The Chemical Homogeneity of Sun-like Stars in the Solar Neighborhood

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.

Temperature Programmed Desorption of Water Ice from the Surface of Amorphous Carbon and Silicate Grains as Related to Planet-forming Disks

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.

GEMS, hydrated chondritic IDPs, CI‐matrix material: Sources of water in 81P/comet Wild 2

1Frans J. M. Rietmeijer
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13201]
1Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
Published by arrangement with John Wiley & Sons

So far there is no conclusive evidence for water in the nucleus of 81P/comet Wild 2. Recently magnetite in collected Wild 2 samples was cited as proxy evidence for parent body aqueous alteration in this comet (Hicks et al. 2017). A potentional source for water of hydration would be layer silicates but unfortunately there is no record, neither texturally nor chemically, for hydrated layer silicates that survived hypervelocity impact in the Wild 2 samples. This paper reports large vesicles in the matrix of allocation C2044,2,41,2,5 from a volatile‐rich type B/C Stardust track. These vesicles were probably caused by boiling water that were generated when hydrated Wild 2 silicates impacted the near‐surface silica aerogel layer. Potential water sources were partially and fully hydrated GEMS (glass with embedded metal and sulfides) and CI carbonaceous chondrite materials among the earliest dusts that experienced hydration and icy‐body formation and long‐range transport and mixing with materials from across the solar system.

A hydrohalite spring deposit in the Canadian high Arctic: A potential Mars analogue

1Melissa K.Ward, 1Wayne H.Pollard
Earth and Planetray Science Letters 504, 126-138 Link to Article [https://doi.org/10.1016/j.epsl.2018.10.001]
1Department of Geography, McGill University, Montreal, Canada
Copyright Elsevier

On Axel Heiberg Island in the Canadian High Arctic, low temperature perennial saline springs occur despite thick permafrost and cold polar desert conditions marked by a mean annual air temperature close to −20 °C. We present the first comprehensive geomorphic study of the Stolz Diapir Spring (79°04′30″N; 87°04′30″W), a unique groundwater system due to its known fresh water source and sodium chloride-dominated chemistry. During winter, spring discharge precipitates hydrohalite (NaCl⋅2H2O) by freezing fractionation that forms a pool and barrage system morphologically similar to carbonate travertines and tufas found in temperate climates. The deposit is the largest hydrohalite accumulation on Earth based on published sources. This system experiences dramatic seasonal differences in hydrology and mineralogy marked by a switch from winter regime of salt deposition and cascading surface flow from pool to pool to a summer regime marked by chemical and mechanical erosion and deposit subsurface flow. The warmer temperatures also cause the decomposition of hydrohalite to halite. Accordingly, this site is a useful analogue for similar structures identified on Mars located in areas rich in evaporite minerals and lacking evidence of volcanic activity.

Compound‐specific carbon isotope compositions of aldehydes and ketones in the Murchison meteorite

1,2,3Danielle N. Simkus, 2,4José C. Aponte, 5Robert W. Hilts, 2Jamie E. Elsila, 1Christopher D. K. Herd 
Meteoritics & Planetary Science (in Press) Link to Article [https://onlinelibrary.wiley.com/doi/10.1111/maps.13202]
1Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2R3, Canada
2Solar System Exploration Division, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
3NASA Postdoctoral Program at NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
4Department of Chemistry, Catholic University of America, Washington, DC, USA
5Department of Physical Sciences, MacEwan University, Edmonton, Alberta T6G 2R3, Canada
Published by arrangement with John Wiley & Sons

Compound‐specific carbon isotope analysis (δ13C) of meteoritic organic compounds can be used to elucidate the abiotic chemical reactions involved in their synthesis. The soluble organic content of the Murchison carbonaceous chondrite has been extensively investigated over the years, with a focus on the origins of amino acids and the potential role of Strecker‐cyanohydrin synthesis in the early solar system. Previous δ13C investigations have targeted α‐amino acid and α‐hydroxy acid Strecker products and reactant HCN; however, δ13C values for meteoritic aldehydes and ketones (Strecker precursors) have not yet been reported. As such, the distribution of aldehydes and ketones in the cosmos and their role in prebiotic reactions have not been fully investigated. Here, we have applied an optimized O‐(2,3,4,5,6‐pentafluorobenzyl)hydroxylamine (PFBHA) derivatization procedure to the extraction, identification, and δ13C analysis of carbonyl compounds in the Murchison meteorite. A suite of aldehydes and ketones, dominated by acetaldehyde, propionaldehyde, and acetone, were detected in the sample. δ13C values, ranging from −10.0‰ to +66.4‰, were more 13C‐depleted than would be expected for aldehydes and ketones derived from the interstellar medium, based on interstellar 12C/13C ratios. These relatively 13C‐depleted values suggest that chemical processes taking place in asteroid parent bodies (e.g., oxidation of the IOM) may provide a secondary source of aldehydes and ketones in the solar system. Comparisons between δ13C compositions of meteoritic aldehydes and ketones and other organic compound classes were used to evaluate potential structural relationships and associated reactions, including Strecker synthesis and alteration‐driven chemical pathways.

Campo del Cielo: A Campo by any other name

1John T. Wasson
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13205]
1Institute of Geophysics, University of California, Los Angeles, California, USA
Published by arrangement with John Wiley & Sons

A sample of Campo del Cielo with any other name would have the same composition. During the last three decades, our instrumental neutron activation analyses (INAA) of many supposedly new iron meteorites have shown an anomalously large fraction to have compositions within the compositional field of the IAB‐MG iron Campo del Cielo. A plot of Ir versus Au provides the best discrimination; only two independent‐fall irons found after 1980 with good recovery documentation fall within the 90% contour ellipse around the centroid of this Campo field, and one of these is from Antarctica. Now (early 2018) a total of 36 other irons attributed to other geographical locations have compositions that cannot be resolved from the Campo compositional field. Because it is possible that some of these are actually independent falls, the Meteoritical Society Nomenclature Committee has chosen to assign about half these meteorites Nova XXX names used for meteorites whose discovery localities are not adequately documented. However, for Campo‐like irons with too little information (e.g., total weight not known) or for which no adequately large type specimens are available, the decision is to call them Campos with the working name used during the UCLA analysis. In the UCLA Meteorite Collection, they are cataloged together with the documented Campos.

Rare, metal micrometeorites from the Indian Ocean

1M. Shyam Prasad, 1N. G. Rudraswami, 1Agnelo Alexandre De Araujo, 1V. D. Khedekar
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13206]
1Geological Oceanography Division, CSIR–National Institute of Oceanography, Dona Paula, Goa, India
Published by arrangement with John Wiley & Sons

Metal in various forms is common in almost all meteorites but considerably rare among micrometeorites. We report here the discovery of two metal micrometeorites, i.e., (1) an awaruite grain similar to those found in the metal nodules of CV chondrites and (2) a metal micrometeorite of kamacite composition enclosing inclusions of chromite and merrillite. This micrometeorite appears to be a fragment of H5/L5 chondrite. These metal micrometeorites add to the inventory of solar system materials that are accreted by the Earth in microscopic form. They also strengthen the argument that a large proportion of material accreted by the Earth that survives atmospheric entry is from asteroidal sources.