Alyssa K. Cobb^1,2 and Ralph E. Pudritz^1,2

¹Origins Institute, McMaster University, ABB 241, 1280 Main Street, Hamilton, ON L8S 4M1, Canada
²Department of Physics and Astronomy, McMaster University, ABB 241, 1280 Main Street, Hamilton, ON L8S 4M1, Canada

The class of meteorites called carbonaceous chondrites are examples of material from the solar system which have been relatively unchanged from the time of their initial formation. These meteorites have been classified according to the temperatures and physical conditions of their parent planetesimals. We collate available data on amino acid abundance in these meteorites and plot the concentrations of different amino acids for each meteorite within various meteorite subclasses. We plot average concentrations for various amino acids across meteorites separated by subclass and petrologic type. We see a predominance in the abundance and variety of amino acids in CM2 and CR2 meteorites. The range in temperature corresponding to these subclasses indicates high degrees of aqueous alteration, suggesting aqueous synthesis of amino acids. Within the CM2 and CR2 subclasses, we identify trends in relative frequencies of amino acids to investigate how common amino acids are as a function of their chemical complexity. These two trends (total abundance and relative frequencies) can be used to constrain formation parameters of amino acids within planetesimals. Our organization of the data supports an onion shell model for the temperature structure of planetesimals. The least altered meteorites (type 3) and their amino acids originated near cooler surface regions. The most active amino acid synthesis likely took place at intermediate depths (type 2). The most altered materials (type 1) originated furthest toward parent body cores. This region is likely too hot to either favor amino acid synthesis or for amino acids to be retained after synthesis.

Reference
Cobb AK and Pudritz RE (2014) Nature’s Starships. I. Observed Abundances and Relative Frequencies of Amino Acids in Meteorites.  The Astrophysical Journal 783:140.
[doi:doi:10.1088/0004-637X/783/2/140132]

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

^aINAF Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere, 00133 Rome, Italy

NASA’s Dawn spacecraft orbited Vesta for approximately one year, collecting thousands of hyperspectral images of its surface. The mission revealed that Vesta’s surface shows the largest variations in surface albedo on asteroids visited thus far, due to the presence of dark and bright materials at the local scale (i.e. 0.1 to 10 km).
The aim of this work is to characterize the photometric properties of bright and dark regions, and thus derive and apply an empirical photometric correction to all the hyperspectral observations of Vesta.
The very large dataset (i.e. more than 20 million spectra) provided by the VIR imaging spectrometer onboard Dawn enabled accurate statistical analysis of the spectral dataset, aimed at retrieving empirical relations between several spectral parameters (i.e. visible and infrared reflectance, band depths, band centers, Band Area Ratio) and the illumination/viewing angles. The derived relations made it possible to derive photometrically corrected maps of these spectral parameters and to infer information on the regolith shadowing effect in the Vestan dark and bright regions. As an additional analysis, we also evaluated the correlation between surface temperature and band center position.
A general conclusion of this analysis is that, from a photometric point of view, the distinction between bright and dark material units lies mainly in the larger contribution due to multiple scattering in the bright units. We observed reflectance and band depth variations over Vesta’s entire surface, but these variations were much larger in the dark regions than in the bright ones.
Band centers have been found to shift to longer wavelengths at increasing temperatures, with a trend that is the same observed for HED meteorites [Reddy et al., 2012. Photometric, spectral phase and temperature effects on 4 Vesta and HED meteorites: Implications for the Dawn mission. Icarus 217, 153-158]. Finally, the Band Area Ratio (i.e. the ratio between areas of the main pyroxene absorption bands located at 1.9 μm and at 0.9 μm, respectively) did not show any dependence on observational geometry, again a behavior similar to laboratory results obtained on HED meteorites [ibid].

Reference
Longobardo et al. (in press) Photometric behavior of spectral parameters in Vesta dark and bright regions as inferred by the Dawn VIR spectrometer. Icarus
[doi:10.1016/j.icarus.2014.02.014]
Copyright Elsevier

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Yutaka Komiya¹, Shimako Yamada², Takuma Suda¹, and Masayuki Y. Fujimoto^3,4

¹National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
²Department of Cosmoscience, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
³Nuclear reaction data center, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
⁴Visiting researcher, Faculty of Engineering, Hokkai-gakuen University, Sapporo, Hokkaido 062-8605, Japan

