Tim Elliott

School of Earth Science, University of Bristol, Queen’s Road, Clifton BS8 1RJ, UK.

As in many building booms, planets were put together pretty rapidly. Transforming nebular dust to fully formed planets took less than ~100 million years of the ~4.5 billion years of solar system history. Accurate determination of the rates of planetary growth is key for understanding these tumultuous beginnings of the solar system, but obtaining high-precision ages on short-lived events that happened so long ago is a formidable challenge. On page 1150 of this issue, Kruijer et al. (1) determine with remarkable accuracy that planetary core formation began less than 1 million years after the first solids condensed—extraordinarily fast on geological time scales.

Reference
Elliot T (in press) Speed metal. Science 344:1086.
[doi:10.1126/science.1254943]
Reprinted with permission from AAAS

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T. S. Kruijer^1,2, M. Touboul³, M. Fischer-Gödde¹, K. R. Bermingham³, R. J. Walker³ and T. Kleine¹

¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, DE-48149 Münster, Germany.
²ETH Zürich, Inst. of Geochemistry and Petrology, Clausiusstrasse 25, CH-8092 Zürich, Switzerland.
³Department of Geology, University of Maryland, College Park, MD 20742, USA.

Understanding core formation in meteorite parent bodies is critical for constraining the fundamental processes of protoplanet accretion and differentiation within the solar protoplanetary disk. We report variations of 5 to 20 parts per million in ¹⁸²W, resulting from the decay of now-extinct ¹⁸²Hf, among five magmatic iron meteorite groups. These ¹⁸²W variations indicate that core formation occurred over an interval of ~1 million years and may have involved an early segregation of Fe-FeS and a later segregation of Fe melts. Despite this protracted interval of core formation, the iron meteorite parent bodies probably accreted concurrently ~0.1 to 0.3 million years after the formation of Ca-Al–rich inclusions. Variations in volatile contents among these bodies, therefore, did not result from accretion at different times from an incompletely condensed solar nebula but must reflect local processes within the nebula.

Reference
Kruijer TS, Touboul M, Fischer-Gödde M, Bermingham KR, Walker RJ and Kleine T (in press) Protracted core formation and rapid accretion of protoplanets. Science 344:1150.
[doi:10.1126/science.1251766]
Reprinted with permission from AAAS

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Seungwon Lee^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aJet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

We present sub-millimeter observations of the ground-state rotational transition (1₁₀–1₀₁) of water vapour from comet C/2002 T7 (LINEAR) obtained with the MIRO Instrument on the ESA Rosetta Spacecraft (s/c) Orbiter on April 30, 2004. At the time of the observations, the comet was at a distance of 0.63 AU from the Sun, 0.68 AU from the MIRO telescope, and about 7.5 days after its perihelion. The ground state rotation transition of ortho-water at 556.936 GHz was observed and integrated for ∼ 8 hours using a frequency switched radiometer to provide short and long term stability. The MIRO beam size is 7.5 arcmin in terms of full width half maximum, corresponding to a radius of 1.1×10⁵ km at the comet location. The observed signal line area of the water line spectrum is 4.3±0.8 K km/s. Using a molecular excitation and radiation transfer model and assuming the spherically symmetric and constant radial expansion of gas in the coma, we estimate that the production rate of water is (1.0±0.2)x10³⁰ molecules/s and the expansion velocity is 1.1±0.2 km/s at the time of the MIRO observation. The present estimation of the water outgassing rate of the comet is in good agreement with other observation-based estimations when the outgassing rates with respect to the time after perihelion are compared. The Doppler-correctd center velocity of the observed line was red-shifted by 0.67±0.13 km/s, of which only 0.18 km/s shift is explained by the model and attributed to a self-absorption effect. The potential sources of the additional red shift are discussed.

