Alex R. Howe and Roman R. Rafikov

Department of Astrophysical Sciences, Princeton University, Ivy Lane, Princeton, NJ 08540, USA

The Oort cloud remains one of the most poorly explored regions of the solar system. We propose that its properties can be constrained by studying a population of dust grains produced in collisions of comets in the outer solar system. We explore the dynamics of μm-sized grains outside the heliosphere (beyond ~250 AU), which are predominantly affected by the magnetic field of the interstellar medium (ISM) flow past the Sun. We derive analytic models for the production and motion of small particles as a function of their birth location in the cloud and calculate the particle flux and velocity distribution in the inner solar system. These models are verified by direct numerical simulations. We show that grains originating in the Oort cloud have a unique distribution of arrival directions, which should easily distinguish them from both interplanetary and interstellar dust populations. We also demonstrate that the distribution of particle arrival velocities is uniquely determined by the mass distribution and dust production rate in the cloud. Cometary collisions within the cloud produce a flux of μm-sized grains in the inner solar system of up to several m⁻² yr⁻¹. The next generation dust detectors may be sensitive enough to detect and constrain this dust population, which will illuminate the Oort cloud’s properties. We also show that the recently detected mysterious population of large (μm-sized) unbound particles, which seems to arrive with the ISM flow, is unlikely to be generated by collisions of comets in the Oort cloud.

Reference
Howe AR and Rafikov RR (2014) Probing Oort Cloud and Local Interstellar Medium Properties via Dust Produced in Cometary Collisions. The Astrophysical Journal 781:52.
[doi:10.1088/0004-637X/781/1/52]

Link to Article

David A. McKeown^1,*, Andrew C. Buechele¹, Ryan Tappero², Timothy J. McCoy³ and Kathryn G. Gardner-Vandy³

¹Vitreous State Laboratory, The Catholic University of America, 620 Michigan Avenue NE, Washington, D.C. 20064, U.S.A.
²Photon Sciences Department, Brookhaven National Laboratory, Upton, New York 11793, U.S.A.
³Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560-0119, U.S.A.

Chromium K-edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg₂SiO₄) as well as sulfides within the MAC 88136 EL3 chondrite to determine Cr valence and to see whether forsterite within this meteorite can be used as a Cr²⁺-silicate standard. Spectra were measured on several areas within a nearly pure 100 × 200 μm forsterite grain containing 0.13 wt% Cr. XANES findings indicate highly reduced Cr²⁺ species, with no clear evidence of Cr³⁺ or Cr⁶⁺. EXAFS data indicate an average 2.02 Å Cr-O nearest-neighbor distance, consistent with Cr-O distances found in square-planar Cr²⁺O₄ sites observed in synthetic crystalline silicates, and an average 2.69 Å Cr-Si second-nearest neighbor distance, consistent with Cr²⁺substituting for Mg²⁺ in the forsterite M(1) site. Nearest-neighbor Debye-Waller factor and coordination number parameters indicate Cr²⁺ is likely entering forsterite in disordered sites that are possible intermediates between M(1) and square-planar Cr²⁺O₄ configurations. Preliminary Cr XAS measurements on sulfides within this meteorite also indicate Cr²⁺ in CrS₆octahedra.

Reference
McKeown DA, Buechele AC, Tappero R, McCoy TJ and Gardner-Vandy KG (2014) X-ray absorption characterization of Cr in forsterite within the MacAlpine Hills 88136 EL3 chondritic meteorite. American Mineralogist 99:190-197.
[doi:10.2138/am.2014.4508]
Copyright: The Mineralogical Society of America

Link to Article

Nachiketa Rai and Wim van Westrenen

Faculty of Earth and Life Sciences, VU University Amsterdam, 1081 HV, Amsterdam, The Netherlands