We investigate the chemical enrichment of r-process elements in the early evolutionary stages of the Milky Way halo within the framework of hierarchical galaxy formation using a semi-analytic merger tree. In this paper, we focus on heavy r-process elements, Ba and Eu, of extremely metal-poor (EMP) stars and give constraints on their astronomical sites. Our models take into account changes of the surface abundances of EMP stars by the accretion of interstellar medium (ISM). We also consider metal-enrichment of intergalactic medium by galactic winds and the resultant pre-enrichment of proto-galaxies. The trend and scatter of the observed r-process abundances are well reproduced by our hierarchical model with ~10% of core-collapse supernovae in low-mass end (~10 M_☉) as a dominant r-process source and the star formation efficiency of ~10^-10 yr–1. For neutron star mergers as an r-process source, their coalescence timescale has to be ~10⁷ yr, and the event rates ~100 times larger than currently observed in the Galaxy. We find that the accretion of ISM is a dominant source of r-process elements for stars with [Ba/H] < –3.5. In this model, a majority of stars at [Fe/H] < –3 are formed without r-process elements, but their surfaces are polluted by the ISM accretion. The pre-enrichment affects ~4% of proto-galaxies, and yet, is surpassed by the ISM accretion in the surface of EMP stars.

Reference
Komiya Y, Yamada S, Suda T and Fujimoto MY (2014) The New Model of Chemical Evolution of r-process Elements Based on the Hierarchical Galaxy Formation. I. Ba and Eu.  The Astrophysical Journal 783:132.
[doi:10.1088/0004-637X/783/2/132]

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F. Scipioni^a, F. Tosi^a, K. Stephan^b, G. Filacchione^a, M. Ciarniello^a, F. Capaccioni^a, P. Cerroni^a, The VIMS Team

^aINAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, I-00133 Roma, Italy;
^bDLR, Institute of Planetary Research, Rutherfordstrasse 2, D-12489 Berlin, Germany.

Rhea is the second largest icy satellites of Saturn and it is mainly composed of water ice. Its surface is characterized by a leading hemisphere slightly brighter than the trailing side. The main goal of this work is to identify homogeneous compositional units on Rhea by applying the Spectral Angle Mapper (SAM) classification technique to Rhea’s hyperspectral images acquired by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini Orbiter in the infrared range (0.88-5.12 μm). The first step of the classification is dedicated to the identification of Rhea’s spectral endmembers by applying the k-meansunsupervised clustering technique to four hyperspectral images representative of a limited portion of the surface, imaged at relatively high spatial resolution. We then identified eight spectral endmembers, corresponding to as many terrain units, which mostly distinguish for water ice abundance and ice grain size. In the second step, endmembers are used as reference spectra in SAM classification method to achieve a comprehensive classification of the entire surface. From our analysis of the infrared spectra returned by VIMS, it clearly emerges that Rhea’ surface units shows differences in terms of water ice bands depths, average ice grain size, and concentration of contaminants, particularly CO₂ and hydrocarbons. The spectral units that classify optically dark terrains are those showing suppressed water ice bands, a finer ice grain size and a higher concentration of carbon dioxide. Conversely, spectral units labeling brighter regions have deeper water ice absorption bands, higher albedo and a smaller concentration of contaminants. All these variations reflect surface’s morphological and geological structures. Finally, we performed a comparison between Rhea and Dione, to highlight different magnitudes of space weathering effects in the icy satellites as a function of the distance from Saturn.

Reference
Scipioni F, Tosi F, Stephan K, Filacchione G, Ciarniello M, Capaccioni F, Cerroni P and The VIMS Team (in press) Spectroscopic classification of icy satellites of Saturn II: Identification of terrain units on Rhea. Icarus
[doi:10.1016/j.icarus.2014.02.010]
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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]
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T. Kelling, G. Wurm, and M. Köster

Faculty of Physics, University Duisburg-Essen, Lotharstr. 1, D-47057 Duisburg, Germany

For dust aggregates in protoplanetary disks, a transition between sticking and bouncing in individual collisions at mm to cm sizes has been observed in the past. This leads to the notion of a bouncing barrier for which growth gets stalled. Here, we present long-term laboratory experiments on the outcome of repeated aggregate collisions at the bouncing barrier. About 100 SiO₂ dust aggregates 1 mm in size were observed interacting with each other. Collisions occurred within a velocity range from below mm s^-1 up to cm s^-1. Aggregates continuously interacted with each other over a period of 900 s. During this time, more than 10⁵ collisions occurred. Nearly 2000 collisions were analyzed in detail. No temporal stable net growth of larger aggregates was observed even though sticking collision occurred. Larger ensembles of aggregates sticking together were formed but were disassembled again during further collisional evolution. The concept of a bouncing barrier supports the formation of planetesimals by seeded collisional growth, as well as by gravitational instability favoring a significant total mass being limited to certain size ranges. Within our parameter set, the experiments confirm that bouncing barriers are one possible and likely evolutionary limit of self-consistent particle growth.

