¹Devin L. Schrader,²Jemma Davidson,³Richard C. Greenwood,³Ian A. Franchi,³Jenny M. Gibson
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th & Constitution NW, Washington, DC 20560-0119, USA
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA
³Planetary and Space Sciences, Department of Physical Sciences, Open University, Milton Keynes MK7 6AA, UK

To better understand the effects of aqueous alteration in the Renazzo-like carbonaceous (CR) chondrite parent asteroid, a minor body in the early Solar System, we studied the petrology and O-isotope compositions of fine-grained matrix from 14 different CR chondrites. The O-isotope compositions of matrix from Queen Alexandra Range 99177 confirm that this sample is the least aqueously altered CR chondrite, provides the best approximation of the primary anhydrous matrix, and suggests matrix is not a byproduct of chondrule formation. Matrix O-isotope compositions within individual CR chondrites are heterogeneous, varying up to ∼5‰∼5‰ in both View the MathML sourceδO18 and View the MathML sourceδO17, as a result of the heterogeneous nature of the matrix and diverse range of aqueous alteration recorded by each sample. Aqueous alteration resulted in matrix that is progressively more 16O-depleted and Ca-carbonate rich. Due to the fine-grained nature of matrix its O-isotope composition is a more sensitive indicator of a chondrite’s overall degree of aqueous alteration than whole-rock O-isotope compositions, which are typically dominated by the compositions of type I (FeO-poor) chondrule phenocrysts. Petrographic signatures correlate with the degree of aqueous alteration and the wide range of matrix O-isotope compositions indicate that some regions of the CR chondrite parent asteroid were relatively dry, while others were heavily hydrated with water. The O-isotope composition of aqueously altered matrix is consistent with asteroidal water being near View the MathML sourceΔO17∼0‰, which suggests an inner Solar System origin for the water. The diverse range of aqueous alteration recorded by a single asteroid has a range of implications for spectral studies of the asteroid belt, and the arrival of Dawn at 1 Ceres, Hayabusa-2 at 162173 1999 JU3, and OSIRIS-REx at 101955 Bennu.

Reference
Schrader DL, Davidson J, Greenwood RC, Franchi IA, Gibson JM (2014) A water–ice rich minor body from the early Solar System: The CR chondrite parent Asteroid. Earth and Planetary Science Letters 407, 48-60
Link to Article [DOI: 10.1016/j.epsl.2014.09.030]

Copyright Elsevier

^1,2Vladimir A. Krasnopolsky
¹Department of Physics, Catholic University of America, Washington, DC 20064, USA
²Moscow Institute of Physics and Technology, Dolgoprudny, Russia

X-rays from comets originate in charge exchange between heavy ions of the solar wind and cometary species. Spectra of nine comets observed by the Chandra X-Ray Observatory (CXO) are analyzed using the time-dependent instrument sensitivity and the energy-dependent spectral resolution. X-ray emissions are extracted from the spectra in the range of 150 to 1100 eV using the χ2-fitting. Production of X-rays varies in the observed comets by a factor of 500 from 4.4×1013 erg s-1 in comet 73P to 2.2×1016 erg s-1 in comet Ikeya-Zhang. The measured solar wind flow varies within a factor of 20, being the weakest in comet 73P and the strongest in 9P/Tempel 1. The retrieved X-ray line intensities vary within a factor of 5×104. These lines above 300 eV are attributed to emissions of the H- and He-like ions, and laboratory data on the excitation cross sections for these emissions (Greenwood et al., 2000, Astrophys. J. 533, L175-L178) are used to convert the observed emissions into abundances of heavy ions in the solar wind. Continuity equations for charge exchange in comets are solved analytically and result in relationships between the X-ray emissions and the ion fluxes. The flux of O7+ scaled to 1 AU varies within a factor of 35 with a mean value of 1.6×104 cm-2 s-1. The retrieved ratios of O8+/O7+, C6+/C5+, Ne10+/Ne9+, C6+/O7+, N6+/O7+, and Ne9+/O7+ demonstrate significant variations, while their mean values for O, C, and N agree with those recommended by Schwadron and Cravens (2000, Astrophys. J. 544, 558-566) for the slow and fast solar wind. (Data on Ne9+ and Ne10+ are lacking in Scwadron and Cravens (2000).) The results are compared with the ion ratios from Bodewits et al. (2007, Astron. Astrophys. 469, 1183-1195) that were obtained from the same CXO spectra of comets, and some significant differences are briefly discussed. CXO X-ray spectroscopy of comets is a diagnostic tool to study the composition of the solar wind and its variations.

Reference
Krasnopolsky VA (2014) CXO X-Ray Spectroscopy of Comets and Abundances of Heavy Ions in the Solar Wind. Icarus (in Press)
Link to Article: [DOI: 10.1016/j.icarus.2014.09.026]

Copyright Elsevier

¹Andreas Nathues et al. (>10)*
¹Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
*Find the extensive, full author and affiliation list on the publishers website

In this paper we present the results of a global survey of olivine-rich lithologies on (4) Vesta. We investigated Dawn Framing Camera (FC) High Altitude Mapping Orbit (HAMO) color cubes (∼60 m/pixel resolution) by using a method described in Thangjam et al. (2014b). In total we identified 15 impact craters exhibiting olivine-rich (> 40 wt.% ol) outcrops on their inner walls, some showing olivine-rich material also in their ejecta and floors. Olivine-rich sites are concentrated in the Bellicia, Arruntia and Pomponia region on Vesta’s northern hemisphere. From our multi-color and stratigraphic analysis, we conclude that most, if not all, of the olivine-rich material identified is of exogenic origin, i.e. remnants of A- or/and S-type projectiles. The olivine-rich lithologies in the north are possibly ejecta of the ∼90 km diameter Albana crater. We cannot draw a final conclusion on their relative stratigraphic succession, but it seems that the dark material (Nathues et al., 2014b) and the olivine-rich lithologies are of a similar age. The origin of some potential olivine-rich sites in the Rheasilvia basin and at crater Portia are ambiguous, i.e. these are either of endogenic or exogenic origin. However, the small number and size of these sites led us to conclude that olivine-rich mantle material, containing more than 40 wt. % of olivine, is basically absent on the present surface of Vesta. In combination with recent impact models of Veneneia and Rheasilvia (Clenet et al., 2014 and Jutzi et al., 2013), which predict an excavation depth of up to 80 km, we are confident that the crust – mantle depth is significantly deeper than predicted by most evolution models (30 km; Mittlefehldt, 2014) or, alternatively, the olivine-content of the (upper) mantle is lower than our detection limit, which would lead to the conclusion that Vesta’s parent material was already depleted in olivine compared to CI meteorites.

Reference
Nathues et al. (2014) Exogenic Olivine on Vesta from Dawn Framing Camera Color Data. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.09.045]

Copyright Elsevier