¹Gene Schmidt, ²Frank Fueten, ³Robert Stesky, ⁴Jessica Flahaut, ⁵Ernst Hauber
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005658]
¹IRSPS, Universita “G.D’Annunzio”, Pescara, Italy
²Department of Earth Sciences, Brock University, St. Catharines, Ontario, Canada
³Pangaea Scientific, Brockville, Ontario, Canada
⁴CNRS (institute)/CRPG Nancy (department), France
⁵Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany
Published by arrangement with John Wiley & Sons

Hebes Chasma is an 8 km deep, 126 by 314 km, isolated basin that is partially filled with massive deposits of water‐altered strata called interior layered deposits (ILD). By analyzing the ILD’s structure, stratigraphy and mineralogy, a depositional history of Hebes Chasma is interpreted. Three distinct ILD units were found and are informally referred to as the Lower, Upper and Late ILD. These units are distinguished by their layer thicknesses, layer attitudes, mineralogies and erosional landforms. The Lower and Upper ILDs comprise the chasma’s 7.5 km tall, 120 by 43 km, central mound and the Late ILD is located in the valley between the central mound and the chasma’s northern wall. A horizontal unconformity separates the Lower and Upper ILDs and layer attitudes revealed large‐scale shallow folding within the Lower ILD. All ILDs are characterized by both monohydrated and polyhydrated sulfates (MHS and PHS) signatures. Erosional landforms such as hummocks, polygons, and debris flows suggest past glacial activity within the chasma. A scenario involving several ash fall events during various stages of chasma formation is proposed as the dominant setting throughout Hebes’ geologic history.

¹C.Xiang, ¹A.Carballido, ²R.D.Hanna, ¹L.S.Matthews, ¹T.W.Hyde
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.10.014]
¹Center for Astrophysics, Space Physics, and Engineering Research, Baylor University, Waco, TX 76798-7316, USA
²Jackson School of Geosciences, University of Texas, Austin, TX 78712, USA
Copyright Elsevier

In order to characterize the early growth of fine-grained dust rims (FGRs) that commonly surround chondrules in carbonaceous chondrites, we perform numerical simulations of dust accretion onto chondrule surfaces. We employ a Monte Carlo algorithm to simulate the collision of dust monomers having radii between 0.5 and 10 μm with chondrules whose radii are between 500 and 1000 μm, in 100-μm increments. The collisions are driven by Brownian motion and solar nebula turbulence. After each collision, the colliding particles either stick at the point of contact, roll or bounce. We limit accretion of dust monomers (and in some cases, dust aggregates) to a small patch of the chondrule surface, for computational expediency. We model the morphology of the dust rim and the trajectory of the dust particle, which are not considered in most of the previous works. Radial profiles of FGR porosity show that rims formed in weak turbulence are more porous (with a porosity of 60-74%) than rims formed in stronger turbulence (with a porosity of 52-60%). The lower end of each range corresponds to large chondrules and the upper end to small chondrules, meaning that the chondrule size also has an impact on FGR porosity. Consistent with laboratory observations of CM chondrites, the thickness of FGRs obtained in the simulations depends linearly on chondrule radius. The collection of single monomers leads to the increase of grain size from the inner to the outer layers of the dust rim. The porosity of FGRs formed by dust aggregates is  ∼ 20% greater on average than that of FGRs formed by single monomers. In general, the relatively high porosities that we obtain are consistent with those calculated by previous authors from numerical simulations, as well as with initial FGR porosities inferred from laboratory measurements of rimmed chondrule samples and rimmed chondrule analogs.

¹K.L.Donaldson Hanna et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.10.018]
¹Atmospheric, Oceanic and Planetary Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom
Copyright Elsevier

We present spectral measurements of a suite of mineral mixtures and meteorites that are possible analogs for asteroid (101955) Bennu, the target asteroid for NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) mission. The sample suite, which includes anhydrous and hydrated mineral mixtures and a suite of chondritic meteorites (CM, CI, CV, CR, and L5), was chosen to characterize the spectral effects due to varying amounts of aqueous alteration and minor amounts of organic material. Our results demonstrate the utility of mineral mixtures for understanding the mixing behavior of meteoritic materials and identifying spectrally dominant species across the visible to near-infrared (VNIR) and thermal infrared (TIR) spectral ranges. Our measurements demonstrate that, even with subtle signatures in the spectra of chondritic meteorites, we can identify diagnostic features related to the minerals comprising each of the samples. Also, the complementary nature of the two spectral ranges regarding their ability to detect different mixture and meteorite components can be used to characterize analog sample compositions better. However, we observe differences in the VNIR and TIR spectra between the mineral mixtures and the meteorites. These differences likely result from (1) differences in the types and physical disposition of constituents in the mixtures versus in meteorites, (2) missing phases observed in meteorites that we did not add to the mixtures, and (3) albedo differences among the samples. In addition to the initial characterization of the analog samples, we will use these spectral measurements to test phase detection and abundance determination algorithms in anticipation of mapping Bennu’s surface properties and selecting a sampling site.

