David L. Cook^a,b, Thomas S. Kruijer^a,b, Ingo Leya^c and Thorsten Kleine^a

^aInstitut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm Str. 10, 48149 Münster, Germany
^bInstitut für Geochemie und Petrologie, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
^cSpace Research and Planetology, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

We measured tungsten isotopes in 23 iron meteorites and the metal phase of the CB chondrite Gujba in order to ascertain if there is evidence for a large-scale nucleosynthetic heterogeneity in the p-process isotope¹⁸⁰W in the solar nebula as recently suggested by Schulz et al. (2013). We observed large excesses in ¹⁸⁰W (up to ≈ 6 ε) in some irons. However, significant within-group variations in magmatic IIAB and IVB irons are not consistent with a nucleosynthetic origin, and the collateral effects on ¹⁸⁰W from an s-deficit in IVB irons cannot explain the total variation. We present a new model for the combined effects of spallation and neutron capture reactions on ¹⁸⁰W in iron meteorites and show that at least some of the observed within-group variability is explained by cosmic ray effects. Neutron capture causes burnout of ¹⁸⁰W, whereas spallation reactions lead to positive shifts in ¹⁸⁰W. These effects depend on the target composition and cosmic-ray exposure duration; spallation effects increase with Re/W and Os/W ratios in the target and with exposure age. The correlation of¹⁸⁰W/¹⁸⁴W with Os/W ratios in iron meteorites results in part from spallogenic production of ¹⁸⁰W rather than from ¹⁸⁴Os decay, contrary to a recent study by Peters et al. (2014). Residual ε¹⁸⁰W excesses after correction for an s-deficit and for cosmic ray effects may be due to ingrowth of ¹⁸⁰W from ¹⁸⁴Os decay, but the magnitude of this ingrowth is at least a factor of ≈ 2 smaller than previously suggested. These much smaller effects strongly limit the applicability of the putative ¹⁸⁴Os-¹⁸⁰W system to investigate geological problems.

Reference
Cook DL, Kruijer TS, Leya I and Kleine T (in press) Cosmogenic 180W Variations in Meteorites and Re-assessment of a Possible 184Os-180W Decay System. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.013]
Copyright Elsevier

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Johanna K. Teske¹, Katia Cunha^1,2, Verne V. Smith³, Simon C. Schuler⁴ and Caitlin A. Griffith⁵

¹Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
²Observatório Nacional, Rua General José Cristino, 77, 20921-400 São Cristóvão, Rio de Janeiro, RJ, Brazil
³National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁴University of Tampa, 401 West Kennedy Boulevard, Tampa, FL 33606, USA
⁵Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA

The relative abundances of carbon and oxygen have long been recognized as fundamental diagnostics of stellar chemical evolution. Now, the growing number of exoplanet observations enable estimation of these elements in exoplanetary atmospheres. In hot Jupiters, the C/O ratio affects the partitioning of carbon in the major observable molecules, making these elements diagnostic of temperature structure and composition. Here we present measurements of carbon and oxygen abundances in 16 stars that host transiting hot Jupiter exoplanets, and we compare our C/O ratios to those measured in larger samples of host stars, as well as those estimated for the corresponding exoplanet atmospheres. With standard stellar abundance analysis we derive stellar parameters as well as [C/H] and [O/H] from multiple abundance indicators, including synthesis fitting of the [O i] λ6300 line and non-LTE corrections for the O i triplet. Our results, in agreement with recent suggestions, indicate that previously measured exoplanet host star C/O ratios may have been overestimated. The mean transiting exoplanet host star C/O ratio from this sample is 0.54 (C/O_☉ = 0.54), versus previously measured C/O_host star means of ~0.65–0.75. We also observe the increase in C/O with [Fe/H] expected for all stars based on Galactic chemical evolution; a linear fit to our results falls slightly below that of other exoplanet host star studies but has a similar slope. Though the C/O ratios of even the most-observed exoplanets are still uncertain, the more precise abundance analysis possible right now for their host stars can help constrain these planets’ formation environments and current compositions.

