Christine Floss^a, Corentin Le Guillou^b,c, Adrian Brearley^b

^aLaboratory for Space Sciences and Physics Department, Washington University, One Brookings Drive, St. Louis, MO 63130, USA
^bDepartment of Earth and Planetary Sciences, MCS03-2040, University of New Mexico, Albuquerque, NM 87106, USA
^cInstitut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany

The organic matter in the primitive CR3 chondrites QUE 99177 and MET 00426 exhibits, as in other CR chondrites, N isotopic compositions characterized by large enrichments in ¹⁵N compared to solar. These enrichments are present in the matrices of these two meteorites as localized hotspots associated with C-rich grains, and larger, more diffuse regions with more modest enrichments in ¹⁵N. Occasionally depletions in ¹⁵N are also observed. FIB-TEM analysis of isotopically anomalous as well as isotopically normal C-rich grains from the matrix of MET 00426 shows that both types of grains consist of highly disordered organic matter that exhibits a variety of morphologies. There are no obvious correlations of isotopic composition with morphology, petrographic association or elemental composition. Large diffuse regions with modest ¹⁵N enrichments may be the result of fluid action that redistributed organic matter (and the associated ¹⁵N enrichments) in veins and cracks along grain boundaries. Grain formation likely occurred in a variety of environments (e.g., molecular clouds or the outer regions of the protosolar nebula) via UV photolysis of simpler precursor ices with variable isotopic compositions.

Reference
Floss C, Le Guillou C and Brearley A (in press) Coordinated NanoSIMS and FIB-TEM Analyses of Organic Matter and Associated Matrix Materials in CR3 Chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.023]
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R. Brunetto^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aInstitut d’Astrophysique Spatiale (IAS), CNRS, UMR-8617, Université Paris-Sud, bâtiment 121, F-91405 Orsay Cedex, France

Little is known about carbonaceous asteroids weathering in space as previous studies have struggled to define a general spectral trend among dark surfaces. Here we present experiments on ion irradiation of the Allende meteorite, performed using 40 keV He⁺ and Ar⁺ ions, as a simulation of solar wind irradiation of primitive bodies surfaces. We used different fluences up to 3×10¹⁶ ions/cm², corresponding to short timescales of ∼10³-10⁴ years in the main asteroid belt. Samples were analyzed before and after irradiation using visible to far-IR (0.4 – 50 μm) reflectance spectroscopy, and Raman micro-spectroscopy. Similarly to what observed in previous experiments, results show a reddening and darkening of VIS-NIR reflectance spectra. These spectral variations are however comparable to other spectral variations due to viewing geometry, grain size, and sample preparation, suggesting an explanation for the contradictory space weathering studies of dark asteroids. After irradiation, the infrared bands of the matrix olivine silicates change profile and shift to longer wavelength, possibly as a consequence of a more efficient sputtering effect on Mg than Fe (lighter and more volatile species are preferentially sputtered backwards) and/or preferential amorphisation of Mg-rich olivine. Spectral variations are compatible with the Hapke weathering model. Raman spectroscopy shows that the carbonaceous component is substantially affected by irradiation: different degrees of de-ordering are produced as a function of dose, to finally end with a highly disordered carbon. All observed modifications seem to scale with the nuclear elastic dose.

Reference
Brunetto et al. (in press) Ion irradiation of Allende meteorite probed by visible, IR, and Raman spectroscopies. Icarus
[doi:10.1016/j.icarus.2014.04.047]
Copyright Elsevier

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John Moriarty¹, Nikku Madhusudhan^2,3 and Debra Fischer¹

¹Department of Astronomy, Yale University, New Haven, CT 06511, USA
²Departments of Physics and Astronomy, Yale University, New Haven, CT 06511, USA
³Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

The composition of planets is largely determined by the chemical and dynamical evolution of the disk during planetesimal formation and growth. To predict the diversity of exoplanet compositions, previous works modeled planetesimal composition as the equilibrium chemical composition of a protoplanetary disk at a single time. However, planetesimals form over an extended period of time, during which elements sequentially condense out of the gas as the disk cools and are accreted onto planetesimals. To account for the evolution of the disk during planetesimal formation, we couple models of disk chemistry and dynamics with a prescription for planetesimal formation. We then follow the growth of these planetesimals into terrestrial planets with N-body simulations of late-stage planet formation to evaluate the effect of sequential condensation on the bulk composition of planets. We find that our model produces results similar to those of earlier models for disks with C/O ratios close to the solar value (0.54). However, in disks with C/O ratios greater than 0.8, carbon-rich planetesimals form throughout a much larger radial range of the disk. Furthermore, our model produces carbon-rich planetesimals in disks with C/O ratios as low as ~0.65, which is not possible in the static equilibrium chemistry case. These results suggest that (1) there may be a large population of short-period carbon-rich planets around moderately carbon-enhanced stars (0.65 < C/O < 0.8) and (2) carbon-rich planets can form throughout the terrestrial planet region around carbon-rich stars (C/O > 0.8).

Reference
Moriarty J, Madhusudhan N and Fischer D (2014) Chemistry in an Evolving Protoplanetary Disk: Effects on Terrestrial Planet Composition. The Astrophysical Journal 787:81.
[doi:10.1088/0004-637X/787/1/81]

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