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]
Copyright Elsevier

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