Mariko Kato¹, Hideyuki Saio², and Izumi Hachisu³
The Astrophysical Journal 863, 125 Link to Article [https://doi.org/10.3847/1538-4357/aad327]
¹Department of Astronomy, Keio University, Hiyoshi, Yokohama 223-8521, Japan
²Astronomical Institute, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan
³Department of Earth Science and Astronomy, College of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

Type Ia supernovae (SNe Ia) often show high-velocity absorption features (HVFs) in their early phase spectra; however, the origin of the HVFs is unknown. We show that a near-Chandrasekhar-mass white dwarf (WD) develops a silicon-rich layer on a carbon–oxygen (CO) core before it explodes as an SN Ia. We calculated the nuclear yields in successive helium shell flashes for 1.0 M _☉, 1.2 M _☉, and 1.35 M _☉ CO WDs accreting helium-rich matter with several mass-accretion rates, ranging from 1 × 10⁻⁷ M _☉ yr⁻¹ to 7.5 × 10⁻⁷ M _☉ yr⁻¹. For the 1.35 M _☉ WD with the accretion rate of 1.6 × 10⁻⁷ M _☉ yr⁻¹, the surface layer developed as helium burning ash and consisted of 40% ²⁴Mg, 33% ¹²C, 23% ²⁸Si, and a few percent of ²⁰Ne by weight. For a higher mass-accretion rate of 7.5 × 10⁻⁷ M _☉ yr⁻¹, the surface layer consisted of 58% ¹²C, 31% ²⁴Mg, and 0.43% ²⁸Si. For the 1.2 M_☉ WDs, silicon is produced only for lower mass-accretion rates (2% for 1.6 × 10⁻⁷ M _☉ yr⁻¹). No substantial silicon (<0.07%) is produced on the 1.0 M _☉ WD independently of the mass-accretion rate. If the silicon-rich surface layer is the origin of Si ii HVFs, its characteristics are consistent with that of mass-increasing WDs. We also discuss possible Ca production on very massive WDs (1.38 M _☉).

^1,2P. Woitke, ^1,2,5Ch. Helling, ^1,2G. H. Hunter, ^1,2J. D. Millard, ^1,2G. E. Turner, ^1,2M. Worters, ³J. Blecic, ⁴J. W. Stock
Astronomy & Astrophysics 614, A1 Link to Article [https://doi.org/10.1051/0004-6361/201732193]
¹SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
²Centre for Exoplanet Science, University of St Andrews, St Andrews, UK
³New York University Abu Dhabi, Abu Dhabi, United Arab Emirates
⁴Department of Chemistry and Environmental Science, Medgar Evers College – City University of New York, 1650 Bedford Avenue, Brooklyn, NY 11235, USA
⁵Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

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^1,2David Morate, ^1,2Julia de León, ³Mário De Prá, ^1,2Javier Licandro, ^1,4Antonio Cabrera-Lavers, ⁵Humberto Campins, ⁶Noemí Pinilla-Alonso
Astronomy & Astrophysics 610, A25 Link to Article [https://doi.org/10.1051/0004-6361/201731407]
¹Instituto de Astrofísica de Canarias (IAC), C/vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: damog@iac.es
²Departamento de Astrofca, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
³Observatório Nacional, Coordenao de Astronomia e Astrofca, 20921-400 Rio de Janeiro, Brazil
⁴GTC Project Office, 38205 La Laguna, Tenerife, Spain
⁵Physics Department, University of Central Florida, PO Box 162385, Orlando, FL 32816-2385, USA
⁵Florida Space Institute, University of Central Florida, Orlando, FL 32816, USA

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¹E. Dartois, ²C. Engrand, ²J. Duprat, ²M. Godard, ²E. Charon, ²L. Delauche, ³C. Sandt, ³F. Borondics
Astronomy & Astrophysics 609, A64 Link to Article [https://doi.org/10.1051/0004-6361/201731322]
¹Institut d’Astrophysique Spatiale (IAS), CNRS, Univ. Paris Sud, Université Paris-Saclay, 91405 Orsay, France
e-mail: emmanuel.dartois@ias.u-psud.fr
²Centre de Sciences Nucléaires et de Sciences de la Matière (CSNSM), CNRS/IN2P3, Univ. Paris Sud, Université Paris-Saclay, 91405 Orsay, France
³Synchrotron SOLEIL, L’Orme des Merisiers, BP48 Saint Aubin, 91192 Gif-sur-Yvette Cedex, France

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¹A.P.Crósta,^2,3W.U.Reimold,⁴M.A.R.Vasconcelos, ²N.Hauser, ¹G.J.G.Oliveira, ¹M.V.Maziviero, ⁵A.M.Góes
Chemie der Erde (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2018.09.002]
¹State University of Campinas, Brazil
²University of Brasília, Brazil
³Natural History Museum—Leibniz Institute for Evolution and Biodiversity Research, Berlin, Germany
⁴Federal University of Bahia, Brazil
⁵University of São Paulo, Brazil
Copyright Elsevier