Analyses of Apollo era seismograms, lunar laser ranging data and the lunar moment of inertia suggest the presence of a small, at least partially molten Fe-rich metallic core in the Moon, but the chemical composition and formation conditions of this core are not well constrained. Here, we assess whether pressure–temperature conditions can be found at which the lunar silicate mantle equilibrated with a Fe-rich metallic liquid during core formation. To this end, we combine measurements of the metal–silicate partitioning behavior of siderophile elements with the estimated depletion due to core formation in those elements in the silicate mantle of the Moon. We also explore how the presence of the light element sulfur (suggested by seismic models to be present in the core at concentrations of up to 6 wt%) in the lunar core affects core formation models.
We use published metal–silicate partitioning data for Ni, Co, W, Mo, P, V and Cr in the lunar pressure range (1 atm–5 GPa) and characterize the dependence of the metal/silicate partition coefficients (D) on temperature, pressure, oxygen fugacity and composition of the silicate melt and the metal. If the core is assumed to consist of pure iron, core–mantle equilibration conditions that best satisfy lunar mantle depletions of five siderophile elements—Ni, Co, W, Mo and P—are a pressure of 4.5(±0.5) GPa and a temperature of 2200 K. The lunar mantle depletions of Cr and V are also consistent with metal–silicate equilibration in this pressure and temperature range if 6 wt% S is incorporated into the lunar core. Our results therefore suggest that metal–silicate equilibrium during lunar core formation occurred at depths close to the present-day lunar core–mantle boundary. This provides independent support for both the existence of a deep magma ocean in the Moon in its early history and the presence of significant amounts of sulfur in the lunar core.

Reference
Rai N and Westrenen W (2014) Lunar core formation: New constraints from metal–silicate partitioning of siderophile elements. Earth and Planetary Science Letters 388:343–352.
[doi:10.1016/j.epsl.2013.12.001]
Copyright Elsevier

Link to Article

Juliane Gross^a,b, Allan H. Treiman^b, Celestine N. Mercer^a,1

^aAmerican Museum of Natural History, New York, NY 10024, United States
^bLunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, United States
¹USGS Denver Federal Center, Denver, CO 80225

The composition of the lunar crust provides clues about the processes that formed it and hence contains information on the origin and evolution of the Moon. Current understanding of lunar evolution is built on the Lunar Magma Ocean hypothesis that early in its history, the Moon was wholly or mostly molten. This hypothesis is based on analyses of Apollo samples of ferroan anorthosites (>90% plagioclase; molarMg/(Mg+Fe)=Mg#<75) and the assumption that they are globally distributed. However, new results from lunar meteorites, which are random samples of the Moonʼs surface, and remote sensing data, show that ferroan anorthosites are not globally distributed and that the Apollo highland samples, used as a basis for the model, are influenced by ejecta from the Imbrium basin. In this study we evaluate anorthosites from all currently available adequately described lunar highland meteorites, representing a more widespread sampling of the lunar highlands than Apollo samples alone, and find that ~80% of them are significantly more magnesian than Apollo ferroan anorthosites. Interestingly, Luna mission anorthosites, collected outside the continuous Imbrium ejecta, are also highly magnesian. If the lunar highland crust consists dominantly of magnesian anorthosites, as suggested by their abundance in samples sourced outside Imbrium ejecta, a reevaluation of the Lunar Magma Ocean model is a sensible step forward in the endeavor to understand lunar evolution. Our results demonstrate that lunar anorthosites are more similar in their chemical trends and mineral abundance to terrestrial massif anorthosites than to anorthosites predicted in a Lunar Magma Ocean. This analysis does not invalidate the idea of a Lunar Magma Ocean, which seems a necessity under the giant impact hypothesis for the origin of the moon. However, it does indicate that most rocks now seen at the Moonʼs surface are not primary products of a magma ocean alone, but are products of more complex crustal processes.

Reference
Gross J, Treiman AH and Mercer CN (2014) Lunar feldspathic meteorites: Constraints on the geology of the lunar highlands, and the origin of the lunar crust. Earth and Planetary Science Letters 388:318–328.
[doi:10.1016/j.epsl.2013.12.006]
Copyright Elsevier

Link to Article

Ben D. Stanley, Marc M. Hirschmann and Anthony C. Withers

Department of Earth Sciences, University of Minnesota, Minneapolis, MN, 55455, USA

