A. Garufi¹, S. P. Quanz¹, H. Avenhaus¹, E. Buenzli^{2, 3}, C. Dominik⁴, F. Meru¹, M. R. Meyer¹, P. Pinilla^{5, 6}, H. M. Schmid¹ and S. Wolf⁷

¹Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
²Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
³Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴Sterrenkundig Instituut Anton Pannekoek, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁶Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷University of Kiel, Institute of Theoretical Physics and Astrophysics, Leibnizstrasse 15, 24098 Kiel, Germany

Context. Transitional disks represent a short stage of the evolution of circumstellar material. Studies of dust grains in these objects can provide pivotal information on the mechanisms of planet formation. Dissimilarities in the spatial distribution of small (μm−size) and large (mm−size) dust grains have recently been pointed out.
Aims. Constraints on the small dust grains can be obtained by imaging the distribution of scattered light at near-infrared wavelengths. We aim at resolving structures in the surface layer of transitional disks (with particular emphasis on the inner 10−50 AU), thus increasing the scarce sample of high-resolution images of these objects.
Methods. We obtained VLT/NACO near-IR high-resolution polarimetric differential imaging observations of SAO 206462 (HD 135344B). This technique allows one to image the polarized scattered light from the disk without any occulting mask and to reach an inner working angle of ~0.1″.
Results. A face-on disk is detected in H and K_s bands between 0.1″ and 0.9″. No significant differences are seen between the H and K_s images. In addition to the spiral arms, these new data allow us to resolve for the first time an inner disk cavity for small dust grains. The cavity size (≃28 AU) is much smaller than what is inferred for large dust grains from (sub-)mm observations (39 to 50 AU). This discrepancy cannot be ascribed to any resolution effect.
Conclusions. The interaction between the disk and potential orbiting companion(s) can explain both the spiral arm structure and the discrepant cavity sizes for small and large dust grains. One planet may be carving out the gas (and, thus, the small grains) at 28 AU, and generating a pressure bump at larger radii (39 AU), which holds back the large grains. We analytically estimate that, in this scenario, a single giant planet (with a mass between 5 and 15 M_J) at 17 to 20 AU from the star is consistent with the observed cavity sizes.

Reference
Garufi A, Quanz SP, Avenhaus H, Buenzli E, Dominik C, Meru F, Meyer MR, Pinilla P, Schmid HM and Wolf S (2013) Small vs. large dust grains in transitional disks: do different cavity sizes indicate a planet? SAO 206462 (HD 135344B) in polarized light with VLT/NACO. Astronomy & Astrophysics 560:A105.
[doi:10.1051/0004-6361/201322429]
Reproduced with permission © ESO

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A. S. Pilski^1,*, J. T. Wasson², A. Muszyński³, R. Kryza⁴, Ł. Karwowski⁵ and M. Nowak³

¹Nicolaus Copernicus Museum, Frombork, Poland
²Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
³Institute of Geology, Adam Mickiewicz University, Poznań, Poland
⁴Institute of Geological Sciences, University of Wrocław, Wrocław, Poland
⁵Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland

Differences in texture and discovery location prompted us to analyze 16 irons from Morasko; one from Seeläsgen, known to have a similar composition; and a new mass found at Jankowo Dolne. These were analyzed in duplicate by instrumental neutron-activation analysis (INAA). The results show that all 18 samples have very similar compositions, distinct from all other IAB irons except Burgavli; we conclude that they are all from a single shower. Eight of the samples were from regions with large amounts of cohenite (but were largely free of inclusions) and six were from samples with very little cohenite; we could find no resolvable difference in composition between these sets, a fact that suggests that the C contents of the metal phases were similar in the two areas. Although Morasko has been classified into the IAB main group (IAB-MG), its Ir plots well outside the main group field on an Ir-Au diagram. We considered the possibility that the low Ir reflected contamination by a melt from a IAB region that ponded and experienced fractional crystallization; however, because Morasko has Pt, W, and Ga values that are the same as the highest values in IAB-MG, we rejected this model. We therefore conclude that Morasko formed from a different melt than the IAB-MG irons; the Morasko melt was produced by impact heating, but one or more of the main Ir carriers did not melt, leaving much of the Ir in the unmelted residue. Copper is the only element that shows resolvable differences among Morasko samples. Most (13 of 18) samples have 149 ± 4 μg g⁻¹ Cu, but three have 213 ± 10 μg g⁻¹; we interpret this to mean that the low-Cu samples have equilibrated with a Cu-rich phase, whereas there was none of the latter phase within a few diffusion lengths of the samples with high Cu contents.

