁹Ar ages recording the progressive cooling of different domains of the impact structure. The Lappajärvi impact structure is the first Phanerozoic impact structure dated by U-Pb analysis of shock-recrystallised zircon to better than, or equal to, 1.0 % uncertainty. This further demonstrates that well-characterised granular zircon grains are likely to have wide utility in the accurate and precise dating of terrestrial impact events.

Axel WITTMANN¹, Randy L. KOROTEV², Bradley L. JOLLIFF², Kunihiko NISHIIZUMI³, A. J. Timothy JULL⁴, Marc W. CAFFEE^5,6, Michael ZANETTI⁷, and Anthony J. IRVING⁸
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13218]
¹Eyring Materials Center, Arizona State University, 901 S. Palm Walk, PSA 213, Tempe, Arizona 85287–1704, USA
²Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University inSt. Louis, One Brookings Drive, St. Louis, Missouri 63130, USA
³Space Sciences Laboratory, University of California Berkeley, Berkeley, California 94720–7450, USA
⁴Department of Geosciences, University of Arizona, 1040 East Fourth St., Tucson, Arizona 85721–0077, USA
⁵Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette,Indiana 47907–2036, USA
⁶Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 525 Northwestern Avenue, West Lafayette,Indiana 47907–2036, USA
⁷Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street N, London, Ontario N6A 5B7, Canada
⁸Department of Earth and Space Sciences, University of Washington, 4000 15thAvenue NE, Seattle, Washington 98195, USA
Published by arrangement with John Wiley & Sons

Oued Awlitis 001 is a highly feldspathic, moderately equilibrated, clast‐rich, poikilitic impact melt rock lunar meteorite that was recovered in 2014. Its poikilitic texture formed due to moderately slow cooling, which judging from textures of rocks in melt sheets of terrestrial impact structures, is observed in impact melt volumes at least 100 m thick. Such coherent impact melt volumes occur in lunar craters larger than ~50 km in diameter. The composition of Oued Awlitis 001 points toward a crustal origin distant from incompatible‐element‐rich regions. Comparison of the bulk composition of Oued Awlitis 001 with Lunar Prospector 5° γ‐ray spectrometer data indicates a limited region of matches on the lunar farside. After its initial formation in an impact crater larger than ~50 km in diameter, Oued Awlitis 001 was excavated from a depth greater than ~50 m. The cosmogenic nuclide inventory of Oued Awlitis 001 records ejection from the Moon 0.3 Ma ago from a depth of at least 4 m and little mass loss due to ablation during its passage through Earth’s atmosphere. The terrestrial residence time must have been very short, probably less than a few hundred years; its exact determination was precluded by a high concentration of solar cosmic ray‐produced ¹⁴C. If the impact that excavated Oued Awlitis 001 also launched it, this event likely produced an impact crater >10 km in diameter. Using petrologic constraints and Lunar Reconnaissance Orbiter Camera and Diviner data, we test Giordano Bruno and Pierazzo as possible launch craters for Oued Awlitis 001.

Yanxia Xie¹, Luis C. Ho^1,2, Aigen Li³, and Jinyi Shangguan^1,2
Astrophysical Journal 867, 91 Link to Article [DOI: 10.3847/1538-4357/aae2b0]
¹Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
²Department of Astronomy, School of Physics, Peking University, Beijing 100871, People’s Republic of China
³Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA

Interstellar dust spans a wide range in size distribution, ranging from ultrasmall grains of a few Ångströms to micrometer-size grains. While the presence of nanometer-size dust grains in the Galactic interstellar medium was speculated six decades ago and was previously suggested based on early infrared observations, systematic and direct analysis of their properties over a wide range of environments has been lacking. Here we report the detection of nanometer-size dust grains that appear to be universally present in a wide variety of astronomical environments, from Galactic high-latitude clouds to nearby star-forming galaxies and galaxies with low levels of nuclear activity. The prevalence of such a grain population is revealed conclusively as prominent mid-infrared continuum emission at λ  10 μm seen in the Spitzer/Infrared Spectrograph data, characterized by temperatures of ~300–400 K that are significantly higher than the equilibrium temperatures of common, submicron-size grains in typical galactic environments. We propose that the optimal carriers of this pervasive, featureless hot dust component are very small carbonaceous (e.g., graphite) grains of nanometer size that are transiently heated by single-photon absorption. This grain population accounts for ~1.4% of the total infrared emission at ~5–3000 μm and ~0.4% of the total interstellar dust mass.

¹Kozyrev, A.S et al. (>10)
Physics of Atomic Nuclei 81, 527-539 Link to Article [DOI: 10.1134/S1063778818040099]
¹Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997, Russian Federation

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