Stuart Ross Taylor

Research School of Earth Sciences, Australian National University. Canberra, Australia

Recent geochemical and geophysical data from the Moon enable a revision of earlier interpretations regarding lunar origin, structure and bulk composition. Earth and Moon show many similarities among their isotopic compositions, but they have evolved in totally dissimilar ways, probably related to the deficiency of water in the Moon as well as the vast differences in size and internal pressure. Although some global geochemical trends such as volatile depletion based on K/U ratios have been established, current lunar samples come from differentiated regions, making the establishment of a bulk composition more reliant on bulk geophysical properties or isotopic similarities although it remains unclear how the latter relate to whole Moon composition. The lack of fractionation effects among the refractory and super-refractory elements indicates that the proto-lunar material seems unlikely to have been vaporized while the depletion of volatile elements may place lower limits on proto-lunar temperatures. The apparent lack of geochemical evidence of an impacting body enables other possible impactors, such as comets, to be considered.

Reference
Taylor SR (in press) The Moon re-examined. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.06.031]
Copyright Elsevier

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José C. Aponte^a,b, Jason P. Dworkin^b, Jamie E. Elsila^b

^aNASA Postdoctoral Program at NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
^bNASA Goddard Space Flight Center and Goddard Center for Astrobiology, Greenbelt, Maryland 20771, USA

The study of meteoritic organic compounds provides a unique window into the chemical inventory of the early Solar System and prebiotic chemistry that may have been important for the origin of life on Earth. Multiple families of organic compounds have been extracted from the Murchison meteorite, which is one of the most thoroughly studied carbonaceous chondrites. The amino acids extracted from Murchison have been extensively analyzed, including measurements of non-terrestrial stable isotopic ratios and discoveries of L-enantiomeric excesses for α-dialkyl amino acids, notably isovaline. However, although the isotopic signatures of bulk amine-containing fractions have been measured, the isotopic ratios and enantiomeric composition of individual aliphatic amines, compounds that are chemically related to amino acids, remain unknown. Here, we report a novel method for the extraction, separation, identification and quantitation of aliphatic monoamines extracted from the Murchison meteorite. Our results show a complete suite of structural isomers, with a larger concentration of methylamine and ethylamine and decreasing amine concentrations with increasing carbon number. The carbon isotopic compositions of fourteen meteoritic aliphatic monoamines were measured, with δ¹³C values ranging from +21 to +129‰, showing a decrease in ¹³C with increasing carbon number, a relationship that may be consistent with the chain elongation mechanism under kinetic control previously proposed for meteoritic amino acids. We also found the enantiomeric composition ofsec-butylamine, a structural analog to isovaline, was racemic within error, while the isovaline extracted from the same Murchison piece showed an L-enantiomeric excess of 9.7%; this result suggested that processes leading to enantiomeric excess in the amino acid did not affect the amine. We used these collective data to assess the primordial synthetic origins of these meteoritic aliphatic amines and their potential linkage to meteoritic amino acids.

Reference
Aponte JC, Dworkin JP and Elsila JE (in press) Assessing the origins of aliphatic amines in the murchison meteorite from their compound-specific carbon isotopic ratios and enantiomeric composition. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.06.035]
Copyright Elsevier

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Kenji Furuya^1,2 and Yuri Aikawa¹

¹Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan
²Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands

We study the influence of the turbulent transport on ice chemistry in protoplanetary disks, focusing on carbon- and nitrogen-bearing molecules. Chemical rate equations are solved with the diffusion term, mimicking the turbulent mixing in the vertical direction. Turbulence can bring ice-coated dust grains from the midplane to the warm irradiated disk surface, and the ice mantles are reprocessed by photoreactions, thermal desorption, and surface reactions. The upward transport decreases the abundance of methanol and ammonia ices at r  30 AU because warm dust temperature prohibits their reformation on grain surfaces. This reprocessing could explain the smaller abundances of carbon and nitrogen bearing molecules in cometary coma than those in low-mass protostellar envelopes. We also show the effect of mixing on the synthesis of complex organic molecules (COMs) in two ways: (1) transport of ices from the midplane to the disk surface and (2) transport of atomic hydrogen from the surface to the midplane. The former enhances the COMs formation in the disk surface, while the latter suppresses it in the midplane. Then, when mixing is strong, COMs are predominantly formed in the disk surface, while their parent molecules are (re)formed in the midplane. This cycle expands the COMs distribution both vertically and radially outward compared with that in the non-turbulent model. We derive the timescale of the sink mechanism by which CO and N2 are converted to less volatile molecules to be depleted from the gas phase and find that the vertical mixing suppresses this mechanism in the inner disks.