To determine the speciation and concentrations of dissolved C-O-H volatiles in graphite-saturated martian primitive magmas, we conducted piston-cylinder experiments on graphite-encapsulated synthetic melt of Adirondack-class Humphrey basaltic composition. Experiments were performed over three orders of magnitude in oxygen fugacity (IW+2.3 to IW-0.8), and at pressures (1-3.2 GPa) and temperatures (1340-1617 °C) similar to those of possible martian source regions. Oxygen fugacities were determined from compositions of coexisting silicate melt+FePt alloy, olivine+pyroxene+FePt alloy, or melt+Fe-C liquid. Infrared spectra of quenched glasses all show carbonate absorptions at 1430 and 1520 cm^-1, with CO₂ concentrations diminishing under more reduced conditions, from 0.50 wt% down to 26 ppm. Carbon contents of silicate glasses and Fe-C liquids were measured using secondary ion mass spectrometry (SIMS) yielding 36-716 ppm and 6.71-7.03 wt%, respectively. Fourier transform infrared (FTIR) and SIMS analysis produced similar H₂O contents of 0.26-0.85 and 0.29-0.40 wt%, respectively. Raman spectra of glasses reveal evidence for OH^– ions, but no indication of methane-related species. FTIR-measured concentrations of dissolved carbonate diminish linearly with oxygen fugacity, but more reduced conditions yield greater dissolved carbonate concentrations than would be expected based on oxidized conditions in previous work. C contents of silicate glasses determined by SIMS are consistently higher than C as carbonate determined by FTIR. Their difference, termed non-carbonate C, correlates well with additional IR absorptions found in reduced glasses (f_O2<IW+0.4) at 1615, 2205, and 3370 cm^-1. These absorption bands are not seen in more oxidized glasses, except B441 (IW+1.7), presumably because they represent reduced C-bearing complexes. The 2205 cm^-1 peak is attributed to a C=O complex, possibly an Fe-carbonyl ion. The 1615 cm^-1 peak does not correlate with that at 2205 cm^-1, but does correlate with non-carbonate C and is in a region commonly associated with C=O bonding. The origin of the peak at 3370 cm^-1 is poorly understood and could potentially be owing to a variety of C-O-H species or to N-H bonding. The intensities of the 1615 and 3370 cm^-1 peaks correlate with each other leading us to provisionally attribute both to an unspecified complex with both C=O and N-H bonds. These results suggest that dissolved species such as carbonyl or other C=O-bearing species could be a significant source of C fluxes to the martian atmosphere, with minor additions of CO₂ and negligible methane contributions. By assuming that degassed, reduced C ultimately becomes atmospheric CO₂, reduced C outgassing may be incorporated in models of martian atmospheric evolution. At Humphrey source region conditions (1350±50 °C, 1.2±0.1 GPa) the total C contents are equivalent to 1200 ppm CO₂ at IW+1 and 475 ppm CO₂ at IW, which are 2 and 4 times higher than the CO₂ derived from CO₃^2- alone. For reasonable magmatic fluxes over the last 4.5 Ga of martian history, such graphite-saturated magmas would produce 0.25 and 0.60 bars from sources at IW and IW+1, significantly more than expected solely from consideration of dissolved CO₂. The carbon contents of Fe-C liquids in this study are consistent with graphite-saturated carbide liquids becoming more C-rich with increasing temperature. Experiments with melt and Fe-C liquid have values of  between 1.3×10³ and 2.2×10³, potentially allowing planetary mantles to retain significant C following core formation.

Reference
Stanley BD, Hirschmann MM and Withers AC (in press) Solubility of C-O-H volatiles in graphite-saturated martian basalts. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.013]
Copyright Elsevier

Link to Article

Thomas F. Nägler¹, Ariel D. Anbar^2,3, Corey Archer⁴, Tatiana Goldberg⁵, Gwyneth W. Gordon², Nicolas D. Greber¹, Christopher Siebert⁶, Yoshiki Sohrin⁷, Derek Vance⁴

¹Institut für Geologie, Universität Bern, Bern, Switzerland
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
³Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ, USA
⁴Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland
⁵Department of Earth Science and Engineering, Imperial College London, London, UK
⁶Helmholtz Centre for Ocean Research, GEOMAR, Kiel, Germany
⁷Institute for Chemical Research, Kyoto University, Uji, Japan

Molybdenum isotopes are increasingly widely applied in Earth Sciences. They are primarily used to investigate the oxygenation of Earth’s ocean and atmosphere. However, more and more fields of application are being developed, such as magmatic and hydrothermal processes, planetary sciences or the tracking of environmental pollution. Here, we present a proposal for a unifying presentation of Mo isotope ratios in the studies of mass-dependent isotope fractionation. We suggest that the δ^98/95Mo of the NIST SRM 3134 be defined as +0.25‰. The rationale is that the vast majority of published data are presented relative to reference materials that are similar, but not identical, and that are all slightly lighter than NIST SRM 3134. Our proposed data presentation allows a direct first-order comparison of almost all old data with future work while referring to an international measurement standard. In particular, canonical δ^98/95Mo values such as +2.3‰ for seawater and −0.7‰ for marine Fe–Mn precipitates can be kept for discussion. As recent publications show that the ocean molybdenum isotope signature is homogeneous, the IAPSO ocean water standard or any other open ocean water sample is suggested as a secondary measurement standard, with a defined δ^98/95Mo value of +2.34 ± 0.10‰ (2s).

