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

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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]

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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]

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