Reference
Pilski AS, Wasson JT, Muszyński A, Kryza R, Karwowski Ł and Nowak M (in press) Low-Ir IAB irons from Morasko and other locations in central Europe: One fall, possibly distinct from IAB-MG. Meteoritics & Planetary Science
[doi:10.1111/maps.12225]
Published by arrangement with John Wiley & Sons

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Thomas S. Kruijer^1,2,*, Peter Sprung^1,2, Thorsten Kleine², Ingo Leya³, Rainer Wieler¹

¹ETH Zürich, Institute of Geochemistry and Petrology, Zürich, Switzerland
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany
³Space Research and Planetary Sciences, University of Bern, Bern, Switzerland

Cadmium is a highly volatile element and its abundance in meteorites may help better understand volatility-controlled processes in the solar nebula and on meteorite parent bodies. The large thermal neutron capture cross section of ¹¹³Cd suggests that Cd isotopes might be well suited to quantify neutron fluences in extraterrestrial materials. The aims of this study were (1) to evaluate the range and magnitude of Cd concentrations in magmatic iron meteorites, and (2) to assess the potential of Cd isotopes as a neutron dosimeter for iron meteorites. Our new Cd concentration data determined by isotope dilution demonstrate that Cd concentrations in iron meteorites are significantly lower than in some previous studies. In contrast to large systematic variations in the concentration of moderately volatile elements like Ga and Ge, there is neither systematic variation in Cd concentration amongst troilites, nor amongst metal phases of different iron meteorite groups. Instead, Cd is strongly depleted in all iron meteorite groups, implying that the parent bodies accreted well above the condensation temperature of Cd (i.e., ≈650 K) and thus incorporated only minimal amounts of highly volatile elements. No Cd isotope anomalies were found, whereas Pt and W isotope anomalies for the same iron meteorite samples indicate a significant fluence of epithermal and higher energetic neutrons. This observation demonstrates that owing to the high Fe concentrations in iron meteorites, neutron capture mainly occurs at epithermal and higher energies. The combined Cd-Pt-W isotope results from this study thus demonstrate that the relative magnitude of neutron capture-induced isotope anomalies is strongly affected by the chemical composition of the irradiated material. The resulting low fluence of thermal neutrons in iron meteorites and their very low Cd concentrations make Cd isotopes unsuitable as a neutron dosimeter for iron meteorites.

Reference
Kruijer TS, Sprung P, Kleine T, Leya I and Wieler R (in press) The abundance and isotopic composition of Cd in iron meteorites. Meteoritics & Planetary Science
[doi:10.1111/maps.12240]
Published by arrangement with John Wiley & Sons

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Maria Lugaro¹, Giuseppe Tagliente^2,8, Amanda I. Karakas³, Paolo M. Milazzo⁴, Franz Käppeler⁵, Andrew M. Davis^6,9,10, and Michael R. Savina^7,9

¹Monash Centre for Astrophysics (MoCA), Monash University, Clayton, VIC 3800, Australia
²Istituto Nazionale di Fisica Nucleare (INFN), Bari, Italy
³Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁴Istituto Nazionale di Fisica Nucleare (INFN), Trieste, Italy
⁵Karlsruhe Institute of Technology, Campus North, D-76021 Karlsruhe, Germany
⁶The Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
⁷Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
⁸Also at University of Ghent, Ghent, Belgium.
⁹Also at Chicago Center for Cosmochemistry, USA.
¹⁰Also at The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA.