Reference
Teske JK, Cunha K, Smith VV, Schuler SC and Griffith CA (2014) C/O ratios of stars with transiting hot Jupiter exoplanets. The Astrophysical Journal  788:39.
[doi:10.1088/0004-637X/788/1/39]

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Michael S. Bramble^1,2, Roberta L. Flemming^1,3, Jeffrey L. Hutter², Melissa M. Battler^1,3, Gordon R. Osinski^1,2,3 and Neil R. Banerjee^1,3

¹Centre for Planetary Science and Exploration, The University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7, Canada
²Department of Physics and Astronomy, The University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 3K7, Canada
³Department of Earth Sciences, The University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7, Canada

A temperature-controlled sample stage with an operational range of ~60 °C above or below ambient laboratory temperature (~ −35 to 85 °C) was constructed for in situ X-ray diffraction of minerals and materials using a Bruker D8 Discover diffractometer with θ-θ geometry. The stage was primarily designed for characterizing mirabilite-bearing samples from a Mars analog High Arctic perennial cold spring at an in situ temperature. Operation of the stage was demonstrated through the analysis of a synthetic sample of the hydrated sodium sulfate, mirabilite (Na₂SO₄·10H₂O). Mirabilite was held at −25 °C for approximately two hours without significant dehydration and then incrementally warmed to ambient laboratory temperature at 5 °C intervals, during the acquisition of in situ diffraction data. At ambient laboratory temperature, the mirabilite dehydrated and only polycrystalline thenardite (Na₂SO₄) remained. Preliminary analysis of the cold spring precipitates demonstrates that when mirabilite is present in the sample the dehydration reaction is occurring between collection and analysis at ambient laboratory temperature. This temperature-controlled stage was designed for versatility and ease of X-ray access, with applications that can extend to many geological and planetary settings, including Mars analog environments.

Reference
Bramble MS, Flemming RL, Hutter JL, Battler MM, Osinski GR and Banerjee NR (2014) A temperature-controlled sample stage for in situ micro-X-ray diffraction: Application to Mars analog mirabilite-bearing perennial cold spring precipitate mineralogy. American Mineralogist  99:943-947.
[doi:10.2138/am.2014.4629]
Copyright: The Mineralogical Society of America

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Ritesh Kumar Mishra^a,b and Marc Chaussidon^a,c

^aCentre de Recherches Pétrographiques et Géochimiques, Boite Postale 20, 15 Rue du Notre Dame des Pauvres, INSU-CNRS, Université de Lorraine, 54501 Vandoeuvre-lès-Nancy, France
^bPhysical Reseach Laboratory, Navrangpura, Ahmedabad 380009, Gujarat, India1
^cInstitut de Physique du Globe de Paris, Université Paris-Diderot, CNRS (UMR 7154), PRES Sorbonne Paris Cité, 1 rue Jussieu, 75005 Paris, France1

The short-lived now-extinct nuclide (SLN) ⁶⁰Fe, which decays to ⁶⁰Ni with a half-life of 2.62 Ma, is uniquely of stellar origin. Hence, its Solar System initial abundance yields information about the source of SLNs and the astrophysical environment in which the Solar System was born. Only a few chondrules (∼19) from unequilibrated ordinary chondrites have reported resolved ⁶⁰Ni excesses using in situ secondary ion mass spectrometry implying  in the early Solar System, and among these very few (3) have higher excesses implying  (Mishra et al., 2010, Mishra and Goswami, 2014 and Telus et al., 2012). At variance, multi-collector inductively coupled plasma mass spectrometer studies of bulk samples and mineral separates from differentiated meteorites, angrites, achondrites, and chondrules suggest a low abundance of ⁶⁰Fe/⁵⁶Fe of ∼1.4×10⁻⁸ which would rule out the need for an external seeding of the early Solar with stellar ⁶⁰Fe (Quitté et al., 2011 and Tang and Dauphas, 2012). Two Semarkona chondrules and one Efremovka chondrule analyzed in the present study have mass fractionation corrected excess of up to ∼75 permil (‰) and give ⁶⁰Fe isochrons with initial ⁶⁰Fe/⁵⁶Fe ratios of(7.8±3.7)×10⁻⁷, (3.8±1.6)×10⁻⁷, and (2.2±1.1)×10⁻⁷ (2σ), for Efremovka Ch 1, Semarkona Ch 12, and Semarkona Ch J5 respectively. The higher values of ⁶⁰Fe/⁵⁶Fe ratios seen in the chondrules of these least altered meteorites samples concur with and lend greater credence to the suggestion of a massive star as the source of ⁶⁰Fe, and possibly of other short-lived nuclides, to the early Solar System. However, no definitive explanation (e.g. sample bias, effects of metamorphism, ⁶⁰Fe heterogeneity) to the apparent disagreement with studies of bulk chondrules and chondrule fragments has been found.

Reference
Mishra RK and Chaussidon M (in press) Fossil records of high level of 60Fe in chondrules from unequilibrated chondrites. Earth and Planetary Science Letters 398:90–100.
[doi:10.1016/j.epsl.2014.04.032]
Copyright Elsevier

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Eric Hand

Planetary scientists have begun to plan the first stage of a Mars sample return mission: a rover to be launched in 2020. Scientists want the rover to drill at least 31 rock samples weighing about 15 grams apiece during its 2-year mission and pack them in a cache that a later mission will retrieve. But the rover will have to work faster than the current rover, Curiosity, which has drilled just three samples in its 20 months on Mars.