¹Patrick J. A. Hill,^1,2Gordon R. Osinski,¹Neil R. Banerjee,³Randy L. Korotev,⁴Sobhi J. Nasir,⁵Christopher D. K. Herd
Meteoritics & Planetray Science (in Press) Link to Article [https://doi.org/10.1111/maps.13207]
¹Department of Earth Science and Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, Canada
²Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
³Department of Earth and Planetary Sciences, Washington University in Saint Louis, St. Louis, Missouri, USA
⁴Earth Sciences Research Center, Sultan Qaboos University, 123 Muscat, Oman
⁵Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
Published by arrangement with John Wiley & Sons

The Dhofar 1673, Dhofar 1983, and Dhofar 1984 meteorites are three lunar regolith breccias classified based on their petrography, mineralogy, oxygen isotopes, and bulk chemistry. All three meteorites are dominated by feldspathic lithic clasts; however, impact melt rock clasts and spherules are also found in each meteorite. The bulk chemistry of these samples is similar to other feldspathic highland meteorites with the Al₂O₃ content only slightly lower than average. Within the lithic clasts, the Mg # of mafic phases versus the anorthite content of feldspars is similar to other highland meteorites and is found to plot intermediate of the ferroan‐anorthositic suite and magnesian suite. The samples lack any KREEPy signature and have only minor indications of a mare basalt component, suggesting that the source region of all three meteorites would have been distal from the Procellarum KREEP Terrane and could have possibly been the Feldspathic Highland Terrane. All three meteorites were found within 500 m of each other in the Dhofar region of Oman. This, together with their similar petrography, stable isotope chemistry, and geochemistry indicates the possibility of a pairing.

¹J.Pape, ¹K.Mezger, ²A.-S.Bouvier, ²L.P.Baumgartner
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.017]
¹Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland
²Institute of Earth Sciences, University of Lausanne, UNIL-Mouline, CH-1015 Lausanne, Switzerland
Copyright Elsevier

Chondrules from unequilibrated ordinary and carbonaceous chondrites belong to the oldest and most primitive materials from the early solar system and record chemical and isotopic signatures relating to their formation and evolution. These signatures allow tracing protoplanetary disk processes that eventually led to the formation of planetary building blocks and rocky planets. ²⁶Al-²⁶Mg ages based on mineral-mesostasis isochrons of 31 porphyritic ferromagnesian chondrules, that belong mainly to type-II, constrain the time of chondrule melting prior to incorporation into the respective chondrite parent bodies. For this study chondrules from the unequilibrated L, L(LL) and LL ordinary chondrites (UOCs) NWA 5206, NWA 8276, MET 96503, MET 00452, MET 00526, NWA 7936 and QUE 97008 were selected, which are of petrologic types 3.00 to 3.15 and were thus least metamorphosed after formation. Magnesium and Al isotopes were measured in-situ by Secondary Ion Mass Spectrometry (SIMS) using a CAMECA 1280 ims. ²⁶Mg excess from in-situ decay of ²⁶Al correlating with ²⁷Al/²⁴Mg has been detected in the mesostasis of all but one chondrule. The initial Al isotopic compositions (²⁶Al/²⁷Al)₀ and ²⁶Mg/²⁴Mg ratios (δ²⁶Mg∗₀) deduced from internal mineral isochron regressions range from (9.5 ± 2.8) × 10^-6 to (3.1 ± 1.2) × 10^-6 and -0.020 ± 0.028‰ to 0.011 ± 0.039‰, respectively. The corresponding chondrule ages (Δt_CAI), calculated relative to calcium-aluminum-rich inclusions (CAIs) using the canonical ²⁶Al/²⁷Al = (5.23 ± 0.13) × 10^-5, are between 1.76-0.27+0.36 and 2.92-0.34+0.51 Ma and date the melt formation and thus primary chondrule formation from dust-like precursors or reprocessing of older chondrules. The age range agrees with those acquired with different short-lived chronometers and with published ²⁶Al-²⁶Mg ages, the majority of which were obtained for chondrules from the Bishunpur and Semarkona meteorites, although no chondrule with (²⁶Al/²⁷Al)₀ > 10^-5 was found.

