¹Takaya Nozawa, ²Shigeru Wakita, ^1,4Yasuhiro Hasegawa, ³Takashi Kozasa
¹Division of Theoretical Astronomy, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
²Center for Computational Astrophysics, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
³Department of Cosmosciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

A few particles of presolar Al2O3 grains with sizes above 0.5 μm are believed to have been produced in the ejecta of core-collapse supernovae (SNe). In order to clarify the formation condition of such large Al2O3 grains, we investigate the condensation of Al2O3 grains for wide ranges of the gas density and cooling rate. We first show that the average radius and condensation efficiency of newly formed Al2O3 grains are successfully described by a non-dimensional quantity ${{\rm{\Lambda }}}_{{\rm{on}}}$, defined as the ratio of the timescale with which the supersaturation ratio increases to the collision timescale of reactant gas species at dust formation. Then we find that the formation of submicron-sized Al2O3 grains requires at least 10 times higher gas densities than those presented by one-dimensional SN models. This indicates that presolar Al2O3 grains identified as having their origin in SNe might be formed in dense gas clumps, allowing us to propose that the measured sizes of presolar grains can be a powerful tool for constraining the physical conditions in which they formed. We also briefly discuss the survival of newly formed Al2O3 grains against destruction in the shocked gas within SN remnants.

Reference
Nozawa T, Wakita S, Hasegawa Y, Kozasa T (2015) PROBING THE PHYSICAL CONDITIONS OF SUPERNOVA EJECTA WITH THE MEASURED SIZES OF PRESOLAR Al2O3 GRAINS. Astrophysical Journal Letters 811, L39
Link to Article [http://dx.doi.org/10.1088/2041-8205/811/2/L39]

¹Philip. J. Carter, ¹Zoë. M. Leinhardt, ²Tim Elliott, ²Michael J. Walter, ³Sarah T. Stewart
¹School of Physics, University of Bristol, H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK
²School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK
³Department of Earth and Planetary Sciences, University of California, One Shields Avenue, Davis, CA 95616, USA

The Earth appears non-chondritic in its abundances of refractory lithophile elements, posing a significant problem for our understanding of its formation and evolution. It has been suggested that this non-chondritic composition may be explained by collisional erosion of differentiated planetesimals of originally chondritic composition. In this work, we present N-body simulations of terrestrial planet formation that track the growth of planetary embryos from planetesimals. We simulate evolution through the runaway and oligarchic growth phases under the Grand Tack model and in the absence of giant planets. These simulations include a state-of-the-art collision model that allows multiple collision outcomes, such as accretion, erosion, and bouncing events, and enables tracking of the evolving core mass fraction of accreting planetesimals. We show that the embryos grown during this intermediate stage of planet formation exhibit a range of core mass fractions, and that with significant dynamical excitation, enough mantle can be stripped from growing embryos to account for the Earth’s non-chondritic Fe/Mg ratio. We also find that there is a large diversity in the composition of remnant planetesimals, with both iron-rich and silicate-rich fragments produced via collisions.

Reference
Carter PJ, Leinhardt ZM, Elliott T, Walter MJ, Stewart ST (2015) COMPOSITIONAL EVOLUTION DURING ROCKY PROTOPLANET ACCRETION. Astrophysical Journal 813, 72
Link to Article [http://dx.doi.org/10.1088/0004-637X/813/1/72]

^1,2Vishnu Reddy et al. (>10)*
¹Planetary Science Institute, 1700 East Fort Lowell Road, Tucson, AZ 85719, USA
²Visiting Astronomer at the Infrared Telescope Facility
*Find the extensive, full author and affiliation list on the publishers website