Reference
Hand E (2014) NASA planners gear up for martian sample return. Science 344:787-788.
[doi:10.1126/science.344.6186.787]
Reprinted with permission from AAAS

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M. Darby Dyar¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.

Phosphate minerals, while relatively rare, show a broad range of crystal structure types with linkages among PO₄ tetrahedra mimicking the hierarchy of polymerization of SiO₄ tetrahedra seen in silicate minerals. To augment previous Mössbauer studies of individual phosphate species and groups of species, this paper presents new Mössbauer data on 63 different phosphate samples, and integrates them with data on more than 37 phosphate species in 62 other studies from the literature. Variations in Mössbauer parameters of different sites in each mineral are then related to both the local polyhedral environment around the Fe cations and the overall structural characteristics of each species. The entire aggregated Mössbauer data set on phosphate minerals is juxtaposed against parameters obtained for spectra from the MIMOS spectrometers on Mars. This comparison demonstrates that signatures from many different phosphate or sulfate mineral species could also be contributing to Mars Mössbauer spectra. Results underscore the conclusion that unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a wealth of new data on Fe-bearing phosphate minerals to bolster future analyses of Mössbauer spectra acquired on Mars.

Reference
Dyar et al. (2014) Mössbauer parameters of iron in phosphate minerals: Implications for interpretation of martian data. American Mineralogist 99:914-942.
[doi:10.2138/am.2014.4701]
Copyright: The Mineralogical Society of America

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P. Riviere-Marichalar^1,2 et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹ Centro de Astrobiología (INTA–CSIC) – Depto. Astrofísica, POB 78, ESAC Campus, 28691 Villanueva de la Cañada, Spain 
² Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands

Context. Debris discs are thought to be formed through the collisional grinding of planetesimals, and then can be considered as the outcome of planet formation. Understanding the properties of gas and dust in debris discs can help us comprehend the architecture of extrasolar planetary systems. Herschel Space Observatory far-infrared (IR) photometry and spectroscopy have provided a valuable dataset for the study of debris discs gas and dust composition. This paper is part of a series of papers devoted to the study of Herschel-PACS observations of young stellar associations.
Aims. This work aims at studying the properties of discs in the beta Pictoris moving group (BPMG) through far-IR PACS observations of dust and gas.
Methods. We obtained Herschel-PACS far-IR photometric observations at 70, 100, and 160 μm of 19 BPMG members, together with spectroscopic observations for four of them. These observations were centred at 63.18 μm and 157 μm, aiming to detect [OI] and [CII] emission. We incorporated the new far-IR observations in the SED of BPMG members and fitted modified blackbody models to better characterise the dust content.
Results. We have detected far-IR excess emission towards nine BPMG members, including the first detection of an IR excess towards HD 29391.The star HD 172555, shows [OI] emission, while HD 181296 shows [CII] emission, expanding the short list of debris discs with a gas detection. No debris disc in BPMG is detected in both [OI] and [CII]. The discs show dust temperatures in the range 55–264 K, with low dust masses (<6.6 × 10^-5 M_⊕ to 0.2 M_⊕) and radii from blackbody models in the range 3 to ~82 AU. All the objects with a gas detection are early spectral type stars with a hot dust component.

Reference
Riviere-Marichalar et al. (2014) Gas and dust in the beta Pictoris moving group as seen by the Herschel Space Observatory. Astronomy & Astrophysics 565:A68.
[doi:10.1051/0004-6361/201322901]
Reproduced with permission © ESO

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A. J. Evans^1,2, M. T. Zuber¹, B. P. Weiss¹ and S. M. Tikoo¹

¹Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
²Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA

While recent analyses of lunar samples indicate the Moon had a core dynamo from at least 4.2–3.56 Ga, mantle convection models of the Moon yield inadequate heat flux at the core-mantle boundary to sustain thermal core convection for such a long time. Past investigations of lunar dynamos have focused on a generally homogeneous, relatively dry Moon, while an initial compositionally stratified mantle is the expected consequence of a postaccretionary lunar magma ocean. Furthermore, recent re-examination of Apollo samples and geophysical data suggests that the Moon contains at least some regions with high water content. Using a finite element model, we investigate the possible consequences of a heterogeneously wet, compositionally stratified interior for the evolution of the Moon. We find that a postoverturn model of mantle cumulates could result in a core heat flux sufficiently high to sustain a dynamo through 2.5 Ga and a maximum surface, dipolar magnetic field strength of less than 1 μT for a 350-km core and near ~2 μT for a 450-km core. We find that if water was transported or retained preferentially in the deep interior, it would have played a significant role in transporting heat out of the deep interior and reducing the lower mantle temperature. Thus, water, if enriched in the lower mantle, could have influenced core dynamo timing by over 1.0 Gyr and enhanced the vigor of a lunar core dynamo. Our results demonstrate the plausibility of a convective lunar core dynamo even beyond the period currently indicated by the Apollo samples.

