Alexandra I. Blinova¹, Thomas J. Zega^2,†, Christopher D. K. Herd¹ and Rhonda M. Stroud²

¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
²Materials Science and Technology Division, Naval Research Laboratory, Washington, District of Columbia, USA
^†Department of of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

Four samples (TL5b, TL11h, TL11i, and TL11v) from the pristine collection of the Tagish Lake meteorite, an ungrouped C2 chondrite, were studied to characterize and understand its alteration history using EPMA, XRD, and TEM. We determined that samples TL11h and TL11i have a relatively smaller proportion of amorphous silicate material than sample TL5b, which experienced low-temperature hydrous parent-body alteration conditions to preserve this indigenous material. The data suggest that lithic fragments of TL11i experienced higher degrees of aqueous alteration than the rest of the matrix, based on its low porosity and high abundance of coarse- and fine-grained sheet silicates, suggesting that TL11i was present in an area of the parent body where alteration and brecciation were more extensive. We identified a coronal, “flower”-like, microstructure consisting of a fine-grained serpentine core and coarse-grained saponite-serpentine radial arrays, suggesting varied fluid chemistry and crystallization time scales. We also observed pentlandite with different morphologies: an exsolved morphology formed under nebular conditions; a nonexsolved pentlandite along grain boundaries; a “bulls-eye” sulfide morphology and rims around highly altered chondrules that probably formed by multiple precipitation episodes during low-temperature aqueous alteration (≥100 °C) on the parent body. On the basis of petrologic and mineralogic observations, we conclude that the Tagish Lake parent body initially contained a heterogeneous mixture of anhydrous precursor minerals of nebular and presolar origin. These materials were subjected to secondary, nonpervasive parent-body alteration, and the samples studied herein represent different stages of that hydrous alteration, i.e., TL5b (the least altered) < TL11h < TL11i (the most altered). Sample TL11v encompasses the petrologic characteristics of the other three specimens.

Reference
Blinova, A. I., Zega, T. J., Herd, C. D. K. and Stroud, R. M. (in press) Testing variations within the Tagish Lake meteorite—I: Mineralogy and petrology of pristine samples. Meteoritics & Planetary Science
[doi:10.1111/maps.12271]
Published by arrangement with John Wiley & Sons

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Matthias M.M. Meier^a,b and Rainer Wieler^b

^aLund University, Department of Geology, Sölvegatan 12, SE-22362 Lund, Sweden
^bETH Zürich, Department of Earth Sciences, NW-C84, Clausiusstrasse 25, CH-8092 Zürich, Switzerland

It has been argued that the decay rates of several radioactive nuclides are slightly lower at Earth’s aphelion than at perihelion, and that this effect might depend on heliocentric distance. It might then be expected that nuclear decay rates be considerably lower at larger distances from the sun, e.g., in the asteroid belt at 2–3 AU from where most meteorites originate. If so, ages of meteorites obtained by analyses of radioactive nuclides and their stable daughter isotopes might be in error, since these ages are based on decay rates determined on Earth. Here we evaluate whether the large data base on nuclear cosmochronology offers any hint for discrepancies which might be due to radially variable decay rates. Chlorine-36 (t_1/2 = 301,000 a) is produced in meteorites by interactions with cosmic rays and is the nuclide for which a decay rate dependence from heliocentric distance has been proposed, which, in principle, can be tested with our approach and the current data base. We show that compilations of ³⁶Cl concentrations measured in meteorites offer no support for a spatially variable ³⁶Cl decay rate. For very short-lived cosmic-ray produced radionuclides (half-lives < 10–100 days), the concentration should be different for meteorites hitting the Earth on the incoming vs. outgoing part of their orbit. However, the current data base of very short-lived radionuclides in freshly fallen meteorites is far from sufficient to deduce solid constraints. Constraints on the age of the Earth and the oldest meteorite phases obtained by the U–Pb dating technique give no hints for radially variable decay rates of the α-decaying nuclides ²³⁵U or ²³⁸U. Similarly, some of the oldest phases in meteorites have U–Pb ages whose differences agree almost perfectly with respective age differences obtained with “short-lived” radionuclides present in the early solar system, again indicating no variability of uranium decay rates in different meteorite parent bodies in the asteroid belt. Moreover, the oldest U–Pb ages of meteorites agree with the main-sequence age of the sun derived from helioseismology within the formal ∼1% uncertainty of the latter. Meteorite ages also provide no evidence for a decrease of decay rates with heliocentric distance for nuclides such as ⁸⁷Rb (decay mode β⁻) ⁴⁰K (β⁻ and electron capture), and ¹⁴⁷Sm (α).

