Heng Chen¹, Bach Mai Nguyen¹ and Frédéric Moynier^1,2,*

¹Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, Missouri, USA
²Institut de Physique du Globe de Paris, Université Paris Diderot, Paris, France

High-precision Zn isotopic compositions measured by MC-ICP-MS are documented for 32 iron meteorites from various fractionally crystallized and silicate-bearing groups. The δ⁶⁶Zn values range from −0.59‰ up to +5.61‰ with most samples being slightly enriched in the heavier isotopes compared with carbonaceous chondrites (0 < δ⁶⁶Zn < 0.5). The δ⁶⁶Zn versus δ⁶⁸Zn plot of all samples defines a common linear fractionation line, which supports the hypothesis that Zn was derived from a single reservoir or from multiple reservoirs linked by mass-dependent fractionation processes. Our data for Redfields fall on a mass fractionation line and therefore refute a previous claim of it having an anomalous isotopic composition due to nonmixing of nucleosynthetic products. The negative correlation between δ⁶⁶Zn and the Zn concentration of IAB and IIE is consistent with mass-dependent isotopic fractionation due to evaporation with preferential loss of lighter isotopes in the vapor phase. Data for the Zn concentrations and isotopic compositions of two IVA samples demonstrate that volatile depletion in the IVA parent body is not likely the result of evaporation. This is important evidence that favors the incomplete condensation origin for the volatile depletion of the IVA parent body.

Reference
Chen H, Nguyen BM and Moynier F (2013) Zinc isotopic composition of iron meteorites: Absence of isotopic anomalies and origin of the volatile element depletion. Meteoritics & Planetary Science 48:2441–2450.
[doi:10.1111/maps.12229]
Published by arrangement with John Wiley & Sons

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Michail I. Petaev^a,b, Shichun Huang^a, Stein B. Jacobsen^a and Alan Zindler^a

^aDepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138; and
^bSolar, Stellar, and Planetary Sciences, Harvard–Smithsonian Center for Astrophysics, Cambridge, MA 02138

Besides providing additional arguments against the Pt depositing event (1) as a cause of the Younger Dryas cooling, Boslough’s letter (2) raises an important question about the scale of this event. Indeed, a localized deposition of Pt by the Cape York meteorite shower is an attractive hypothesis considered by us initially (3), but abandoned because of (i) a large difference in the …

Reference
Petaev MI, Huang S, Jacobsen SB and Zindler A (2013) Reply to Boslough: Is Greenland Pt anomaly global or local?. PNAS 110:E5036.
[doi:10.1073/pnas.1320772111]

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Mark Boslough

Sandia National Laboratories, Albuquerque, NM 87185

Petaev et al. (1) tested the suite of hypotheses (collectively known as the “impact hypothesis”) that a swarm of impacts or airbursts from comets, chondritic, or stony asteroids caused an abrupt climate change, continental-scale wildfires, mass extinctions, and collapse of the Clovis culture at or near the Younger Dryas Boundary (YDB). The authors identify a large Pt anomaly in the Greenland Ice Sheet Project 2 (GISP2) core and suggest that it hints at an extraterrestrial source. Because there is no corresponding Ir spike, Petaev et al. challenge the impact hypothesis by proposing a highly fractionated iron meteorite. The Pt anomaly predates ammonimum and nitrate peaks in the GISP2 core by decades, eliminating …

Reference
Boslough M (2013) Greenland Pt anomaly may point to noncataclysmic Cape York meteorite entry. PNAS 110:E5035.
[doi:10.1073/pnas.1320328111]

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T. Sabri¹, L. Gavilan², C. Jäger¹, J. L. Lemaire², G. Vidali^2,5, H. Mutschke³ and T. Henning⁴

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
²Observatoire de Paris/Université de Cergy-Pontoise, 5 mail Gay Lussac, F-95000 Cergy-Pontoise, France
³Laboratory Astrophysics Group of the Astrophysical Institute and University Observatory, Friedrich Schiller University Jena Schillergässchen 3, D-07743 Jena, Germany
⁴Max Planck Institute for Astronomy Königstuhl 17, D-69117 Heidelberg, Germany
⁵Physics Department, Syracuse University, Syracuse, NY 13244-1320, USA.

Amorphous, astrophysically relevant silicates were prepared by laser ablation of siliceous targets and subsequent quenching of the evaporated atoms and clusters in a helium/oxygen gas atmosphere. The described gas-phase condensation method can be used to synthesize homogeneous and astrophysically relevant silicates with different compositions ranging from nonstoichiometric magnesium iron silicates to pyroxene- and olivine-type stoichiometry. Analytical tools have been used to characterize the morphology, composition, and spectral properties of the condensates. The nanometer-sized silicate condensates represent a new family of cosmic dust analogs that can generally be used for laboratory studies of cosmic processes related to condensation, processing, and destruction of cosmic dust in different astrophysical environments. The well-characterized silicates comprising amorphous Mg₂SiO₄ and Fe₂SiO₄, as well as the corresponding crystalline silicates forsterite and fayalite, produced by thermal annealing of the amorphous condensates, have been used as real grain surfaces for H₂ formation experiments. A specifically developed ultra-high vacuum apparatus has been used for the investigation of molecule formation experiments. The results of these molecular formation experiments on differently structured Mg₂SiO₄ and Fe₂SiO₄ described in this paper will be the topic of the next paper of this series.

