Holmbeck^1,2 et al. (>10)
Astrophysical Journal Letters 859, L24 Link to Article [DOI: 10.3847/2041-8213/aac722]
¹Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

We report the discovery of a new actinide-boost star, 2MASS J09544277+5246414, originally identified as a very bright (V = 10.1), extremely metal-poor ([Fe/H] = −2.99) K giant in the LAMOST survey, and found to be highly r-process-enhanced (r-II; [Eu/Fe] = +1.28]), during the snapshot phase of the R-Process Alliance (RPA). Based on a high signal-to-noise ratio (S/N), high-resolution spectrum obtained with the Harlan J. Smith 2.7 m telescope, this star is the first confirmed actinide-boost star found by RPA efforts. With an enhancement of [Th/Eu] = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhanced r-II star yet discovered, and only the sixth metal-poor star with a measured uranium abundance ([U/Fe] = +1.40). Using the Th/U chronometer, we estimate an age of 13.0 ± 4.7 Gyr for this star. The unambiguous actinide-boost signature of this extremely metal-poor star, combined with additional r-process-enhanced and actinide-boost stars identified by the RPA, will provide strong constraints on the nature and origin of the r-process at early times.

^1,2Shuai Li, ¹Paul G. Lucey, ²Ralph E. Milliken, ³Paul O. Hayne, ²Elizabeth Fisher, ⁴Jean-Pierre Williams, ⁵Dana M. Hurley, ⁶Richard C. Elphic
Processdings of the National Academy of Sciences of the United States of America (PNAS) 115, 8907-8912 Link to Article [https://doi.org/10.1073/pnas.1802345115]
¹Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822
²Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912
³Department of Astrophysical & Planetary Sciences, University of Colorado Boulder, Boulder, CO 80309
⁴Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095
⁵Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723
⁶Ames Research Center, NASA, Mountain View, CA 94035

Water ice may be allowed to accumulate in permanently shaded regions on airless bodies in the inner solar system such as Mercury, the Moon, and Ceres [Watson K, et al. (1961) J Geophys Res 66:3033–3045]. Unlike Mercury and Ceres, direct evidence for water ice exposed at the lunar surface has remained elusive. We utilize indirect lighting in regions of permanent shadow to report the detection of diagnostic near-infrared absorption features of water ice in reflectance spectra acquired by the Moon Mineralogy Mapper [M (3)] instrument. Several thousand M (3) pixels (∼280 × 280 m) with signatures of water ice at the optical surface (depth of less than a few millimeters) are identified within 20° latitude of both poles, including locations where independent measurements have suggested that water ice may be present. Most ice locations detected in M (3) data also exhibit lunar orbiter laser altimeter reflectance values and Lyman Alpha Mapping Project instrument UV ratio values consistent with the presence of water ice and also exhibit annual maximum temperatures below 110 K. However, only ∼3.5% of cold traps exhibit ice exposures. Spectral modeling shows that some ice-bearing pixels may contain ∼30 wt % ice that is intimately mixed with dry regolith. The patchy distribution and low abundance of lunar surface-exposed water ice might be associated with the true polar wander and impact gardening. The observation of spectral features of H2O confirms that water ice is trapped and accumulates in permanently shadowed regions of the Moon, and in some locations, it is exposed at the modern optical surface.

¹Tabea Bogdan, ¹Jens Teiser, ¹Nikolai Fischer, ¹Maximilian Kruss, ¹Gerhard Wurm
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.09.011]
University of Duisburg-Essen, Faculty of Physics, Lotharstr. 1-21, Duisburg, 47057, Germany
Copyright Elsevier

In laboratory experiments, spherical 1-mm-wide glass and basalt particles are heated, and the hot particles collide at about 1 m/s with a flat glass target that is at room temperature. When the particles are heated below 900 K, the collisions are essentially elastic with coefficients of restitution of about 0.9, but above 900 K collisions become increasingly inelastic and the coefficient of restitution decreases with increasing temperature. At 1100 K the glass particles approach sticking but, simultaneously, at the same temperature the particles melt on timescales of minutes. The basalt particles approach sticking at 1200 K. Only above 1400 K do basalt grains in contact with each other fuse together, forming compounds on timescales of hours, and at 1500 K basalt grains completely fuse together. Therefore, cooling basalt grains only have a 100 K window for compound formation, and velocities very likely have to be below 1 m/s for sticking in the first place. We predict that this puts constraints on compound chondrule formation and particle densities in the solar nebula.