Reference
Meier MMM and Wieler R (in press) No evidence for a decrease of nuclear decay rates with increasing heliocentric distance based on radiochronology of meteorites. Astroparticle Physics
[doi:10.1016/j.astropartphys.2014.01.004]
Copyright Elsevier

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Rebecca G. Martin^1,3 and Mario Livio²

¹JILA, University of Colorado & NIST, UCB 440, Boulder, CO 80309, USA
²Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³Sagan Fellow.

CO is thought to be a vital building block for prebiotic molecules that are necessary for life. Thus, understanding where CO existed in a solid phase within the solar nebula is important for understanding the origin of life. We model the evolution of the CO snow line in a protoplanetary disk. We find that the current observed location of the CO snow line in our solar system, and in the solar system analog TW Hydra, cannot be explained by a fully turbulent disk model. With time-dependent disk models we find that the inclusion of a dead zone (a region of low turbulence) can resolve this problem. Furthermore, we obtain a fully analytic solution for the CO snow line radius for late disk evolutionary times. This will be useful for future observational attempts to characterize the demographics and predict the composition and habitability of exoplanets.

Reference
Martin RG and Livio M (2014) On the Evolution of the CO Snow Line in Protoplanetary Disks. The Astrophysical Journal – Letters 783:L28.
[doi:10.1088/2041-8205/783/2/L28]

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R. Le Gal¹, P. Hily-Blant^1,2, A. Faure¹, G. Pineau des Forêts^3,4, C. Rist¹ and S. Maret¹

¹Université Joseph Fourier/CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
²Institut Universitaire de France, France 
³Université de Paris-Sud/CNRS, IAS (UMR 8617), 91405 Orsay Cedex, France
⁴LERMA/CNRS (UMR 8112)/Observatoire de Paris, 75014 Paris, France

Nitrogen, amongst the most abundant metals in the interstellar medium, has a peculiar chemistry that differs from those of carbon and oxygen. Recent observations of several nitrogen-bearing species in the interstellar medium suggest abundances in sharp disagreement with current chemical models. Although some of these observations show that some gas-grain processes are at work, gas-phase chemistry needs first to be revisited. Strong constraints are provided by recent Herschel observations of nitrogen hydrides in cold gas. The aim of the present work is to comprehensively analyse the interstellar chemistry of nitrogen, focussing on the gas-phase formation of the smallest polyatomic species and, in particular, on nitrogen hydrides. We present a new chemical network in which the kinetic rates of critical reactions have been updated based on recent experimental and theoretical studies, including nuclear spin branching ratios. Our network thus treats the different spin symmetries of the nitrogen hydrides self-consistently, together with the ortho and para forms of molecular hydrogen. This new network is used to model the time evolution of the chemical abundances in dark cloud conditions. The steady-state results are analysed, with special emphasis on the influence of the overall amounts of carbon, oxygen, and sulphur. Our calculations are also compared withHerschel/HIFI observations of NH, NH₂, and NH₃ detected towards the external envelope of the protostar IRAS 16293-2422. The observed abundances and abundance ratios are reproduced for a C/O gas-phase elemental abundance ratio of ~0.8, provided that the sulphur abundance be depleted by a factor greater than 2. The ortho-to-para ratio of H₂ in these models is ~ 10^-3. Our models also provide predictions for the ortho-to-para ratios of NH₂ and NH₃ of ~2.3 and ~0.7, respectively. We conclude that the abundances of nitrogen hydrides in dark cloud conditions are consistent with the gas-phase synthesis predicted with our new chemical network.

Reference
Le Gal R, Hily-Blant P, Faure A, des Forêts GP, Rist C and Maret S (2014) Interstellar chemistry of nitrogen hydrides in dark clouds. Astronomy & Astrophysics 562:A83.
[doi:10.1051/0004-6361/201322386]
Reproduced with permission © ESO

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