Reference
Sabri T, Gavilan L, Jäger C, Lemaire JL, Vidali G, Mutschke H and Henning T (2014) Interstellar Silicate Analogs for Grain-surface Reaction Experiments: Gas-phase Condensation and Characterization of the Silicate Dust Grains. The Astrophysical Journal Volume 780:180.
[doi:10.1088/0004-637X/780/2/180]

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Myriam Lemelin^1,2,*, Caroline-Emmanuelle Morisset³, Mickaël Germain¹, Victoria Hipkin³, Kalifa Goïta¹ and Paul G. Lucey⁴

¹Département de Géomatique Appliquée, Université de Sherbrooke, Sherbrooke, Québec, Canada
²Now at Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawai‘i at Manoa, Honolulu, Hawaii, USA
³Department of Space Exploration, Canadian Space Agency, Saint-Hubert, Québec, Canada
⁴Hawai‘i Institute of Geophysics and Planetology, Honolulu, Hawaii, USA

We model lunar ilmenite abundances over Mare Australe and Mare Ingenii regions using a new approach; we integrate Lunar Reconnaissance Orbiter Wide Angle Camera (WAC) and Clementine UV-visible/near-infrared (UVVIS/NIR) data to obtain a 14-band mosaic (320-2000 nm). We use Hapke’s radiative transfer equations to compute spectra for various mixtures of orthopyroxene, clinopyroxene, plagioclase, olivine, and ilmenite, with varying grain size, chemistry, and degree of maturity, and find the closest match between the modeled spectra and the spectra of the less mature pixels (optical maturity ≥ 0.2) in the 14-band mosaic. We calculate a “maximum stoichiometrically possible ilmenite content”, using Clementine-derived TiO₂ abundances and the amount of TiO₂ in stoichiometric ilmenite, and use it as a constraint in our model. We validate our methodology with lunar soil spectra of known composition. Our results show that the integrated WAC-UVVIS/NIR data and the UVVIS/NIR data overestimate ilmenite abundances by 8.80 wt % and 7.97 wt %, respectively, when a fixed maximum of 20 wt % ilmenite is used. When the maximum stoichiometrically possible ilmenite content is used as a constraint, the integrated WAC-UVVIS/NIR data give slightly more accurate ilmenite abundance estimation (±2.87 wt %) than when using only UVVIS/NIR data (± 3.04 wt %). We find ilmenite concentrations of 0−11 wt % in Mare Australe and 0−6 wt % in Mare Ingenii region. Ilmenite abundances between 4 and 7 wt % are exposed in Mare Australe, whereas ilmenite abundances between 7 and 11 wt % are found on the walls of 0.6-11.8 km diameter craters within Mare Australe.

Reference
Lemelin M, Morisset C-E, Germain M, Hipkin V, Goïta K and Lucey PG (in press) Ilmenite mapping of the lunar regolith over Mare Australe and Mare Ingenii regions: An optimized multisource approach based on Hapke radiative transfer theory. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004392]
Published by arrangement with John Wiley & Sons

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Adam J. McKay^a, Nancy J. Chanover^a, Michael A. DiSanti^b, Jeffrey P. Morgenthaler^c, Anita L. Cochran^d, Walter M. Harris^e and Neil Dello Russo^f

^aAstronomy Department, New Mexico State University, 1320 Frenger Mall, Las Cruces, NM 88001 (U.S.A.)
^bGoddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771 (U.S.A.)
^cPlanetary Science Institute, 1700 E. Fort Lowell, Ste 106, Tucson, AZ, 85719 (U.S.A)
^dUniverisity of Texas Austin/McDonald Observatory, 1 University Station, Austin, TX 78712, (U.S.A)
^eDepartment of Applied Science, University of California Davis, 1 Shields Ave., Davis, CA, 95616 (U.S.A)
^fJohns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD, 20723 (U.S.A.)

We present analysis of high spectral resolution optical spectra of comet 103P/Hartley taken during its Fall 2010 apparition. These spectra include transitions belonging to CN, C₂, CH, NH₂, and OI. We measure production rates and mixing ratios from these spectra. We find evidence for large changes in production rates (factors of a few) over the course of a nucleus rotation, in agreement with other measurements. We also measure variability with rotational phase in the CN/H₂O and C₂/CN ratios, which has not been previously reported for any comet. There may also be variability in the NH₂/H₂O ratio with rotational phase, but this trend is not as clear as for CN/H₂O. We interpret the changing mixing ratios as due to H₂O and C₂ being released primarily from the icy grain halo, while the CN parent molecule comes directly from the nucleus. There is evidence that the CH/CN ratio is higher pre-perihelion than post-perihelion. We conclude that the observed CN and NH₂ abundances are consistent with HCN and NH₃ being the dominant parent molecules for these species. The C₂ and CH abundances are higher than those of candidate parent molecules (C₂H₂ and CH₄respectively), so there must be another source for these molecules in 103P’s coma. Carbonaceous dust grains could serve as this source.

Reference
McKay AJ, Chanover NJ, DiSanti MA, Morgenthaler JP, Cochran AL, Harris WM and Russo ND (2013) Rotational Variation of Daughter Species Production Rates in Comet 103P/Hartley: Implications for the Progeny of Daughter Species and the Degree of Chemical Heterogeneity. Icarus 48:2441–2450.
[doi:10.1111/maps.12229]
Copyright Elsevier

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