^1,2M.S.Thompson, ^3,4M.J.Loeffler, ¹R.V.Morris, ¹L.P.Keller, ⁵R.Christoffersen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.09.022]
¹ARES, NASA Johnson Space Center, Houston, TX 77058
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907
³NASA Goddard Space Flight Center, Greenbelt, MD 20771
⁴Northern Arizona University, Department of Physics and Astronomy, Flagstaff, AZ 86011
⁵Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, TX
Copyright Elsevier

We performed pulsed-laser irradiation of a chip of the CM2 Murchison carbonaceous chondrite meteorite to simulate micrometeorite impacts on carbonaceous asteroids. Optical reflectance spectroscopy and by transmission electron microscopy were performed to characterize the unirradiated and irradiated samples and vapor and melt deposits collected on a glass slide ∼7 mm from the surface of the sample. The spectrum of the deposit on the glass slide shows a red slope between 0.35-2.5 µm, while the irradiated surface of the meteorite shows only slight darkening over the same spectral range. We identified predominant melt products and vesiculated textures in the glass slide deposit, in the fine-grained matrix of the meteorite, and in individual mineral phases of the meteorite chip. Extracted focused ion beam (FIB) sections from the matrix material, an olivine grain, a pentlandite grain, and from the glass slide deposit were analyzed by scanning transmission electron microscopy (STEM). Microstructural and chemical analyses based on the STEM observations show widespread melting and the formation of Fe-bearing nanoparticles (including prevalent Fe-Ni-sulfides) across the surface of the meteorite. The section extracted from the glass slide revealed nanoparticles embedded in a chemically and microstructurally complex deposit, which likely formed as a result of both melting and vaporization processes. These analyses reveal a significantly more compositionally diverse population of nanoparticles compared to what is observed in lunar or ordinary chondritic space weathered samples. We discuss the implications these results have for the space weathering of carbonaceous asteroids and their importance for understanding the surface processes on primitive bodies.

¹S. V. S. Murty, ¹S. Ghosh, ¹Dwijesh Ray
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13195]
¹Planetary Sciences Division, Physical Research Laboratory, Ahmedabad, India, Vastrapur, Ahmedabad, India
Published by arrangement with John Wiley & Sons

Noble gases and nitrogen were measured in two adjacent samples each from the Raghunathpura (IIAB) and the Nyaung (IIIAB) iron meteorite falls. Light noble gases in both the meteorites were of pure cosmogenic origin. Using (3He/4He)c ratios and the production systematic of Ammon et al. (2009), we estimated the sample depth and meteoroid size for Nyaung (~8 cm depth in a ~15 cm radius object) and Raghunathpura (~12–14 cm depth in a ~25 cm object). We derived cosmic ray exposure ages of 1710 ± 256 Ma (for Nyaung, the highest reported so far for the IIIAB group) and 224 ± 34 Ma (for Raghunathpura). Variable amounts of trapped Kr and Xe were found in both meteorites. The phase Q‐like elemental ratio (84Kr/132Xe) suggests that the trapped component is of indigenous origin, and most likely hosted in the heterogeneously distributed micro‐inclusions of troilite/schreibersite. Trapped phase Q component is being reported for the first time, for a IIAB iron meteorite. Both meteorites showed light isotopic composition for nitrogen, and need at least two N components to explain the observed N isotopic systematic. Variable amounts of trapped noble gases and the presence of more than one N component suggest that the magmatic process that formed the parent body of these meteorites either could not completely homogenize or completely degas all the phases.