Reference
Furuya K and Aikawa Y (2014) Reprocessing of Ices in Turbulent Protoplanetary Disks: Carbon and Nitrogen Chemistry. The Astrophysical Journal 790:97.
[doi:10.1088/0004-637X/790/2/97]

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Harry Y. McSween Jr.¹, Theodore C. Labotka¹ and Christina E. Viviano-Beck²

¹Department of Earth and Planetary Sciences and Planetary Geoscience Institute, University of Tennessee, Knoxville, Tennessee, USA
²The Johns Hopkins Applied Physics Laboratory, Laurel, Maryland, USA

Compositions of basaltic and ultramafic rocks analyzed by Mars rovers and occurring as Martian meteorites allow predictions of metamorphic mineral assemblages that would form under various thermophysical conditions. Key minerals identified by remote sensing roughly constrain temperatures and pressures in the Martian crust. We use a traditional metamorphic approach (phase diagrams) to assess low-grade/hydrothermal equilibrium assemblages. Basaltic rocks should produce chlorite + actinolite + albite + silica, accompanied by laumontite, pumpellyite, prehnite, or serpentine/talc. Only prehnite-bearing assemblages have been spectrally identified on Mars, although laumontite and pumpellyite have spectra similar to other uncharacterized zeolites and phyllosilicates. Ultramafic rocks are predicted to produce serpentine, talc, and magnesite, all of which have been detected spectrally on Mars. Mineral assemblages in both basaltic and ultramafic rocks constrain fluid compositions to be H₂O-rich and CO₂-poor. We confirm the hypothesis that low-grade/hydrothermal metamorphism affected the Noachian crust on Mars, which has been excavated in large craters. We estimate the geothermal gradient (>20 °C km⁻¹) required to produce the observed assemblages. This gradient is higher than that estimated from radiogenic heat-producing elements in the crust, suggesting extra heating by regional hydrothermal activity.

Reference
McSween HY, Labotka TC and Viviano-Beck CE (in press) Metamorphism in the Martian crust. Meteoritics & Planetary Science
[doi:10.1111/maps.12330]
Published by arrangement with John Wiley & Sons

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A. Sezer¹ and F. Gök²

¹TÜBİTAK Space Technologies Research Institute, ODTU Campus, Ankara, 06531, Turkey
²Akdeniz University, Faculty of Education, Department of Secondary Science and Mathematics Education, Antalya, 07058, Turkey

In this work, we present results from a ~201.6 ks observation of G352.7–0.1 using the X-ray Imaging Spectrometer on board SuzakuX-ray Observatory. The X-ray emission from the remnant is well described by two-temperature thermal models of non-equilibrium ionization with variable abundances with a column density of N_H ~ 3.3 × 10²² cm^-2. The soft component is characterized by an electron temperature of kT_e ~ 0.6 keV, an ionization timescale of τ ~ 3.4 × 10¹¹ cm^-3 s, and enhanced Si, S, Ar, and Ca abundances. The hard component has kT_e ~ 4.3 keV, τ ~ 8.8 × 10⁹ cm^-3 s, and enhanced Fe abundance. The elemental abundances of Si, S, Ar, Ca, and Fe are found to be significantly higher than the solar values that confirm the presence of ejecta. We detected strong Fe K-shell emission and determined its origin to be the ejecta for the first time. The detection of Fe ejecta with a lower ionization timescale favors a Type Ia origin for this remnant.

Reference
Sezer A and Gök F (2014) Fe-rich Ejecta in the Supernova Remnant G352.7–0.1 with Suzaku. The Astrophysical Journal 790:81.
[doi:10.1088/0004-637X/790/1/81]

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Francis M. McCubbin¹, Charles K. Shearer¹, Paul V. Burger¹, Erik H. Hauri², Jianhua Wang², Stephen M. Elardo¹ and James J. Papike¹

¹Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, D.C. 20015, U.S.A.