Reference
Lee et al. (in press) Sub-millimeter Observation of Water Vapor at 557 GHz in Comet C/2002 T7 (LINEAR). Icarus
[doi:10.1016/j.icarus.2014.05.004]
Copyright Elsevier

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Yayoi N. Miura¹, Keisuke Nagao² and Makoto Kimura³

¹Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo, Japan
²Geochemical Research Center, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan
³Faculty of Science, Ibaraki University, Mito, Ibaraki, Japan

We analyzed noble gases in nine individual chondrules, an assemblage of small chondrules, and four whole-rock samples of the Allende CV3 chondrite. Major elements were also determined for five chondrules. The cosmic ray exposure ages are calculated from cosmogenic ³He to be 5.17 ± 0.38 and 5.15 ± 0.25 Myr for the averages of the chondrules and whole rocks, respectively, showing no significant pre-exposure evidence for the studied chondrules. Large amounts of ³⁶Ar, ^80,82Kr, and ¹²⁸Xe produced by neutron capture are observed in most samples; the abundances of these nuclides are correlated among the samples. The epithermal neutron flux and neutron slowing down density are calculated based on [⁸⁰Kr]_n, from which a sample depth of about 30 cm can be calculated. The measured chondrules contain variable amounts of radiogenic ¹²⁹Xe. The abundance ratios of radiogenic ¹²⁹Xe to neutron capture–produced ¹²⁸Xe are rather constant among the studied chondrules; four chondrules give more precise ratios at the high-temperature fractions, ranging from 1920 ± 80 to 2280 ± 140, which corresponds to a time difference of 3.9 ± 2.4 Myr. It is noticeable that most chondrules also contain ²⁴⁴Pu-derived fission Xe. The average²⁴⁴Pu/²³⁸U ratio for nine chondrules is 0.0069 ± 0.0018, which agrees well with the preferred ratio reported for chondrites.

Reference
Miura YN, Nagao K and Kimura M (in press) Noble gases in individual chondrules of the Allende CV3 chondrite. Meteoritics & Planetary Science
[doi:10.1111/maps.12313]
Published by arrangement with John Wiley & Sons

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Masato Kiuchi and Akiko M. Nakamura

Department of Earth and Planetary Sciences, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501,Japan

We obtain an empirical relationship between porosity and the interparticle force of granular media based on measurement data on the ground. We apply the relationship to the condition of the surface of small bodies to estimate the porosity and the particle size of the regolith.

Reference
Kiuchi M and Nakamura AM (in press) Relationship between Regolith Particle Size and Porosity on Small Bodies. Icarus
[doi:10.1016/j.icarus.2014.05.029]
Copyright Elsevier

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Andrea Isella¹, Claire J. Chandler¹, John M. Carpenter¹, Laura M. Pérez^2,3 and Luca Ricci¹

³Department of Astronomy, California Institute of Technology, MC 249-17, Pasadena, CA 91125, USA
³National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, USA
³Jansky Fellow.

We present Karl G. Jansky Very Large Array (VLA) observations of the 7 mm continuum emission from the disk surrounding the young star LkCa 15. The observations achieve an angular resolution of 70 mas and spatially resolve the circumstellar emission on a spatial scale of 9 AU. The continuum emission traces a dusty annulus of 45 AU in radius that is consistent with the dust morphology observed at shorter wavelengths. The VLA observations also reveal a compact source at the center of the disk, possibly due to thermal emission from hot dust or ionized gas located within a few AU from the central star. No emission is observed between the star and the dusty ring and, in particular, at the position of the candidate protoplanet LkCa 15 b. By comparing the observations with theoretical models for circumplanetary disk emission, we find that if LkCa 15 b is a massive planet (>5 M_J ) accreting at a rate greater than 10⁶ M_J  yr^-1, then its circumplanetary disk is less massive than 0.1 M_J , or smaller than 0.4 Hill radii. Similar constraints are derived for any possible circumplanetary disk orbiting within 45 AU from the central star. The mass estimates are uncertain by at least one order of magnitude due to the uncertainties on the mass opacity. Future ALMA observations of this system might be able to detect circumplanetary disks down to a mass of 5 × 10^-4 M_J and as small as 0.2 AU, providing crucial constraints on the presence of giant planets in the act of forming around this young star.