The physical characterization of potentially hazardous asteroids (PHAs) is important for impact hazard assessment and evaluating mitigation options. Close flybys of PHAs provide an opportunity to study their surface photometric and spectral properties that enable the identification of their source regions in the main asteroid belt. We observed PHA (357439) 2004 BL86 during a close flyby of the Earth at a distance of 1.2 million km (0.0080 AU) on 2015 January 26, with an array of ground-based telescopes to constrain its photometric and spectral properties. Lightcurve observations showed that the asteroid was a binary and subsequent radar observations confirmed the binary nature and gave a primary diameter of 300 m and a secondary diameter of 50–100 m. Our photometric observations were used to derive the phase curve of 2004 BL86 in the V-band. Two different photometric functions were fitted to this phase curve, the IAU H–G model and the Shevchenko model. From the fit of the H–G function we obtained an absolute magnitude of H = 19.51 ± 0.02 and a slope parameter of G = 0.34 ± 0.02. The Shevchenko function yielded an absolute magnitude of H = 19.03 ± 0.07 and a phase coefficient b = 0.0225 ± 0.0006. The phase coefficient was used to calculate the geometric albedo (Ag) using the relationship found by Belskaya & Schevchenko, obtaining a value of Ag = 40% ± 8% in the V-band. With the geometric albedo and the absolute magnitudes derived from the H–G and the Shevchenko functions we calculated the diameter (D) of 2004 BL86, obtaining D = 263 ± 26 and D = 328 ± 35 m, respectively. 2004 BL86 spectral band parameters and pyroxene chemistry are consistent with non-cumulate eucrite meteorites. A majority of these meteorites are derived from Vesta and are analogous with surface lava flows on a differentiated parent body. A non-diagnostic spectral curve match using the Modeling for Asteroids tool yielded a best-match with non-cumulate eucrite Bereba. Three other near-Earth asteroids (1993 VW, 1998 KK17, and 2000 XH44) that were observed by Burbine et al. also have spectral properties similar to 2004 BL86. The presence of eucrites with anomalous oxygen isotope ratios compared to the howardites, eucrites, and diogenites meteorites from Vesta suggests the possible presence of multiple differentiated bodies in the inner main belt or the contamination of Vesta’s surface with exogenic material. The spectral properties of both anomalous and Vestan eucrites are degenerate, making it difficult to identify the parent bodies of anomalous eucrites in the main belt and the NEO population using remote sensing. This makes it difficult to link 2004 BL86 directly to Vesta, although the Vesta family is the largest contributor of V-types to near-Earth space.

Reference
Reddy V et al. (2015) THE PHYSICAL CHARACTERIZATION OF THE POTENTIALLY HAZARDOUS ASTEROID 2004 BL86: A FRAGMENT OF A DIFFERENTIATED ASTEROID. Astrophysical Journal (in Press)
Link to Article [http://dx.doi.org/10.1088/0004-637X/811/1/65]

¹Pia Friend,^1,2Dominik C. Hezel, ¹Daniel Mucerschi
¹University of Cologne, Department of Geology and Mineralogy, Zülpicher Str. 49b, 50674 Köln, Germany
²Natural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD, London, UK

We studied the texture of 256 chondrules in thin sections of 16 different carbonaceous (CV, CR, CO, CM, CH) and Rumuruti chondrites. In a conservative count ∼75% of all chondrules are mineralogically zoned, i.e. these chondrules have an olivine core, surrounded by a low-Ca pyroxene rim. A realistic estimate pushes the fraction of zoned chondrules to >90% of all chondrules. Mineralogically zoned chondrules are the dominant and typical chondrule type in carbonaceous and Rumuruti chondrites. The formation of the mineralogical zonation represents a fundamentally important process of chondrule formation. The classic typification of chondrules into PO, POP and PP might in fact represent different sections through mineralogically zoned chondrules. On average, the low-Ca pyroxene rims occupy 30 vol.% of the entire chondrule. The low-Ca pyroxene most probably formed by reaction of an olivine rich chondrule with SiO from the surrounding gas. This reaction adds 3-15 wt.% of material, mainly SiO2, to the chondrule. Chondrules were open systems and interacted substantially with the surrounding gas. This is in agreement with many previous studies on chondrule formation. This open system behaviour and the exchange of material with the surrounding gas can explain bulk chondrule compositional variations in a single meteorite and supports the findings from complementarity that chondrules and matrix formed from the same chemical reservoir.

Reference
Friend P, Hezel DC, Mucerschi D (2015) The conditions of chondrule formation, Part II: Open System. Geochimica et Cosmochimica (in Press)
Link to Article [doi:10.1016/j.gca.2015.10.026]
Copyright Elsevier

¹A. K. Speck, ^2,3K. M. Pitman, ⁴A. M. Hofmeister
¹Department of Physics and Astronomy, University of Missouri-Columbia, Columbia, MO 65211, USA
²Planetary Science Institute, Tucson, AZ 85719, USA
³Space Science Institute, Boulder, CO 80301, USA
⁴Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA

“Astronomical” or “circumstellar” silicate optical functions (real and imaginary indices of refraction $n(\lambda )$ and $k(\lambda )$) have previously been derived from compositionally and structurally disparate samples; past values were compiled from different sources in the literature, and are essentially kluges of observational, laboratory, and extrapolated or interpolated values. These synthetic optical functions were created because astronomers lack the quantitative data on amorphous silicates at all wavelengths needed for radiative transfer modeling. This paper provides optical functions that (1) are created with a consistent methodology, (2) use the same sample across all wavelengths, and (3) minimize interpolation and extrapolation wherever possible. We present electronic data tables of optical functions derived from mid-ultraviolet to far-infrared (FIR) laboratory transmission spectra for two materials: iron-free glass with chondritic/solar atmospheric abundances, and metallic iron. We compare these optical functions to other popular n, k data used to model amorphous silicates (e.g., “astronomical” or “circumstellar” silicate), both directly and in application to a simple system: the dust shell of the post-AGB star HD 161796. Using the new optical functions, we find that the FIR profile of model spectral energy distributions are significantly affected by the ratio of glass to iron. Our case study on HD 161796 shows that in modeling with our new optical functions, the mineralogy is markedly different from that derived using synthetic optical functions and suggests a new scenario of crystalline silicate formation.

Reference
Speck AK, Pitman KM, Hofmeister AM (2015) Better Alternatives to “Astronomical Silicate”: Laboratory-Based Optical Functions of Chondritic/Solar Abundance Glass with Application to HD161796. The Astrophysical Journal 809
Link to Article [http://dx.doi.org/10.1088/0004-637X/809/1/65]

¹T.E. Johnson, ^1,2G.K. Benedix, ¹P.A. Bland
¹Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA 6845, Australia
²Department of Earth and Planetary Sciences, Western Australia Museum, 49 Kew Street, Welshpool, WA 6986, Australia

Constraining the metamorphic pressures (P) and temperatures (T) recorded by meteorites is key to understanding the size and thermal history of their asteroid parent bodies. New thermodynamic models calibrated to very low P for minerals and melt in terrestrial mantle peridotite permit quantitative investigation of high-T metamorphism in ordinary chondrites using phase equilibria modelling. Isochemical P–T phase diagrams based on the average composition of H, L and LL chondrite falls and contoured for the composition and abundance of olivine, ortho- and clinopyroxene, plagioclase and chromite provide a good match with values measured in so-called equilibrated (petrologic type 4–6) samples. Some compositional variables, in particular Al in orthopyroxene and Na in clinopyroxene, exhibit a strong pressure dependence when considered over a range of several kilobars, providing a means of recognising meteorites derived from the cores of asteroids with radii of several hundred kilometres, if such bodies existed at that time. At the low pressures (<1 kbar) that typify thermal metamorphism, several compositional variables are good thermometers. Although those based on Fe–Mg exchange are likely to have been reset during slow cooling, those based on coupled substitution, in particular Ca and Al in orthopyroxene and Na in clinopyroxene, are less susceptible to retrograde diffusion and are potentially more faithful recorders of peak conditions. The intersection of isopleths of these variables may allow pressures to be quantified, even at low P, permitting constraints on the minimum size of parent asteroid bodies. The phase diagrams predict the onset of partial melting at 1050–1100 °C by incongruent reactions consuming plagioclase, clinopyroxene and orthopyroxene, whose compositions change abruptly as melting proceeds. These predictions match natural observations well and support the view that type 7 chondrites represent a suprasolidus continuation of the established petrologic types at the extremes of thermal metamorphism. The results suggest phase equilibria modelling has potential as a powerful quantitative tool in investigating, for example, progressive oxidation during metamorphism, the degree of melting and melt loss or accumulation required to produce the spectrum of differentiated meteorites, and whether the onion shell or rubble pile model best explains the metamorphic evolution of asteroid parent bodies in the early solar System.

Reference
Johnson TE, Benedix GK, Bland PA (2015) Metamorphism and partial melting of ordinary chondrites: Calculated phase equilibria. Earth and Planetary Science Letters 433, 21–30
Link to Article [doi:10.1016/j.epsl.2015.10.035]
Copyright Elsevier

¹Audrey Bouvier, ²Janne Blichert-Toft, ³Maud Boyet,²Francis Albarède
¹Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, London, ON, Canada
²Laboratoire de Géologie de Lyon, CNRS UMR 5276, Ecole Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, Lyon, France
³Laboratoire Magmas et Volcans, CNRS UMR 6524, Université Blaise Pascal, Clermont-Ferrand, France

Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present 147Sm-143Nd and 176Lu-176Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a 147Sm-143Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous 147Sm-143Nd and 176Lu-176Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.