Reference
Kelling T, Wurm G and Köster M (2014) Experimental Study on Bouncing Barriers in Protoplanetary Disks.  The Astrophysical Journal 783:111.
[doi:10.1088/0004-637X/783/2/111]

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J.P. Emery^a, Y.R. Fernández^b, M.S.P. Kelley^c, K.T. Warden (nèe Crane)^a,1, C. Hergenrother^d, D.S. Lauretta^d, M.J. Drake^d, H. Campins^b, J. Ziffer^e

^aEarth and Planetary Science Dept & Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996
^bPhysics Department, University of Central Florida, Orlando, FL 32816
^cDepartment of Astronomy, University of Maryland, College Park, MD 20742-2421
^dDepartment of Planetary Sciences, University of Arizona, Tucson, AZ 85721
^eDepartment of Physics, University of Southern Maine, Portland, ME 04104
¹Department of Earth, Atmospheric, and Planetary Science, Purdue University, West Lafayette, IN 47907.

Near-Earth asteroids (NEAs) have garnered ever increasing attention over the past few years due to the insights they offer into Solar System formation and evolution, the potential hazard they pose, and their accessibility for both robotic and human spaceflight missions. Among the NEAs, carbonaceous asteroids hold particular interest because they may contain clues to how the Earth got its supplies of water and organic materials, and because none has yet been studied in detail by spacecraft. (101955) Bennu is special among NEAs in that it will not only be visited by a spacecraft, but the OSIRIS-REx mission will also return a sample of Bennu’s regolith to Earth for detailed laboratory study. This paper presents analysis of thermal infrared photometry and spectroscopy that test the hypotheses that Bennu is carbonaceous and that its surface is covered in fine-grained (sub-cm) regolith. The Spitzer Space Telescope observed Bennu in 2007, using the Infrared Spectrograph (IRS) to obtain spectra over the wavelength range 5.2 – 38 μm and images at 16 and 22 μm at 10 different longitudes, as well as the Infrared Array Camera (IRAC) to image Bennu at 3.6, 4.5, 5.8, and 8.0 μm, also at 10 different longitudes. Thermophysical analysis, assuming a spherical body with the known rotation period and spin-pole orientation, returns an effective diameter of 484±10 m, in agreement with the effective diameter calculated from the radar shape model at the orientation of the Spitzer observations (492±20 m, Nolan et al. 2013) and a visible geometric albedo of 0.046±0.005 (using H_v=20.51, Hergenrother et al. 2013). Including the radar shape model in the thermal analysis, and taking surface roughness into account, yields a disk-averaged thermal inertia of 310±70 J m^-2K^-1s^-1/2, which is significantly lower than several other NEAs of comparable size. There may be a small variation of thermal inertia with rotational phase (±60 J m^-2K^-1s^-1/2). The spectral analysis is inconclusive in terms of surface mineralogy; the emissivity spectra have a relatively low signal-to-noise ratio and no spectral features are detected. The thermal inertia indicates average regolith grain size on the scale of several millimeters to about a centimeter. This moderate grain size is also consistent with low spectral contrast in the 7.5 – 20 μm spectral range. If real, the rotational variation in thermal inertia would be consistent with a change in average grain size of only about a millimeter. The thermophysical properties of Bennu’s surface appear to be fairly homogeneous longitudinally. A search for a dust coma failed to detect any extended emission, putting an upper limit of about 10⁶ g of dust within 4750 km of Bennu. Three common methodologies for thermal modeling are compared, and some issues to be aware of when interpreting the results of such models are discussed. We predict that the OSIRIS-REx spacecraft will find a low albedo surface with abundant sub-cm sized grains, fairly evenly distributed in longitude.

Reference
Emery JP, Fernández YR, Kelley MSP, Warden (nèe Crane) KT, Hergenrother C, Lauretta DS, Drake MJ, Campins H and Ziffer J (in press) Thermal Infrared Observations and Thermophysical Characterization of OSIRIS-REx Target Asteroid (101955) Bennu. Icarus
[doi:10.1016/j.icarus.2014.02.005]
Copyright Elsevier

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