Whitlockite and merrillite are two Ca-phosphate minerals found in terrestrial and planetary igneous rocks, sometimes coexisting with apatite. Whitlockite has essential structural hydrogen, and merrillite is devoid of hydrogen. Whitlockite components have yet to be discovered in samples of extraterrestrial merrillite, despite evidence for whitlockite-merrillite solid solution in terrestrial systems. The observation of merrillite in meteoritic and lunar samples has led many to conclude that the magmas from which the merrillite formed were “very dry.” However, the Shergotty martian meteorite has been reported to contain both apatite and merrillite, and recently the apatite has been shown to contain substantial OH abundances, up to the equivalent of 8600 ppm H₂O. In the present study, we determined the abundances of F, Cl, H₂O, and S in merrillite from Shergotty using secondary ion mass spectrometry (SIMS). We determined that the merrillite in Shergotty was properly identified (i.e., no discernible whitlockite component), and it coexists with OH-rich apatite. The absence of a whitlockite component in Shergotty merrillite and other planetary merrillites may be a consequence of the limited thermal stability of H in whitlockite (stable only at T <1050 °C), which would prohibit merrillite-whitlockite solid-solution at high temperatures. Consequently, the presence of merrillite should not be used as evidence of dry magmatism without a corresponding estimate of the T of crystallization. In fact, if a whitlockite component in extraterrestrial merrillite is discovered, it may indicate formation by or equilibration with hydrothermal or aqueous fluids.

Reference
McCubbin FM, Shearer CK, Burger PV, Hauri EH, Wang J, Elardo SM and Papike JJ (2014) Volatile abundances of coexisting merrillite and apatite in the martian meteorite Shergotty: Implications for merrillite in hydrous magmas. American Mineralogist 99:1347.
[doi:10.2138/am.2014.4782]
Copyright: The Mineralogical Society of America

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Gongjie Li and Konstantin Batygin

Harvard-Smithsonian Center for Astrophysics, The Institute for Theory and Computation, 60 Garden Street, Cambridge, MA 02138, USA

The variation of a planet’s obliquity is influenced by the existence of satellites with a high mass ratio. For instance, Earth’s obliquity is stabilized by the Moon and would undergo chaotic variations in the Moon’s absence. In turn, such variations can lead to large-scale changes in the atmospheric circulation, rendering spin-axis dynamics a central issue for understanding climate. The relevant quantity for dynamically forced climate change is the rate of chaotic diffusion. Accordingly, here we re-examine the spin-axis evolution of a Moonless Earth within the context of a simplified perturbative framework. We present analytical estimates of the characteristic Lyapunov coefficient as well as the chaotic diffusion rate and demonstrate that even in absence of the Moon, the stochastic change in Earth’s obliquity is sufficiently slow to not preclude long-term habitability. Our calculations are consistent with published numerical experiments and illustrate the putative system’s underlying dynamical structure in a simple and intuitive manner.

Reference
Li G and Batygin K (2014) On the Spin-axis Dynamics of a Moonless Earth. The Astrophysical Journal 790:69.
[doi:10.1088/0004-637X/790/1/69]

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Aaron S. Bell¹, Paul V. Burger¹, Loan Le², Charles K. Shearer¹, James J. Papike¹, Steve R. Sutton³, Matthew Newville³ and John Jones⁴

¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
²Jacobs, NASA Johnson Space Center, 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

In this work we present a series of experiments that examine the relationship between oxygen fugacity and Cr valence ratio in olivine grown from a basaltic liquid. These experiments are specifically targeted for an olivine-rich martian basalt composition that was modeled after the bulk chemistry of the meteorite Yamato 980459 (i.e., Y-98). The chromium valence ratio in the olivine crystals was measured with X-ray absorption near edge spectroscopy (XANES) at the Advanced Photon Source, Argonne National Laboratory. Results from the XANES measurements indicate that the ratio of divalent to trivalent Cr in the olivine is not only systematically correlated with f_O2, but is also reflective of the molar Cr³⁺/Cr²⁺ in the silicate liquid from which it
grew. In this way, measurements of Cr valence in olivine phenocrysts can yield important information about the oxygen fugacity and molar Cr³⁺/Cr²⁺ of its parental liquid in the absence of a quenched melt phase. Although the results from the experiments presented in this work specifically apply to the Y-98 parental melt, the concepts and XANES analytical techniques discussed within the text present a novel, generalized methodology that may be applicable to any olivine-bearing basalt. Furthermore, the XANES-based measurements are made on a micrometer-scale, thus potential changes of the Cr³⁺/Cr²⁺ in the melt during crystallization could be examined with a great deal of spatial detail.