Reference
Nägler TF, Anbar AD, Archer C, Goldberg T, Gordon GW, Greber ND, Siebert C, Sohrin Y and Vance D (in press) Proposal for an International Molybdenum Isotope Measurement Standard and Data Representation. Geostandards and Geoanalytical Research
[doi:10.1111/j.1751-908X.2013.00275.x]
Published by arrangement with John Wiley & Sons

Link to Article

Ai-Cheng Zhang^a,b, Shoichi Itoh^b,1, Naoya Sakamoto^c, Ru-Cheng Wang^a, Hisayoshi Yurimoto^b,c

^aState Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
^bDepartment of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
^cIsotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
¹Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606- 8502, Japan

Aluminum-rich chondrules are one of the most interesting components of primitive chondrites, because they have characteristics that are similar to both CAIs and ferromagnesian chondrules. However, their precursor and formation history remain poorly constrained, especially with respect to their oxygen isotopic distributions. In this study, we report on the petrography, mineralogy, oxygen isotope ratios, and rare-earth-element compositions of a sapphirine-bearing Al-rich chondrule (SARC) in the ungrouped chondrite Dar al Gani (DaG) 978. The SARC has a complex core-mantle-rim texture; while both the core and the mantle are mainly composed of Al-rich enstatite and anorthite with minor amounts of mesostasis, these regions are distinguished by the presence of Fe-rich spinel and sapphirine in the core and their absence in the mantle. The rim of the SARC consists mainly of Fe-rich olivine, enstatite, and Fe-Ni metal. Spinel and some olivine grains in the SARC are ¹⁶O-rich, with Δ¹⁷O values down to –20‰ and –23‰, respectively. Enstatite, sapphirine, and most olivine grains have similar Δ¹⁷O values (∼ –7‰), which are lower than those of anorthite and the mesostasis (including augite therein) (Δ¹⁷O: ∼ –3‰). Mesostasis from both the core and mantle have Group II rare-earth-element (REE) patterns; however, the core mesostasis has higher REE concentrations than the mantle mesostasis. These observations provide a strong indication that the SARC formed by the melting and crystallization of a mixture of materials from Group II Ca,Al-rich inclusions (CAIs) and ferromagnesian chondrules. Both spinel and olivine with ¹⁶O-rich features could be of relict origin. The ¹⁶O-poor isotopic compositions of most components in Al-rich chondrules can be explained by oxygen isotopic exchange between the melt and ¹⁶O-poor nebular gas (Δ¹⁷O: ∼ –7‰) during melting in chondrule-forming regions; whereas the anorthite and mesostasis could have experienced further oxygen isotopic exchange with a relatively ¹⁶O-poor reservoir (Δ¹⁷O: ∼ –3‰) on the parent body, likely during fluid-assisted thermal metamorphism. During the same thermal metamorphism event, spinel, olivine, some enstatite, and the mesostasis experienced Mg-Fe exchange to various extents.

Reference
Zhang A-C, Itoh S, Naoya Sakamoto N, Wang R-C and Hisayoshi Yurimoto H (in press) Origins of Al-rich chondrules: Clues from a compound Al-rich chondrule in the Dar al Gani 978 carbonaceous chondrite. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.026]
Copyright Elsevier

Link to Article

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

¹Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstr. 2, D-12489 Berlin, Germany

The near-Earth object (NEO) population, which mainly consists of fragments from collisions between asteroids in the main asteroid belt, is thought to include contributions from short-period comets as well. One of the most promising NEO candidates for a cometary origin is near-Earth asteroid (3552) Don Quixote, which has never been reported to show activity. Here we present the discovery of cometary activity in Don Quixote based on thermal-infrared observations made with the Spitzer Space Telescope in its 3.6 and 4.5 μm bands. Our observations clearly show the presence of a coma and a tail in the 4.5 μm but not in the 3.6 μm band, which is consistent with molecular band emission from CO₂. Thermal modeling of the combined photometric data on Don Quixote reveals a diameter of 18.4 km and an albedo of , which confirms Don Quixote to be the third-largest known NEO. We derive an upper limit on the dust production rate of 1.9 kg s^–1 and derive a CO₂ gas production rate of (1.1 ± 0.1) × 10²⁶ molecules s^–1. SpitzerInfrared Spectrograph spectroscopic observations indicate the presence of fine-grained silicates, perhaps pyroxene rich, on the surface of Don Quixote. Our discovery suggests that CO₂ can be present in near-Earth space over a long time. The presence of CO₂ might also explain that Don Quixote’s cometary nature remained hidden for nearly three decades.