Reference
Bouvier A, Blichert-Toft J, Boyet M, Albarède F (2015) 147Sm-143Nd and 176Lu-176Hf systematics of eucrite and angrite meteorites. Meteoritics & Planetary Science (in Press).
Link to Article [DOI: 10.1111/maps.12553]
Published by arrangement with John Wiley & Sons

¹Piero D’Incecco et al. (>10)*
¹Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, D-12489 Berlin, Germany
*Find the extensive, full author and affiliation list on the publishers website

We have performed a combined geological and spectral analysis of two impact craters on Mercury: the 15 km diameter Waters crater (106 °W; 9 °S) and the 62.3 km diameter Kuiper crater (30 °W; 11 °S). Using the Mercury Dual Imaging System (MDIS) Narrow Angle Camera (NAC) dataset we defined and mapped several units for each crater and for an external reference area far from any impact related deposits. For each of these units we extracted all spectra from the MESSENGER Atmosphere and Surface Composition Spectrometer (MASCS) Visible-InfraRed Spectrograph (VIRS) applying a first order photometric correction. For all the mapped units, we analyzed the spectral slope in two wavelength ranges, 350–450 nm and 450–650 nm, and the absolute reflectance in the 700-750 nm range. Normalized spectra of Waters crater display a generally bluer spectral slope than the external reference area over both wavelength windows. Normalized spectra of Kuiper crater generally display a redder slope than the external reference area in the 350–450 nm window, while they display a bluer slope than the external reference area in the 450–650 nm wavelength range. The combined use of geological and spectral analyses enables reconstruction of the local scale stratigraphy beneath the two craters, providing insight into the properties of the shallower crust of Mercury. Kuiper crater, being ~4 times larger than Waters crater, exposes deeper layers with distinctive composition, while the result for Waters crater might indicate substantial compositional homogeneity with the surrounding intercrater plains, though we can’t exclude the occurrence of horizontal compositional heterogeneities in the shallow sub-surface.

Reference
D’Incecco P. et al. (2015) Shallow crustal composition of Mercury as revealed by spectral properties and geological units of two impact craters. Icarus (in Press)
Link to Article [doi:10.1016/j.pss.2015.10.007]

Copyright Elsevier

The most popular papers on Cosmochemistry Papers in October were:

1.Renne PR, Sprain CJ, Richards MA, Self S, Vanderkluysen L, Pande K (2015) State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact. Science 6256:76-78. Link to Article [doi:10.1126/science.aac7549]

2.Defouilloy C, Cartigny P, Assayag N, Moynier F, Barrat J-A (2015) High-precision sulfur isotope composition of enstatite meteorites and implications of the formation and evolution of their parent bodies. Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2015.10.009]

3.Becker M, Hezel DC, Schulz T, Elfers B-M, Münker C (2015) Formation timescales of CV chondrites from component specific Hf–W systematics. Earth and Planetary Science Letters (in Press) Link to Article [doi:10.1016/j.epsl.2015.09.049]

4.Laurent B, Roskosz M, Remusat L, Robert F, Leroux H, Vezin H, Depecker C, Nuns N, Lefebvre J-M (2015) The deuterium/hydrogen distribution in chondritic organic matter attests to early ionizing Irradiation. Nature Communications 6, 8567 Link to Article [doi:10.1038/ncomms9567]

5. Goderis S, Brandon AD, Mayer B, Humayun M (2015) s-Process Os isotope enrichment in ureilites by planetary processing. Earth and Planetary Science Letters 431, 110–118 Link to Article [doi:10.1016/j.epsl.2015.09.021]

¹de Vet, S. J.
¹Earth Surface Science, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, the Netherlands

Two meteorites impacted in 1925 around the town of Serooskerke on the isle of Schouwen, the Netherlands. The largest mass is widely known as the “Ellemeet” diogenite, while a second mass, heavily weathered due to environmental exposure, also survived until the present day. This work aims to reconstruct the history of the 1925 fall and for the first time documents the second mass, known as the “Serooskerke,” by integrating a historical and experimental approach. The study of historical news archives and cadastral records redefined the 1925 impact site at N 51°42.086′ E 3°49.789′. Environmental exposure experiments reproducing the effects of rainfall and frost weathering identified the latter as the main cause for the second mass’ reported disintegration in the field sometime during the 1925–1926 winter. The bulk mineralogy of the second mass was established using XRD powder diffraction for a 2θ range of 3–70° and was found to be identical to an Ellemeet reference sample. UV/VIS/nIR spectroscopy (300–2500 nm) was subsequently used to broadly compare the second mass to HED clan meteorites Bouvante, EET87503, Johnstown and asteroid 4 Vesta in order to corroborate its vestan origin. The historical and geographic relationship of the two masses and the comparable bulk mineralogy supported the pairing of these two meteorites. This makes the Serooskerke a valuable legacy of the 1925 fall, especially as the location of ~50% of the remaining Ellemeet mass is presently unknown.

Reference
de Vet SJ (2015) The 1925 meteorite fall near Ellemeet and Serooskerke, the Netherlands. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12554]
Published by arrangement with John Wiles & Sons