In the first part of this review of the impact record of South America, we have presented an up-to-date introduction to impact processes and to the criteria to identify/confirm an impact structure and related deposits, as well as a comprehensive examination of Brazilian impact structures. The current paper complements the previous one, by reviewing the impact record of other countries of South America and providing current information on a number of proposed impact structures. Here, we also review those structures that have already been discarded as not being formed by meteorite impact. In addition, current information on impact-related deposits is presented, focusing on impact glasses and tektites known from this continent, as well as on the rare K–Pg boundary occurrences revealed to date and on reports of possible large airbursts. We expect that this article will not only provide systematic and up-to-date information on the subject, but also encourage members of the South American geoscientific community to be aware of the importance of impact cratering and make use of the criteria and tools to identify impact structures and impact deposits, thus potentially contributing to expansion and improvement of the South American impact record.

¹Michael C. Bouchard,¹Bradley L. Jolliff
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005631]
¹Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri
Published by arrangement with John Wiley & Sons

The Mars rover Opportunity has collected in‐situ compositional data with the Alpha Particle X‐Ray Spectrometer at almost 500 sites. To analyze these data, hierarchical clustering analysis and an error‐weighted similarity index are applied to a subset of 57 APXS target compositions and selected martian meteorites. Hierarchical clustering provides a rapid first approximation of compositional relationships whereas the error‐weighted similarity index provides an in‐depth and quantifiable comparison of individual composition pairs. These analyses are combined into a statistical grouping model that provides insight into lithologic relationships and is critically informed by examination of Panoramic Camera and Microscopic Imager images. Major lithologies are (1) the Burns formation sulfate sandstones, (2) Shoemaker impact breccias (Endeavour crater ejecta/rim deposits), (3) the morphologically distinct Grasberg formation, associated with Endeavour crater rim deposits, (4) the Matijevic formation, an exposure interpreted to be Endeavour crater target rocks, and (5) erratics or other rocks that do not cluster with groups 1‐4. The Grasberg formation is more similar to the Shoemaker formation than any other formation, and thus likely incorporated eroded Shoemaker material. The lowest Shoemaker member (Copper Cliff breccia) may contain material from the pre‐ impact Matijevic formation. The Matijevic formation is the most chemically distinct formation and is most similar to the volcanic erratic rock “Marquette Island.” Clustering and similarity index values also show that regolith breccia martian meteorites (represented by the NWA 7475/7034 paired meteorites) are similar in bulk composition to Mars surface materials at Meridiani, especially the Matijevic formation.

¹C. B. Kiddell, ¹E. A. Cloutis, ¹B. R. Dagdick, ¹J. M. Stromberg, ¹D. M. Applin, ¹J. P. Mann
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005600]
¹Dept. of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
Published by arrangement with John Wiley & Sons

Carbonaceous chondrite meteorites (CCs) are among the most primitive materials in the solar system and provide important insights into solar system history and evolution. A number of planetary spacecraft missions will visit asteroids that are thought to compositionally resemble these meteorites. To better assist sample acquisition in terms of how the physical properties of CCs affect their reflectance spectra, we investigated the spectral reflectance properties of solid and powdered CCs, and powder coatings on slabs of a number of CCs, including CB, CH, CK, CM, CO, CR, and CV classes. We found that decreasing grain size leads to increasing reflectance across the ~500‐2500 nm range and steeper spectral slope, regardless of CC type. Powdered CC reflectance spectra are brighter beyond ~500 nm and redder than bare roughened slabs. For powders sprinkled on slabs, as the powder coating gets thicker, spectral slopes get redder.

Optically thick fine‐grained powders are brighter beyond ~500 nm and are as red or redder, than slabs covered with airfall dust (for dust thicknesses up to a few hundred microns). Diagnostic absorption features of CC minerals, particularly those in the 1000 nm region attributable to Fe‐bearing silicates, are ubiquitous regardless of physical properties. Reflectance spectra of terrestrially weathered (i.e., “rusty”) CCs are strongly modified below ~700 nm and in the 900 and 1900 nm regions by these Fe oxyhydroxides. Their effects can be mitigated through chemical treatment, but this may also affect pre‐terrestrial ferric iron‐bearing phases. Some spectral characteristics, such as hydrous and anhydrous silicate absorption bands in CC spectra, are present regardless of physical properties (fine‐grained dust, powders, slabs, dust on slabs). Other spectral characteristics (such as albedo and spectral slope) vary as a function of physical properties, indicates that reflectance spectroscopy could be used to ascribe spectral variations across an asteroid’s surface to either physical or compositional causes. This information can, in turn, be used to inform site selection for asteroid sample return missions, where both composition and physical properties are important drivers. When searching for fine‐grained areas on an asteroid to sample, the best indication would be the brightest and reddest‐sloped spectra.

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.