Reference
Isella A, Chandler CJ, Carpenter JM, Pérez LM and Ricci L (in press) Searching for Circumplanetary Disks around LkCa 15. The Astrophysical Journal 788:129.
[doi:10.1088/0004-637X/788/2/129]

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Joseph I. Goldstein^a, Jijin Yang^b and Edward R.D. Scott^c

^aDepartment of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA, USA
^bCarl Zeiss Microscopy, LLC. One Zeiss Drive, Thornwood, NY, USA
^cHIGP, University of Hawaii, Honolulu, HI, USA

Analyses and modeling of Ni zoning in taenite in differentiated meteorites provide metallographic cooling rates at ∼500 °C that are inconsistent with conventional formation models. Group IVA iron meteorites have very diverse cooling rates of 100-6600 °C/Myr indicating that they cooled inside a large metallic body with little or no silicate mantle (Yang et al., 2007). Wasson and Hoppe (2012) have questioned these diverse cooling rates on the basis of their ion probe measurements of Ni/Co ratios at the kamacite-taenite interface in two group IVA and in two group IIIAB iron meteorites. To investigate their claims and to assess methods for determining relative cooling rates from kamacite-taenite interface compositions, we have analyzed 38 meteorites—13 IVA, 14 IIIAB irons, 4 IAB complex irons, 6 pallasites and a mesosiderite—using the electron probe microanalyzer (EPMA). Ni concentrations in taenite (Niγ) and kamacite (Niα) at kamacite-taenite interfaces are well correlated with metallographic cooling rates: Niγ values increase from 30 to 52 wt.% while Niα decreases from 7 to 4 wt.% as cooling rates decrease. EPMA measurements of Niγ, Niα, and Niα/ Niγ, can therefore be used to provide order-of-magnitude estimates of relative cooling rates. Concentrations of Co in kamacite and taenite at their interface (Coα, Coγ) are controlled by bulk Ni and Co composition, as well as cooling rate. The ratios Coα/Coγ and (Co/Ni)α/(Co/Ni)γ are correlated with cooling rate, but because of significant scatter, these parameters should not be used to estimate cooling rates. Our analyses of 13 group IVA irons provide robust support for diverse cooling rates that decrease with increasing bulk Ni, consistent with measurements of cloudy zone size and tetrataenite width. Apparent equilibration temperatures, which are inferred from Niγ values and the Fe-Ni-P phase diagram and Ni diffusion rates in taenite, show that cooling rates of IVA irons vary by a factor of ≈100, in excellent agreement with the metallographic cooling rates. Similar calculations using NiγNiα and Coα/Coγ ratios and phase diagram data give factors that are an order of magnitude lower but have larger uncertainties. Thus we strongly disagree with the conclusion ofWasson and Hoppe (2012) that interface concentrations of Ni and Co are in any way in conflict with the cooling rates of Yang et al. (2008). Our measurements confirm that the IVA irons could not have cooled in an asteroidal core surrounded by a silicate mantle, and also that main-group pallasites cooled slower than IIIAB irons and did not cool at the boundary between the mantle and core from which the IIIAB irons originated. Our data provide additional evidence that mesosiderites, which formed by impact mixing of Fe-Ni melt and crustal rocks, cooled at uniquely slow rates.

Reference
Goldstein JI, Yang J and Scott ERD (in press) Determining cooling rates of iron and stony-iron meteorites from measurements of Ni and Co at kamacite-taenite interfaces. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.025]
Copyright Elsevier

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C. Pirim^a, M.A. Pasek^b, D.A. Sokolov^a, A.N. Sidorov^a, R. Gann^a and T.M. Orlando^a

^aSchool of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA
^bDepartment of Geology, University of South Florida, Tampa, FL 33620, USA

This work presents a comprehensive investigation of schreibersite inclusions within iron-poor and iron-rich meteorites, and of the associated intrinsic low-temperature oxidation products observed after exposure to terrestrial weathering. First, a thermodynamic equilibrium modeling of the oxidation of schreibersite was carried out and showed that oxidation is mostly limited to the surface in the absence of other ions and/or water. This oxidation occurs rapidly (less than a few weeks) and is mediated by the atmosphere, forming primarily iron oxides and iron phosphates. Second, detailed analyses of meteorite schreibersite inclusions and synthetic schreibersite surrogates (Fe₃P and Fe₂NiP) were performed using surface characterization techniques such as micro-Raman spectroscopy, atomic force microscopy (AFM), electrostatic force microscopy (EFM), electron microprobe analysis (EPMA), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Such thorough analyses are required as all prebiotic reactivity studies performed nowadays use meteoritic samples that have been somewhat exposed to Earth weathering. We find that, while the meteorite samples have not been introduced directly into water, they all bear significant oxidation signatures that appear to be similar for both studied short-term and long-term natural weathering corrosion processes. In addition, we find that synthetic schreibersite samples have similar surface and sub-surface chemistry and are reasonable chemical proxies for natural schreibersite. The thorough analytical studies detailed in this paper provide a chemical model for schreibersite oxidation products.