Reference
Mommert et al. (2014) The Discovery of Cometary Activity in Near-Earth Asteroid (3552) Don Quixote. The Astrophysical Journal 781:25.
[doi:10.1088/0004-637X/781/1/25]

Link to Article

L. J. Karssemeijer¹, S. Ioppolo^1,2, M. C. van Hemert³, A. van der Avoird¹, M. A. Allodi⁴, G. A. Blake^2,4 and H. M. Cuppen¹

¹Theoretical Chemistry, Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
²Division of Geological and Planetary Science, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
³Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
⁴Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA

The long-timescale behavior of adsorbed carbon monoxide on the surface of amorphous water ice is studied under dense cloud conditions by means of off-lattice, on-the-fly, kinetic Monte Carlo simulations. It is found that the CO mobility is strongly influenced by the morphology of the ice substrate. Nanopores on the surface provide strong binding sites, which can effectively immobilize the adsorbates at low coverage. As the coverage increases, these strong binding sites are gradually occupied leaving a number of admolecules with the ability to diffuse over the surface. Binding energies and the energy barrier for diffusion are extracted for various coverages. Additionally, the mobility of CO is determined from isothermal desorption experiments. Reasonable agreement on the diffusivity of CO is found with the simulations. Analysis of the 2152 cm⁻¹ polar CO band supports the computational findings that the pores in the water ice provide the strongest binding sites and dominate diffusion at low temperatures.

Reference
Karssemeijer LJ, Ioppolo S, van Hemert MC, van der Avoird A, Allodi Ma, Blake GA and Cuppen HM (2014) Dynamics of CO in Amorphous Water-ice Environments. The Astrophysical Journal 781:16.
[doi:10.1088/0004-637X/781/1/16]

Link to Article

Fara N. Lindsay^a, Gregory F. Herzog^a, Jisun Park^a,b, Jeremy S. Delaney^a, Brent Turrin^c, Carl C. Swisher III^c

^aDepartment of Chemistry and Chemical Biology Rutgers University 610 Taylor Rd. Piscataway, NJ 8854
^bLunar and Planetary Institute 3600 Bay Area Blvd. Houston, TX 77058
^cDepartment of Earth and Planetary Sciences Rutgers University 610 Taylor Rd. Piscataway, NJ 08854

⁴⁰Ar/³⁹Ar ages of single feldspar grains from the paired meteorites Graves Nunatak 06128 (GRA8; 8 grains) and 06129 (GRA9; 26 grains) are presented. Plateau ages (⩾70% of the ³⁹Ar released) ranged from 4000 Ma to 4600 Ma with an average 1-σ uncertainty of ± 90 Ma. The most precise ages obtained were 4267±17 Ma for a grain from GRA8 and 4437±19 Ma and 4321±18 Ma for two grains from GRA9. Isotope correlation diagrams yield less precise ages ranging from 3800 Ma to 5200 Ma with an average 1-σ uncertainty of 250 Ma; they indicate a negligible trapped component. Plateau ages, integrated total fusion ages, and isochron ages are internally concordant at the 95% confidence level.
The distribution of the plateau ages for GRA9 is bimodal with peaks at 4400 and 4300 Ma. In contrast, the plateau age distribution for GRA8 peaks at about 4260 Ma with broad wings extending toward younger and older ages. To explain the distributions of grain ages we prefer a scenario that includes a major post-formation event about 4400 Ma ago and a later melt intrusion event that heated GRA8 more than some parts of GRA9.

Reference
Lindsay FN, Herzog GF, Park J, Delaney JS, Turrin B and Swisher III CC (in press) 40Ar/39Ar dating of microgram feldspar grains from the paired feldspathic achondrites GRA 06128 and 06129. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.023]
Copyright Elsevier

Link to Article