Reference
Evans AJ, Zuber MT, Weiss BP and Tikoo SM (in press) A wet, heterogeneous lunar interior: Lower mantle and core dynamo evolution. Journal of Geophysical Research: Planets 
[doi:10.1002/2013JE004494]
Published by arrangement with John Wiley & Sons

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T. Tsujimoto¹ and T. Shigeyama²

¹National Astronomical Observatory of Japan, Mitaka-shi, 181-8588 Tokyo Japan
²Research Center for the Early Universe, Graduate School of Science, University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

The origin of r-process elements remains unidentified and still puzzles us. The recent discovery of evidence for the ejection of r-process elements from a short-duration γ-ray burst singled out neutron star mergers (NSMs) as their origin. In contrast, core-collapse supernovae are ruled out as the main origin of heavy r-process elements (A > 110) by recent numerical simulations. However, the properties characterizing NSM events – their rarity and high yield of r-process elements per event – have been claimed to be incompatible with the observed stellar records on r-process elements in the Galaxy. We add to this picture with our results, which show that the observed constant [r-process/H] ratio in faint dwarf galaxies and one star unusually rich in r-process in the Sculptor galaxy agree well with this rarity of NSM events. Furthermore, we found that a large scatter in the abundance ratios of r-process elements to iron in the Galactic halo can be reproduced by a scheme that incorporates an assembly of various protogalactic fragments, in each of which r-process elements supplied by NSMs pervade the whole fragment while supernovae distribute heavy elements only inside the regions swept up by the blast waves. Our results demonstrate that NSMs occurring at Galactic rate of 12–23 Myr^-1 are the main site of r-process elements, and we predict the detection of gravitational waves from NSMs at a high rate with upcoming advanced detectors.

Reference
Tsujimoto T and Shigeyama T (2014) Enrichment history of r-process elements shaped by a merger of neutron star pairs.  Astronomy & Astrophysics 565:AL5.
[doi:10.1051/0004-6361/201322175]
Reproduced with permission © ESO

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Thomas M. McCollom¹, Bethany L. Ehlmann^3,4, Alian Wang⁵, Brian M. Hynek^1,2, Bruce Moskowitz^6,7 and Thelma S. Berquó⁸

¹Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80309, U.S.A.
²Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A.
³California Institute of Technology, Pasadena, California 91125, U.S.A.
⁴Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, U.S.A.
⁵Deptartment of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, Missouri 63130, U.S.A.
⁶Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.
⁷Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.
⁸Department of Physics, Concordia College, Moorhead, Minnesota 56562, U.S.A.

Natroalunite containing substantial amounts of Fe occurs as a prominent secondary phase during acid-sulfate alteration of pyroclastic basalts in volcanic fumaroles in Nicaragua and elsewhere, and has been observed in laboratory simulations of acid-sulfate alteration as well. Reaction path models constrained by field and experimental observations predict that Fe-rich natroalunite should also form as a major secondary phase during alteration of martian basalt under similar circumstances. Here, we evaluate the potential to use spectroscopic methods to identify minerals from the alunite group with chemical compositions intermediate between natroalunite and natrojarosite on the surface of Mars, and to remotely infer their Fe contents. X-ray diffraction and spectroscopic measurements (Raman, visible/near infrared, mid-infrared, Mössbauer) were obtained for a suite of synthetic solid solutions with a range of Fe contents ranging from natroalunite to natrojarosite. In the visible/near infrared, minerals with intermediate compositions display several spectral features not evident in end-member spectra that could be used to remotely identify these minerals and infer their composition. In addition, Raman spectra, mid-infrared spectra, and X-ray diffraction peaks all show systematic variation with changing Fe content, indicating that these methods could potentially be used to infer mineral compositions as well. The results suggest that alunite group minerals with intermediate Fe compositions may be able to account for some visible/near-infrared and Mössbauer spectral features from Mars that had previously been unidentified or attributed to other phases. Overall, our findings indicate that consideration of solid solutions may lead to new identifications of alunite group minerals on the surface of Mars, and raise the possibility that minerals with compositions intermediate between natroalunite and natrojarosite may be widely distributed on the planet.

Reference
McCollom TM, Ehlmann BL, Wang A, Hynek BM, Moskowitz B and Berquó TS (2014) Detection of iron substitution in natroalunite-natrojarosite solid solutions and potential implications for Mars. American Mineralogist 99:948-964.
[doi:10.2138/am.2014.4617]
Copyright: The Mineralogical Society of America

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