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

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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

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