Edward D. Young

Department of Earth, Planetary, and Space Sciences, University of California Los Angeles, 595 Charles E. Young Drive East, Los Angeles, CA 90095, United States

Apparent excesses in early-solar ²⁶Al, ³⁶Cl, ⁴¹Ca, and ⁶⁰Fe disappear if one accounts for ejecta from massive-star winds concentrated into dense phases of the ISM in star-forming regions. The removal of apparent excesses is evident when wind yields from Wolf–Rayet stars are included in the plot of radionuclide abundances vs. mean life. The resulting trend indicates that the solar radionuclides were inherited from parental molecular clouds with a characteristic residence time of 10⁸ yr. This residence time is of the same order as the present-day timescale for conversion of molecular cloud material into stars. The concentrations of these extinct isotopes in the early solar system need not signify injection from unusual proximal stellar sources, but instead are well explained by normal concentrations in average star-forming clouds. The results imply that the efficiency of capture is greater for stellar winds than for supernova ejecta proximal to star-forming regions.

Reference
Young ED (2014) Inheritance of solar short- and long-lived radionuclides from molecular clouds and the unexceptional nature of the solar system. Earth and Planetary Science Letters 392:16–27.
[doi:10.1016/j.epsl.2014.02.014]
Copyright Elsevier

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Alexandra I. Blinova¹, Thomas J. Zega^2,†, Christopher D. K. Herd¹, Rhonda M. Stroud²

¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
²Materials Science and Technology Division, Naval Research Laboratory, Washington, District of Columbia, USA
^†Department of of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

Four samples (TL5b, TL11h, TL11i, and TL11v) from the pristine collection of the Tagish Lake meteorite, an ungrouped C2 chondrite, were studied to characterize and understand its alteration history using EPMA, XRD, and TEM. We determined that samples TL11h and TL11i have a relatively smaller proportion of amorphous silicate material than sample TL5b, which experienced low-temperature hydrous parent-body alteration conditions to preserve this indigenous material. The data suggest that lithic fragments of TL11i experienced higher degrees of aqueous alteration than the rest of the matrix, based on its low porosity and high abundance of coarse- and fine-grained sheet silicates, suggesting that TL11i was present in an area of the parent body where alteration and brecciation were more extensive. We identified a coronal, “flower”-like, microstructure consisting of a fine-grained serpentine core and coarse-grained saponite-serpentine radial arrays, suggesting varied fluid chemistry and crystallization time scales. We also observed pentlandite with different morphologies: an exsolved morphology formed under nebular conditions; a nonexsolved pentlandite along grain boundaries; a “bulls-eye” sulfide morphology and rims around highly altered chondrules that probably formed by multiple precipitation episodes during low-temperature aqueous alteration (≥100 °C) on the parent body. On the basis of petrologic and mineralogic observations, we conclude that the Tagish Lake parent body initially contained a heterogeneous mixture of anhydrous precursor minerals of nebular and presolar origin. These materials were subjected to secondary, nonpervasive parent-body alteration, and the samples studied herein represent different stages of that hydrous alteration, i.e., TL5b (the least altered) < TL11h < TL11i (the most altered). Sample TL11v encompasses the petrologic characteristics of the other three specimens.

Reference
Blinova AI, Zega TJ, Herd CDK and Stroud RM (in press) Testing variations within the Tagish Lake meteorite—I: Mineralogy and petrology of pristine samples. Meteoritics & Planetary Science
[doi:10.1111/maps.12271]
Published by arrangement with John Wiley & Sons

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Michael D. Hicks^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aJet Propulsion Laboratory, California Inst. of Technology, 4800 Oak Grove Dr. 183-501, Pasadena, CA 91109, USA.

In anticipation of the Dawn Mission to 4 Vesta, we conducted a ground-based campaign of Bessel BVRI filter photometry of five V-type near-Earth asteroids over a wide range of solar phase angles. We also obtained medium-resolution optical spectroscopy (0.38 μm < λ < 0.92 μm; R∼500) of sixteen near-Earth and main-belt V-type asteroids in order to investigate their spectral diversity and to draw connections between spacecraft data of Vesta and V-type asteroids. Our disk-integrated photometry extended the excursion in solar phase angle beyond the maximum of 24° available from Earth for Vesta to 87 degrees, which is more typical of the geometry during the Dawn approach and mapping phases. The majority of our broad-band observations were obtained at the JPL 0.6-m Table Mountain Observatory but multiple nights were also contributed by the Calar Alto 1.2-m and 2.2-m telescopes, as well as by the Purple Mountain 1-m Schmidt. Our results include a determination of rotation periods for 4 asteroids, identification of a binary candidate and four new V-type asteroids, including a confirmation of two main-belt V-type asteroids beyond the Jupiter 1:3 resonance (Cruikshank et al., 1991, Lazzaro et al., 2000, Roig and Gil-Hutton, 2006 and Moskovitz et al., 2008). This latter finding supports the hypothesis that some vestoids may be crustal fragments of a disrupted basaltic parent body compositionally similar to 4 Vesta. We also obtained rotationally resolved medium resolution spectra of Vesta during the Dawn orbit insertion phase, which will be valuable for calibration and comparison of spacecraft data. Modeling of a composite V-type asteroid phase curve yielded a generic photometric model for V asteroids. We also find that a significant amount of the spectral diversity in the V class comes from changes in solar phase angle. A fit of a composite solar phase curve containing our vestoid observations, previously published groundbased observations of Vesta, and early disk-integrated Dawn observations show important differences with other asteroids. The macroscopic surface roughness of V-type asteroids is significantly larger than that of C-type or S-types ( Helfenstein and Veverka, 1989). This result is consistent with radar studies showing that igneous rocky asteroids – the E and V types – exhibit the largest surface roughness ( Benner et al., 2008). The effects of what appears to be space weathering can be largely explained by phase reddening in our collection of V-type NEOs, but our finding that smaller vestoids, which have shorter lifetimes, are more similar to Vesta suggests that some type of alteration of the surface through time occurs. Our observations confirm that the south polar region of Vesta has a more diogenitic composition than its equatorial regions. The south pole, which is dominated by a large impact feature, thus may offer a view into the interior of Vesta. We derive a visible phase integral of 0.44±0.02 and a corresponding Bond albedo of 0.15±0.03 from our composite V-type asteroid solar phase curve.

Reference
Hicks et al. (in press) Spectral diversity and photometric behavior of main-belt and near-Earth vestoids and (4) Vesta: a study in preparation for the Dawn encounter. Icarus
[doi:10.1016/j.icarus.2013.11.011]
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David A. Williams^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aSchool of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287-14047996

We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (~63 km diam.) and Calpurnia (~53 km diam.) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between ~40-60 Ma (depending upon choice of chronology system), and Marcia’s ejecta blanket ranges in age between ~120-390 Ma (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of ~280-990 Ma based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta’s ancient crust preserved on Vestalia Terra.

Reference
Williams et al. (in press) The Geology of the Marcia Quadrangle of Asteroid Vesta: Assessing the Effects of Large, Young Craters. (101955) Bennu. Icarus
[doi:10.1016/j.icarus.2014.01.033]
Copyright Elsevier

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