Chondrules in single chondrite samples or between different chondrite groups show no distinct age distributions. The initial ²⁶Al/²⁷Al of the oldest chondrules in the L(LL)/LL and L chondrite samples are identical within their 1σ uncertainties and yield a mean age of 1.99-0.08+0.08 Ma and 1.81-0.10+0.11 Ma, respectively. The oldest chondrules from six of the seven studied samples record a mean age of 1.94-0.06+0.07 Ma. Since heating events in the protoplanetary disk could have partially reset the Al-Mg systematics in pre-existing chondrules and this would have shifted recorded ²⁶Al-²⁶Mg ages toward younger dates, the oldest mean age of 1.81-0.10+0.11 Ma recorded in L chondrite chondrules is interpreted to date the rapid and punctuated onset of chondrule formation. The density distribution of chondrule ages from this study, which comprises the largest single dataset of OC chondrule ages, combined with published ages for chondrules from ordinary and carbonaceous chondrites reveals major age peaks for OC chondrules at 2.0 and 2.3 Ma. Chondrules in ordinary and carbonaceous chondrites formed almost contemporaneously (with a possible distinction between CC groups) in two chemically distinct reservoirs, probably in density-enriched regions at the edges of Jupiter’s orbit. The young formation ages of chondrules suggest that they do not represent precursors but rather by-products of planetesimal accretion.

¹Addi Bischoff, ^1,2Maximilian Schleiting, ¹Markus Patzek
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13208]
¹Institut für Planetologie, University of Münster, Münster, Germany
²Institut für konstruktiven Ingenieurbau, Universität Kassel, Kassel, Germany
Published by arrangement with John Wiley & Sons

The brecciation and shock classification of 2280 ordinary chondrites of the meteorite thin section collection at the Institut für Planetologie (Münster) has been determined. The shock degree of S3 is the most abundant shock stage for the H and LL chondrites (44% and 41%, respectively), while the L chondrites are on average more heavily shocked having more than 40% of rocks of shock stage S4. Among the H and LL chondrites, 40–50% are “unshocked” or “very weakly shocked.” Considering the petrologic types, in general, the shock degree is increasing with petrologic type. This is the case for all meteorite groups. The main criteria to define a rock as an S6 chondrite are the solid‐state recrystallization and staining of olivine and the melting of plagioclase often accompanied by the formation of high‐pressure phases like ringwoodite. These characteristics are typically restricted to local regions of a bulk chondrite in or near melt zones. In the past, the identification of high‐pressure minerals (e.g., ringwoodite) was often taken as an automatic and practical criterion for a S6 classification during chondrite bulk rock studies. The shock stage classification of many significantly shocked chondrites (>S3) revealed that most ringwoodite‐bearing rocks still contain more than 25% plagioclase (74%). Thus, these bulk chondrites do not even fulfill the S5 criterion (e.g., 75% of plagioclase has to be transformed into maskelynite) and have to be classified as S4. Studying chondrites on typically large thin sections (several cm2) and/or using samples from different areas of the meteorites, bulk chondrites of shock stage S6 should be extremely rare. In this respect, the paper will discuss the probability of the existence of bulk rocks of S6.

^1,2Maree McGregor, ²Christopher R.M.McFarlane, ^1,2John G.Spray
Earth and Planetary Science Letters 504, 185-197 Link to Article [https://doi.org/10.1016/j.epsl.2018.10.006]
¹Planetary and Space Science Centre, Canada
²Department of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
Copyright Elsevier

In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been deployed to determine the U–Pb ages of impact metamorphosed apatite and zircon associated with the ∼12.5 km diameter Nicholson Lake impact structure, Northwest Territories, Canada. The dated phases occur within both impact melt-bearing breccias and clast-laden impact melts. A total of 84 laser ablation analyses from 57 apatite grains within seven rock samples yield a minimum refined lower intercept at 387 ± 5 Ma (MSWD = 0.87, 2σ, n=26), and maximum age with lower intercept of ∼1740 Ma. A total of 90 laser ablation analyses on 52 zircon grains from two rock samples yield a minimum intercept age of 384 ± 8 Ma (MSWD = 1.3, 2σ, n=22), with a maximum upper intercept age of 2679 ± 14 Ma, and a second discordia with a ∼1740 Ma upper intercept age. The results are consistent with the target rocks comprising Archean Snow Island Suite (∼2.7 Ga) and Paleoproterozoic Nueltin plutonic suite (∼1.74 Ga), with the impact event occurring at approximately 385 Ma. The degree of resetting of inherited apatite can be related to its proximity to impact melt (now partly devitrified glass) within the host impact melt-bearing breccias and clast-laden impact melt rocks. Those grains in direct contact with melt bodies are reset, while those occurring as inclusions in lithic or mineral clasts partly or wholly retain their original isotopic compositions. The youngest (impact-reset) apatite ages are most closely associated with the highest shock levels (i.e., granular textured and thermally dissociated zircons), which we relate to juxtaposition of apatite with superheated melt. Due to accumulated radiation damage prior to impact, many of the relic zircons are metamict, which facilitated enhanced Pb diffusion and their resetting to the impact age. Granular zircons record impact ages while those exhibiting planar fractures or experiencing negligible shock record basement ages. We link apatite and zircon geochronology to the textural and structural states of ZrSiO4 and its associated shock levels via field emission scanning electron microscopy and micro-Raman spectrometry.