We present model predictions for the Zr isotopic ratios produced by slow neutron captures in C-rich asymptotic giant branch (AGB) stars of masses 1.25-4 M_☉ and metallicities Z = 0.01-0.03, and compare them to data from single meteoritic stardust silicon carbide (SiC) and high-density graphite grains that condensed in the outflows of these stars. We compare predictions produced using the Zr neutron-capture cross sections from Bao et al. and from n_TOF experiments at CERN, and present a new evaluation for the neutron-capture cross section of the unstable isotope ⁹⁵Zr, the branching point leading to the production of ⁹⁶Zr. The new cross sections generally present an improved match with the observational data, except for the ⁹²Zr/⁹⁴Zr ratios, which are on average still substantially higher than predicted. The ⁹⁶Zr/⁹⁴Zr ratios can be explained using our range of initial stellar masses, with the most ⁹⁶Zr-depleted grains originating from AGB stars of masses 1.8-3 M_☉ and the others from either lower or higher masses. The ^90,91Zr/⁹⁴Zr variations measured in the grains are well reproduced by the range of stellar metallicities considered here, which is the same needed to cover the Si composition of the grains produced by the chemical evolution of the Galaxy. The ⁹²Zr/⁹⁴Zr versus ²⁹Si/²⁸Si positive correlation observed in the available data suggests that stellar metallicity rather than rotation plays the major role in covering the ^90,91,92Zr/⁹⁴Zr spread.

Reference
Lugaro M, Tagliente G, Karakas AI, Milazzo PM, Käppeler F, Davis AM and Savina MR (2014) The Impact of Updated Zr Neutron-capture Cross Sections and New Asymptotic Giant Branch Models on Our Understanding of the S Process and the Origin of Stardust. The Astrophysical Journal 780:95.
[doi:10.1088/0004-637X/780/1/95]

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I. Strashnov^1,2 and J. D. Gilmour¹

¹School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK
²School of Physics and Astronomy, The University of Manchester, Manchester, UK

⁸¹Kr-Kr cosmic ray exposure (CRE) ages of individual chondrules (6–10 mg) and adjacent matrix samples (5–10 mg) from the Allegan H5 chondrite have been measured using a new highly sensitive resonance ionization mass spectrometer. No conclusive evidence of variations among the CRE ages of individual chondrules or between chondrules and matrix has been observed—average CRE ages of 5.90 ± 0.42 Ma (⁸¹Kr-⁷⁸Kr) and 5.04 ± 0.37 Ma (⁸¹Kr-⁸⁰⁺⁸²Kr) are identical within error to those determined for the matrix (7.42 ± 1.27 Myr, ⁸¹Kr-⁸⁰⁺⁸²Kr) and agree well with the literature value for bulk Allegan. If any accumulation of cosmogenic krypton in the early solar system took place, either it was below our detection limit in these samples (<100 atoms), or any such gas was lost during parent body metamorphism. However, this demonstration that useful ⁸¹Kr-Kr ages can be obtained from few milligram samples of chondritic material has clear relevance to the analysis of samples returned by planned missions to asteroids and to the search for a signature of pre-exposure in other, less processed meteorites.

Reference
Strashnov I and Gilmour JD (in press) 81Kr-Kr cosmic ray exposure ages of individual chondrules from Allegan. Meteoritics & Planetary Science
[doi:10.1111/maps.12228]
Published by arrangement with John Wiley & Sons

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N. Tosi^1,*, M. Grott², A.-C. Plesa^2,3 and D. Breuer²

¹Department of Planetary Geodesy, Technische Universität Berlin, Berlin, Germany
²Department of Planetary Physics, German Aerospace Center, Berlin, Germany
³Department of Planetology, Westfälische Universität Münster, Münster, Germany

