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

V. G. Shevchenko^1,2,*, I. G. Slyusarev¹, I. N. Belskaya¹

¹Astronomical Institute, Kharkiv V. N. Karazin National University, Kharkiv, Ukraine
²Department of Astronomy, Kharkiv V. N. Karazin National University, Kharkiv, Ukraine

CCD-photometry was performed for two Jupiter Trojan asteroids (911) Agamemnon and (4709) Ennomos for which the diameters were obtained from occultation events. New data on rotation periods, lightcurve amplitudes, color indices, magnitude–phase slopes, and absolute magnitudes were obtained for these asteroids. We have used the diameters from occultations (166 and 99 km) and new data on absolute magnitudes at the instant occultation (7.95 and 8.85 mag) to revise their albedos to 0.042 (911 Agamemnon) and 0.052 (4709 Ennomos).

Reference
Shevchenko VG, Slyusarev IG and Belskaya IN (in press) Revised albedos of Trojan asteroids (911) Agamemnon and (4709) Ennomos. Meteoritics & Planetary Science
[doi:10.1111/maps.12234]
Published by arrangement with John Wiley & Sons

Link to Article

And will be back tomorrow.

D.J. Cherniak^a and J.A. Van Orman^b

^aDepartment of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
^bDepartment of Earth, Environmental and Planetary Sciences, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA

Diffusion of tungsten has been characterized in synthetic forsterite and natural olivine (Fo₉₀) under dry conditions. The source of diffusant was a mixture of magnesium tungstate and olivine powders. Experiments were prepared by sealing the source material and polished olivine under vacuum in silica glass ampoules with solid buffers to buffer at NNO or IW. Prepared capsules were annealed in 1 atm furnaces for times ranging from 45 minutes to several weeks, at temperatures from 1050 to 1450°C. Tungsten distributions in the olivine were profiled by Rutherford Backscattering Spectrometry (RBS).
The following Arrhenius relation is obtained for W diffusion in forsterite:
D_W=1.0×10-8exp(-365±28kJ mol^-1/RT)m²sec^-1
Diffusivities for the synthetic forsterite and natural Fe-bearing olivine are similar, and tungsten diffusion in olivine shows little dependence on crystallographic orientation or oxygen fugacity.
The slow diffusivities measured for W in olivine indicate that Hf-W ages in olivine-metal systems will close to diffusive exchange at higher temperatures than other chronometers commonly used in cosmochronology, and that tungsten isotopic signatures will be less likely to be reset by subsequent thermal events.

Reference
Cherniak DJ and Van Orman JA (in press) Tungsten Diffusion in Olivine. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.020]
Copyright Elsevier

Link to Article

E. Bowell¹, D. A. Oszkiewicz^2,3,*, L. H. Wasserman¹, K. Muinonen^2,4, A. Penttilä², D. E. Trilling⁵

¹Lowell Observatory, Flagstaff, Arizona, USA
²Department of Physics, University of Helsinki, Helsinki, Finland
³Institute Astronomical Observatory, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland
⁴Finnish Geodetic Institute, Masala, Finland

By analyzing brightness variation with ecliptic longitude and using the Lowell Observatory photometric database, we estimate spin-axis longitudes for more than 350,000 asteroids. Hitherto, spin-axis longitude estimates have been made for fewer than 200 asteroids. We investigate longitude distributions in different dynamical groups and asteroid families. We show that asteroid spin-axis longitudes are not isotropically distributed as previously considered. We find that the spin-axis longitude distribution for Main Belt asteroids is clearly nonrandom, with an excess of longitudes from the interval 30°–110° and a paucity between 120° and 180°. The explanation of the nonisotropic distribution is unknown at this point. Further studies have to be conducted to determine if the shape of the distribution can be explained by observational bias, selection effects, a real physical process, or other mechanism.