Reference
Bell AS, Burger PV, Le L, Shearer CK, Papike JJ, Sutton SR, Newville M and Jones J (2014) XANES measurements of Cr valence in olivine and their applications to planetary basalts. American Mineralogist 99:1404.
[doi:10.2138/am.2014.4646]
Copyright: The Mineralogical Society of America

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J. Drążkowska, F. Windmark and C. P. Dullemond

Heidelberg University, Center for Astronomy, Institute of Theoretical Astrophysics, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany

Context. Dust coagulation in protoplanetary disks is one of the initial steps toward planet formation. Simple toy models are often not sufficient to cover the complexity of the coagulation process, and a number of numerical approaches are therefore used, among which integration of the Smoluchowski equation and various versions of the Monte Carlo algorithm are the most popular.
Aims. Recent progress in understanding the processes involved in dust coagulation have caused a need for benchmarking and comparison of various physical aspects of the coagulation process. In this paper, we directly compare the Smoluchowski and Monte Carlo approaches to show their advantages and disadvantages.
Methods. We focus on the mechanism of planetesimal formation via sweep-up growth, which is a new and important aspect of the current planet formation theory. We use realistic test cases that implement a distribution in dust collision velocities. This allows a single collision between two grains to have a wide range of possible outcomes but also requires a very high numerical accuracy.
Results. For most coagulation problems, we find a general agreement between the two approaches. However, for the sweep-up growth driven by the “lucky” breakthrough mechanism, the methods exhibit very different resolution dependencies. With too few mass bins, the Smoluchowski algorithm tends to overestimate the growth rate and the probability of breakthrough. The Monte Carlo method is less dependent on the number of particles in the growth timescale aspect but tends to underestimate the breakthrough chance due to its limited dynamic mass range.
Conclusions. We find that the Smoluchowski approach, which is generally better for the breakthrough studies, is sensitive to low mass resolutions in the high-mass, low-number tail that is important in this scenario. To study the low number density features, a new modulation function has to be introduced to the interaction probabilities. As the minimum resolution needed for breakthrough studies depends strongly on setup, verification has to be performed on a case by case basis.

Reference
Drążkowska J, Windmark F and Dullemond CP(2014) Modeling dust growth in protoplanetary disks: The breakthrough case. Astronomy & Astrophysics 567:A38.
[doi:10.1051/0004-6361/201423708]
Reproduced with permission © ESO

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Jean-David Bodénan^a,b, Natalie A. Starkey^a, Sara S. Russell^b, Ian P. Wright^a, Ian A. Franchi^a

^aPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
^aDepartment of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

Calcium–aluminium-rich Inclusions (CAIs) and the thin Wark–Lovering (WL) rims of minerals surrounding them offer a record of the nature of changing conditions during the earliest stages of Solar System formation. Considerable heterogeneity in the gas composition in the immediate vicinity of the proto-Sun had previously been inferred from oxygen isotopic variations in the WL rim of a CAI from Allende (Simon et al., 2011). However, high precision and high spatial resolution oxygen isotope measurements presented in this study show that WL rim and pristine core minerals of individual CAIs from meteorites that had experienced only low degrees of alteration or low grade metamorphism (one from Léoville (reduced CV3), two in QUE 99177 (CR3.0) and two in ALHA 77307 (CO3.0)) are uniformly ¹⁶O-rich. This indicates that the previously observed variations are the result of secondary processes, most likely on the asteroid parent body, and that there were no temporal or spatial variations in oxygen isotopic composition during CAI and WL rim formation. Such homogeneity across three groups of carbonaceous chondrites lends further support for a common origin for the CAIs in all chondrites. ¹⁶O-poor oxygen reservoirs such as those associated with chondrule formation, were probably generated by UV photo-dissociation involving self-shielding mechanisms and must have occurred elsewhere in outer regions of the solar accretion disk.

Reference
Bodénan J-D, Starkey NA, Russell SS, Wright IP and Franchi IA (2014) An oxygen isotope study of Wark–Lovering rims on type A CAIs in primitive carbonaceous chondrites. Earth and Planetary Science Letters 401:327.
[doi:10.1016/j.epsl.2014.05.035]
Copyright Elsevier

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