^1,2Queenie H.S. Chan et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13193]
¹ARES, NASA Johnson Space Center, Houston, Texas, USA
²Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

We present in this study the effects of short‐term heating on organics in the Tagish Lake meteorite and how the difference in the heating conditions can modify the organic matter (OM) in a way that complicates the interpretation of a parent body’s heating extent with common cosmothermometers. The kinetics of short‐term heating and its influence on the organic structure are not well understood, and any study of OM is further complicated by the complex alteration processes of the thermally metamorphosed carbonaceous chondrites—potential analogues of the target asteroid Ryugu of the Hayabusa2 mission—which had experienced posthydration, short‐duration local heating. In an attempt to understand the effects of short‐term heating on chondritic OM, we investigated the change in the OM contents of the experimentally heated Tagish Lake meteorite samples using Raman spectroscopy, scanning transmission X‐ray microscopy utilizing X‐ray absorption near edge structure spectroscopy, and ultraperformance liquid chromatography fluorescence detection and quadrupole time of flight hybrid mass spectrometry. Our experiment suggests that graphitization of OM did not take place despite the samples being heated to 900 °C for 96 h, as the OM maturity trend was influenced by the heating conditions, kinetics, and the nature of the OM precursor, such as the presence of abundant oxygenated moieties. Although both the intensity of the 1s−σ* exciton cannot be used to accurately interpret the peak metamorphic temperature of the experimentally heated Tagish Lake sample, the Raman graphite band widths of the heated products significantly differ from that of chondritic OM modified by long‐term internal heating.

¹Shijie Li et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.09.004]
¹Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
Copyright Elsevier

We report a petrography, mineral chemistry, oxygen and chromium isotopic study of Grove Mountains (GRV) 020043 together with a subset of other acapulcoites and lodranites. GRV 020043 is a petrologic type 4 chondrite, with chondrules of diverse types and sizes, and is composed of low-Ca pyroxene (40 vol.%), olivine (24 vol.%), diopside (8 vol.%), plagioclase (10 vol.%), Fe-Ni metal (kamacite and taenite), troilite and some accessory minerals (chromite and apatite). The olivine in GRV 020043 has an average fayalite content (Fa) of 10.7 mol.% with the low-Ca pyroxene having an average ferrosilite (Fs) content of 10.8 mol.%. The whole rock oxygen isotopic composition of GRV 020043 is +3.226 ± 0.267‰, +0.797 ± 0.131‰, and -0.927 ± 0.017‰ for δ18O, δ17O, and Δ17O, respectively, with a bulk chromium isotopic compositions of ε54Cr = -0.48 ± 0.10. These characteristics of GRV 020043 are different from all established or ungrouped chondrites but agree with those of the acapulcoite-lodranite clan. We therefore suggest that GRV 020043 represents the chondritic precursor of acapulcoite-lodranite parent body.
The similarity of bulk oxygen and chromium isotopic compositions among GRV 020043, Acapulco, Northwest Africa (NWA) 468 (metal-rich lodranite), NWA 8118 (lodranite), NWA 8287 (acapulcoite), and NWA 8422 (lodranite) indicates that they originated from a common oxygen and chromium reservoir in the protoplanetary disk or may have derived from a parent body with a differentiated multilayer structure.