Reference
Pirim C, Pasek MA, Sokolov DA, Sidorov AN, Ganna R and Orlando TM (in press) Investigation of schreibersite and intrinsic oxidation products from Sikhote-Alin, Seymchan, and Odessa meteorites and Fe3P and Fe2NiP synthetic surrogates. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.027]
Copyright Elsevier

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Amy M. Gaffney and Lars E. Borg

Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore, CA 94550

The time at which the moon solidified can be determined from the Lu-Hf isotope systematics of lunar rocks derived from magma sources that formed during crystallization of the lunar magma ocean. The final magma ocean crystallization product, termed urKREEP, is enriched in incompatible trace elements including K, REE and P. We have determined the initial Hf isotopic compositions of four samples, two KREEP basalts and two Mg-suite norites. The incompatible trace element compositions of these samples are controlled by an urKREEP component, and therefore the initial Hf isotopic compositions of these samples represent the Hf isotopic evolution of urKREEP. In order to correct the effects of neutron irradiation on the Hf isotopic compositions of these samples, we have developed a model that uses the stable Hf and Sm isotopic compositions measured on an irradiated sample to determine and correct for the thermal and epithermal neutron fluence that has modified the Hf isotopic composition of the sample. We use our corrected results to calculate a ¹⁷⁶Lu-¹⁷⁶Hf urKREEP model age of 4353 ± 37 Ma and the ¹⁷⁶Lu/¹⁷⁷Hf of urKREEP to be 0.0153 ± 0.0033. The Lu-Hf model age is concordant with the re-calculated Sm-Nd urKREEP model age of 4389 ± 45 Ma, and we take the average of these ages, 4368 ± 29 Ma, to represent the time at which urKREEP formed. This age is concordant with the age of the most reliably dated ferroan noritic anorthosite as well as ¹⁴²Nd model ages for the formation or re-equilibration of mare basalt sources. Taken together, these ages indicate that the Moon experienced a widespread, large-scale magmatic event around 4370 Ma, most plausibly attributed to solidification of the lunar magma ocean.

Reference
Gaffney AM and Borg LE (in press) A young solidification age for the lunar magma ocean. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.028]
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Winfried H. Schwarz^1,2 andHans J. Lippolt²

¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Laboratorium für Geochronologie, Universität Heidelberg, Heidelberg, Germany

Seven impact melts from various places in the Nördlinger Ries were dated by ⁴⁰Ar-³⁹Ar step-heating. The aim of these measurements was to increase the age data base for Ries impact glasses directly from the Ries crater, because there is only one Ar-Ar step-heating spectrum available in the literature. Almost all samples display saddle-shaped age spectra, indicating the presence of excess argon in most Ries glass samples, most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases. In contrast, moldavites usually contain no inherited argon, probably due to their different formation process implying solidification during ballistic transport. The plateau age of the only flat spectrum is 14.60 ± 0.16 (0.20) Ma (2σ), while the total age of this sample is 14.86 ± 0.20 (0.22) Ma (isochron age: 14.72 ± 0.18 [0.22] Ma [2σ]), proofing the chronological relationship of the Ries impact and moldavites. The total ages of the other samples range between 15.77 ± 0.52 and 20.4 ± 1.0 Ma (2σ), implying approximately 2–40% excess⁴⁰Ar (compared to the nominal age of the Ries crater) in respective samples. Thus, the age of 14.60 ± 0.16 (0.20) (2σ) (14.75 ± 0.16 [0.20 Ma] [2σ], calculated using the most recent suggestions for the K decay constants) can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites.

Reference
Schwarz WH and Lippolt HJ  (in press) 40Ar-39Ar step-heating of impact glasses from the Nördlinger Ries impact crater—Implications on excess argon in impact melts and tektites. Meteoritics & Planetary Science
[doi:10.1111/maps.12309]
Published by arrangement with John Wiley & Sons

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