Reference
Bowell E, Oszkiewicz DA, Wasserman LH, Muinonen K, Penttilä A and Trilling DE (in press) Asteroid spin-axis longitudes from the Lowell Observatory database. Meteoritics & Planetary Science
[doi:10.1111/maps.12230]
Published by arrangement with John Wiley & Sons

Link to Article

Joshua L. Bandfield^a, Eugenie Song^b, Paul O. Hayne^c, Brittany D. Brand^d, Rebecca R. Ghent^e, Ashwin R. Vasavada^c, David A. Paige^f

^aSpace Science Institute
^bHawai’i Institute of Geophysics and Planetology, University of Hawai’i
^cJet Propulsion Laboratory, California Institute of Technology
^dDepartment of Geosciences, Boise State University
^eDepartment of Geology, University of Toronto
^fEarth and Space Sciences, UCLA

Systematic temperature mapping and high resolution images reveal a previously unrecognized class of small, fresh lunar craters. These craters are distinguished by near-crater deposits with evidence for lateral, ground-hugging transport. More distal, highly insulating surfaces surround these craters and do not show evidence of either significant deposition of new material or erosion of the substrate. The near-crater deposits can be explained by a laterally propagating granular flow created by impact in the lunar vacuum environment. Further from the source crater, at distances of ∼10–100 crater radii, the upper few to 10’s of centimeters of regolith appear to have been “fluffed-up” without the accumulation of significant ejecta material. These properties appear to be common to all impacts, but quickly degrade in the lunar space weathering environment. Cratering in the vacuum environment involves a previously unrecognized set of processes that leave prominent, but ephemeral, features on the lunar surface.

Reference
Bandfield JL, Song E, Hayne PO, Brand BD, Ghent RR, Vasavad AR and Paige DA (in press) Lunar cold spots: Granular flow features and extensive insulating materials surrounding young craters. Icarus
[doi:10.1016/j.icarus.2013.12.017]
Copyright Elsevier

Link to Article

James F.J. Bryson^a, Nathan S. Church^a, Takeshi Kasama^b, Richard J. Harrison^a

^aDepartment of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
^bCenter for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, Denmark

Nanoscale intergrowths unique to the cloudy zones (CZs) of meteoritic metal display novel magnetic behaviour with the potential to reveal new insight into the early development of magnetic fields on protoplanetary bodies. The nanomagnetic state of the CZ within the Tazewell IIICD iron meteorite has been imaged using off-axis electron holography. The CZ is revealed to be a natural nanocomposite of magnetically hard islands of tetrataenite (ordered FeNi) embedded in a magnetically soft matrix of ordered Fe₃Ni. In the remanent state, each tetrataenite island acts as a uniaxial single domain particle with its [001] magnetic easy axis oriented along one of three 〈100〉 crystallographic directions of the parent taenite phase. Micromagnetic simulations demonstrate that switching occurs via the nucleation and propagation of domain walls through individual tetrataenite particles. The switching field (H_s) varies with the length scale of the matrix phase (L_m), with H_s > 1 T for L_m ∼10 nm (approaching the intrinsic switching field for isolated single domain tetrataenite) and 0.2<H_s<0.6 T for L_m ∼30 nm. The reduction in H_s with increasing L_c is caused by exchange coupling between the hard tetrataenite islands and the soft magnetic matrix, which lowers the critical field for domain wall nucleation, providing an explanation for previously observed coercivity variations throughout the CZ. Non-random distributions of the tetrataenite easy axes are observed locally throughout the CZ, suggesting a magnetic field could have been present during nanostructure formation. This observation demonstrates the potential for stable chemical transformation remanent magnetisation to be encoded by the nanostructure, with variations in the proportions of the six possible magnetisation states reflecting the intensity and relative direction of the magnetic fields present during cooling. According to recent cooling models, the cooling rate of meteoritic metal originating near the surface of differentiated planetesimals was such that the magnetic signal across the CZ could potentially record dynamo field intensity and direction variations over time (10–100 Ma), which would enable events such as magnetic reversals and the decay of an asteroid dynamo to be observed.

Reference
Bryson JFJ, Church NS, Kasama T and Harrison RJ (2014) Nanomagnetic intergrowths in Fe–Ni meteoritic metal: The potential for time-resolved records of planetesimal dynamo fields. Earth and Planetary Science Letters 388:237–248.
[doi:10.1016/j.epsl.2013.12.004]
Copyright Elsevier

Link to Article