¹E.Dobrică, ²C.Le Guillou, ¹A.J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.10.002]
¹Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, NM 87131-0001, USA
²Unité Matériaux et Transformations, UMR-CNRS 8207, University of Lille, F-59655 Villeneuve d’Ascq, France
Copyright Elsevier

Porous, igneous glassy microchondrules are relatively common in Semarkona and other unequilibrated ordinary chondrites (UOC), occurring embedded within the fine-grained matrices and the chondrule rims. We have investigated the effects of aqueous alteration on two porous microchondrules and associated amorphous silicate materials in the fine-grained matrix of the Semarkona (LL3.00) ordinary chondrite. The iron valency was measured by synchrotron-based scanning transmission X-ray microscopy (STXM) and detailed compositional and mineralogical data were obtained by transmission electron microscopy (TEM). Our STXM data show that the iron in both microchondrules and the amorphous matrix materials is highly oxidized, with elevated ferric iron contents (up to Fe³⁺/∑Fe ratio 81%). The oxidation process appears to be the result of the interaction of aqueous fluids produced by melting of water ice that accreted into the LL chondrite parent body. The lack of secondary phases such as phyllosilicates or FeO-rich olivines in the microchondrules and the surrounding fine-grained matrix indicates that aqueous alteration was extremely limited. The distribution of major and minor elements indicates limited elemental exchange between the porous microchondrules and the adjacent fine-grained matrix, suggesting that the chemical composition of the two porous microchondrules was not extensively modified during the aqueous alteration. Therefore, these results strongly support the recent conclusions that porous and nonporous microchondrules have different precursor compositions, since their bulk compositions are different and have not been extensively modified by secondary processes.

^1,2Megan Bedell et al. (>10)
The Astrophysical Journal 865, 68 Link to Article [https://doi.org/10.3847/1538-4357/aad908]
¹Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA
²Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA

The compositions of stars are a critical diagnostic tool for many topics in astronomy such as the evolution of our Galaxy, the formation of planets, and the uniqueness of the Sun. Previous spectroscopic measurements indicate a large intrinsic variation in the elemental abundance patterns of stars with similar overall metal content. However, systematic errors arising from inaccuracies in stellar models are known to be a limiting factor in such studies, and thus it is uncertain to what extent the observed diversity of stellar abundance patterns is real. Here we report the abundances of 30 elements with precisions of 2% for 79 Sun-like stars within 100 pc. Systematic errors are minimized in this study by focusing on solar twin stars and performing a line-by-line differential analysis using high-resolution, high-signal-to-noise spectra. We resolve [X/Fe] abundance trends in galactic chemical evolution at precisions of 10−3 dex Gyr−1 and reveal that stars with similar ages and metallicities have nearly identical abundance patterns. Contrary to previous results, we find that the ratios of carbon-to-oxygen and magnesium-to-silicon in solar-metallicity stars are homogeneous to within 10% throughout the solar neighborhood, implying that exoplanets may exhibit much less compositional diversity than previously thought. Finally, we demonstrate that the Sun has a subtle deficiency in refractory material relative to >80% of solar twins (at 2σ confidence), suggesting a possible signpost for planetary systems like our own.

¹Alexey Potapov, ¹Cornelia Jäger, ²Thomas Henning
The Astrophysical Journal 865, 58 Link to Article [https://doi.org/10.3847/1538-4357/aad803]
¹Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena, Germany
²Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany

Understanding the history and evolution of small bodies, such as dust grains and comets, in planet-forming disks is very important to reveal the architectural laws responsible for the creation of planetary systems. These small bodies in cold regions of the disks are typically considered to be mixtures of dust particles with molecular ices, where ices cover the surface of a dust core or are actually physically mixed with dust. While the first case, ice-on-dust, has been intensively studied in the laboratory in recent decades, the second case, ice-mixed-with-dust, presents uncharted territory. This work is the first laboratory study of the temperature-programmed desorption of water ice mixed with amorphous carbon and silicate grains. We show that the kinetics of desorption of H2O ice depends strongly on the dust/ice mass ratio, probably due to the desorption of water molecules from a large surface of fractal clusters composed of carbon or silicate grains. In addition, it is shown that water ice molecules are differently bound to silicate grains in contrast to carbon. The results provide a link between the structure and morphology of small cosmic bodies and the kinetics of desorption of water ice included in them.