A number of observations performed by the MESSENGER spacecraft can now be employed to better understand the evolution of Mercury’s interior. Using recent constraints on interior structure, surface composition, volcanic and tectonic histories, we modeled the thermal and magmatic evolution of the planet. We ran a large set of Monte Carlo simulations based on one-dimensional parametrized models, spanning a wide range of parameters. We complemented these simulations with selected calculations in 2-D cylindrical and 3-D spherical geometry, which confirmed the validity of the parametrized approach and allowed us to gain additional insight into the spatiotemporal evolution of mantle convection. Core radii of 1940 km, 2040 km, and 2140 km have been considered, and while in the first two cases several models satisfy the observational constraints, no admissible models were found for a radius of 2140 km. A typical thermal evolution scenario consists of an initial phase of mantle heating accompanied by planetary expansion and the production of a substantial amount of partial melt. The evolution subsequent to 2 Gyr is characterized by secular cooling that proceeds approximately at a constant rate and implies that planetary contraction should be ongoing today. Most of the models predict mantle convection to cease after 3–4 Gyr, indicating that Mercury may be no longer dynamically active. Finally, assuming the observed surface abundance of radiogenic elements to be representative for the entire crust, we determined bulk silicate concentrations of 35–62 ppb Th, 20–36 ppb U, and 290–515 ppm K, similar to those of other terrestrial planets.

Reference
Tosi N, Grott M, Plesa A-C and Breuer D (in press) Thermochemical evolution of Mercury’s interior. Journal of Geophysical Research – Planets
[doi:10.1002/jgre.20168]
Published by arrangement with John Wiley & Sons

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J.J. Papike¹, P.V. Burger^1,*, A.S. Bell¹, L. Le², C.K. Shearer¹, S.R. Sutton³, J. Jones⁴ and M. Newville³

¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque New Mexico 87131, U.S.A.
²JSC Engineering, Technology and Science (JETS), NASA Johnson Space Center, Mail Code JE-23, Building 31, Houston, Texas 77058, U.S.A.
³Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁴NASA/Johnson Space Center, Houston, Texas 77058, U.S.A.

A spiked (with REE, V, Sc) martian basalt Yamato 980459 (Y98) composition was used to synthesize olivine, spinel, and pyroxene at 1200 °C at five oxygen fugacities: IW−1, IW, IW+1, IW+2, and QFM. These run products were analyzed by electron microprobe, ion microprobe, and X-ray absorption near-edge spectroscopy to establish four oxybarometers based on vanadium partitioning behavior between the following pairs of phases: V spinel-melt, V/(Cr+Al) spinel-melt, olivine-melt, and spinel-olivine. The results for the spinel-melt, olivine-melt, and V/(Cr+Al) spinel-melt are applicable for the entire oxygen fugacity range while the spinel-olivine oxybarometer is only applicable between IW−1 and IW+1. The oxybarometer based on V partitioning between spinel-olivine is restricted to basalts that crystallized under low oxygen fugacities, some martian, all lunar, as well as samples from 4 Vesta. The true potential and power of the new spinel-olivine oxybarometer is that it does not require samples representative of a melt composition or samples with some remnant of quenched melt present. It just requires that the spinel-olivine pairs were in equilibrium when the partitioning of V occurred. We have applied the V spinel-olivine oxybarometer to the Y98 meteorite as a test of the method.

Reference
Papike JJ, Burger PV, Bell AS, Le L, Shearer CK, Sutton SR, Jones J and Newville M (2013) Developing vanadium valence state oxybarometers (spinel-melt, olivine-melt, spinel-olivine) and V/(Cr+Al) partitioning (spinel-melt) for martian olivine-phyric basalts. American Mineralogist 98:2193-2196.
[doi:10.2138/am.2013.4622]
Copyright: The Mineralogical Society of America

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Pavithra Sekhar and Scott D. King

Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, United States

While most of Tharsis rise was in place by end of the Noachian period, at least one volcano on Tharsis swell (Arsia Mons) has been active within the last 2 Ma. This places an important constraint on mantle convection and on the thermal evolution of Mars. The existence of recent volcanism on Mars implies that adiabatic decompression melting and, hence, upwelling convective flow in the mantle remains important on Mars at present. The thermal history on Mars can be constrained by the history of melt production, specifically generating sufficient melt in the first billion years of the planets history to produce Tharsis rise as well as present day melt to explain recent volcanism. In this work, mantle convection simulations were performed using finite element code CitcomS in a 3D sphere starting from a uniformly hot mantle and integrating forward in time for the age of the solar system. We implement constant and decaying radioactive heat sources; and vary the partitioning of heat sources between the crust and mantle, and consider decreasing core–mantle boundary temperature and latent heat of melting. The constant heat source calculations produce sufficient melt to create Tharsis early in Martian history and continue to produce significant melt to the present. Calculations with decaying radioactive heat sources generate excessive melt in the past, except when all the radiogenic elements are in the crust, and none produce melt after 2 Gyr. Producing a degree-1 or degree-2 structure may not be pivotal to explain the Tharsis rise: we present multi-plume models where not every plume produces melt. The Rayleigh number controls the timing of the first peak of volcanism while late-stage volcanism is controlled more by internal mantle heating. Decreasing the Rayleigh number increases the lithosphere thickness (i.e., depth), and increasing lithosphere thickness increases the mean mantle temperature. Increasing pressure reduces melt production while increasing temperature increases melt production; hence predicting melt production from convection parameters is not straightforward. Generating enough melt in the mantle to create Tharsis early on and also to explain recent volcanism may require other mechanisms such as small-scale convection or lowering the thermal conductivity of the crust.

Reference
Sekhar P and King SD (2013) 3D spherical models of Martian mantle convection constrained by melting history. Earth and Planetary Science Letters 388:27–37.
[doi:10.1016/j.epsl.2013.11.047]
Copyright Elsevier

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Richard A. Kerr

Recent results from the Curiosity Mars rover have helped scientists formulate a plan for the next phase of its mission: looking for possible “molecular fossils” left by ancient martian microbes. Analyses of rocks show that Curiosity landed near a former lake that at least intermittently held enough water to have supported life. Now, papers published online in Science show that rocks that once formed the lakebed and bottom mud layer are high in organic carbon molecules. Researchers can’t tell yet whether the molecules came from ancient life or rained down from space. But future analyses—especially of recently eroded rocks that spent most of their history shielded from the cosmic rays thought to sterilize the top meter or so nearest the martian surface—should help researchers determine whether Mars ever harbored life.

Reference
Kerr RA (2013) New Results Send Mars Rover on a Quest for Ancient Life. Science 342:1300-1301.
[doi:10.1126/science.342.6164.1300]
Reprinted with permission from AAAS

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Stuart J. Mills^1,*, Fabrizio Nestola², Volker Kahlenberg³, Andrew G. Christy⁴, Clivia Hejny³ and Günther J. Redhammer⁵

¹Geosciences, Museum Victoria, GPO Box 666, Melbourne 3001, Victoria, Australia
²Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, Padova I-35131, Italy
³Institut für Mineralogie und Petrographie der Universität Innsbruck, Innrain 52, 6020 Innsbruck, Austria
⁴Centre for Advanced Microscopy, Australian Natioanl University, Canberra, ACT 0200, Australia
⁵Department of Materials Engineering and Physics, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria

Single-crystal diffraction of jarosite, KFe₃³⁺(SO₄)₂(OH)₆, has been undertaken at low temperatures that proxy for martian surface conditions. Room-temperature data are consistent with literature data [a = 7.2913(5), c = 17.1744(17), and V = 790.72(11) in R3̄m], while the first low-temperature data for the mineral is presented (at 253, 213, 173, and 133 K). Data collections between 297 and 133 K show strongly anisotropic thermal expansion, with the c axis much more expandable than the a axis. Much of the anisotropy is due to strong distortion of the KO₁₂ polyhedron, which increases by 8% between 297 and 133 K. The data sets can aid in the identification of jarosite by X-ray diffraction of martian soils using the Curiosity Rover’s CheMin instrument.

Reference
Mills SJ, Nestola F, Kahlenberg V, Christy AG, Hejny C and Redhammer GJ (2013) Looking for jarosite on Mars: The low-temperature crystal structure of jarosite. American Mineralogist 98:1966-1971.
[doi:10.2138/am.2013.4587]
Copyright: The Mineralogical Society of America

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