¹Yuki Hibiya et al. (>10)*
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.04.031]
¹Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
Copyright Elsevier

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re–187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55–57Wo3–4Fs40–42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50–53; Fe/Mn = 1.1–2.1), chromite (Cr# ∼ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–102 °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high super-saturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ17O = −1.052 ± 0.004, 2 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

Aki Takigawa^1,2, Rhonda M. Stroud³, Larry R. Nittler⁴, Conel M. O’D Alexander⁴, and Akira Miyake²
Astrophysical Journal Letters 862, L13 Link to Article [DOI: 10.3847/2041-8213/aad1f5]
¹The Hakubi Center for Advanced Research, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo, Kyoto 606-8502, Japan
²Division of Earth and Planetary Sciences, Kyoto University Kitashirakawa-Oiwakecho, Sakyo, Kyoto 606-8502, Japan
³Naval Research Laboratory, Code 6360, Washington, DC 20375, USA
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA

Corundum (α-Al₂O₃) and amorphous or metastable Al₂O₃ are common components of circumstellar dust observed around O-rich asymptotic giant branch (AGB) stars and found in primitive meteorites. We report a detailed isotopic and microstructural investigation of a unique presolar corundum grain, QUE060, identified in an acid residue of the Queen Alexandra Range 97008 (LL3.05) meteorite. Based on its O and Mg isotopic compositions, this 1.4 μm diameter grain formed in a low- or intermediate-mass AGB star. It has four developed rhombohedral {011} faces of corundum and a rough, rounded face with cavities. High Mg contents (Mg/Al > 0.004) are due to the decay of radioactive ²⁶Al. No spinel (MgAl₂O₄) inclusions that might have exsolved from the corundum are observed, but there are several high-Mg domains with modulated structures. The subhedral shape of grain QUE060 is the first clear evidence that corundum condenses and grows to micrometer sizes in the extended atmospheres around AGB stars. The flat faces indicate that grain QUE060 experienced little modification by gas–grain and grain–grain collisions in the interstellar medium (ISM) and solar nebula. The Mg distribution in its structure indicates that grain QUE060 has not experienced any severe heating events since the exhaustion of ²⁶Al. However, it underwent at least one very transient heating event to form the high-Mg domains. A possible mechanism for producing this transient event, as well as the one rough surface and cavity, is a single grain–grain collision in the ISM. These results indicate that grain QUE060 is the most pristine circumstellar corundum studied to date.

Silvia Protopapa^1,2,6, Michael S. P. Kelley^1,6, Bin Yang^3,6, James M. Bauer¹, Ludmilla Kolokolova¹, Charles E. Woodward^4,6, Jacqueline V. Keane⁵, and Jessica M. Sunshine¹
Astrophysical Journal Letters 862, L16 Link to Article [DOI: 10.3847/2041-8213/aad33b]
¹Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
²Southwest Research Institute, Boulder, CO 80302, USA
³European Southern Observatory, Santiago, Chile
⁴Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, MN 55455, USA
⁵Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁶Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration.

We present Infrared Telescope Facility/SpeX and NEOWISE observations of the dynamically new comet C/2013 US₁₀ (Catalina), hereafter US10, from 5.8 au inbound, to near perihelion at 1.3 au, and back to 5.0 au outbound. We detect water ice in the coma of US10, assess and monitor the physical properties of the ice as insolation varies with heliocentric distance, and investigate the relationship between water ice and CO₂. This set of measurements is unique in orbital coverage and can be used to infer both the physical evolution of the ice, and, potentially, the nucleus composition. We report (1) nearly identical near-infrared spectroscopic measurements of the coma at −5.8 au, −5.0 au, +3.9 au (where <0 au indicates pre-perihelion epochs), all presenting evidence of water-ice grains, (2) a dust-dominated coma at 1.3 and 2.3 au and, (3) an increasing CO₂/Afρ ratio from −4.9 to 1.8 au. We propose that sublimation of the hyper-volatile CO₂ is responsible for dragging water-ice grains into the coma throughout the orbit. Once in the coma, the observability of the water-ice grains is controlled by the ice grain sublimation lifetime, which seems to require some small dust contaminant (i.e., non-pure ice grains). At , the ice grains are long-lived and may be unchanged since leaving the comet nucleus. We find that the nucleus of comet US10 is made of, among other components, ~1 μm water-ice grains containing up to 1% refractory materials.