Timothy A. Goudge¹ et al. (>10)*
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¹Department of Geological Sciences, Brown University, Providence, Rhode Island, USA

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than ~400 nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

Reference
Goudge et al. (in press) Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004480]
Published by arrangement with John Wiley & Sons

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William B. McKinnon

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Reference
McKinnon WB (2014) Planetary science: Shrinking wrinkling Mercury. Nature Geoscience 7:251.
[doi:10.1038/ngeo2123]

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Paul K. Byrne, Christian Klimczak, A. M. Celâl Şengör, Sean C. Solomon, Thomas R. Watters & Steven A. Hauck, II

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Reference
Byrne PK, Klimczak C, Şengör AMC, Solomon SC, Watters TR & Hauck, II SA (2014) Mercury’s global contraction much greater than earlier estimates. Nature Geoscience 7:301.
[doi:10.1038/ngeo2097]

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Y. J. Liu¹, K. F. Tan¹, L. Wang¹, G. Zhao¹, Bun’ei Sato² Y. Takeda³ and H. N. Li¹

¹Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, China
²Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
³National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

The lithium abundances for 378 G/K giants are derived with non-local thermodynamic equilibrium correction considered. Among these are 23 stars that host planetary systems. The lithium abundance is investigated, as a function of metallicity, effective temperature, and rotational velocity, as well as the impact of a giant planet on G/K giants. The results show that the lithium abundance is a function of metallicity and effective temperature. The lithium abundance has no correlation with rotational velocity at v sin  i < 10 km s^-1. Giants with planets present lower lithium abundance and slow rotational velocity (v sin  i < 4 km s^-1). Our sample includes three Li-rich G/K giants, 36 Li-normal stars, and 339 Li-depleted stars. The fraction of Li-rich stars in this sample agrees with the general rate of less than 1% in the literature, and the stars that show normal amounts of Li are supposed to possess the same abundance at the current interstellar medium. For the Li-depleted giants, Li-deficiency may have already taken place at the main sequence stage for many intermediate mass (1.5–5 M_☉) G/K giants. Finally, we present the lithium abundance and kinematic parameters for an enlarged sample of 565 giants using a compilation of the literature, and confirm that the lithium abundance is a function of metallicity and effective temperature. With the enlarged sample, we investigate the differences between the lithium abundance in thin-/thick-disk giants, which indicate that the lithium abundance in thick-disk giants is more depleted than that in thin-disk giants.

Reference
Liu YJ, Tan KF, Wang L, Zhao G, Sato B, Takeda Y and Li HN (2014) The Lithium Abundances of a Large Sample of Red Giants. The Astrophysical Journal 785:94.
[doi:10.1088/0004-637X/785/2/94]

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Joseph A. Burns

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Reference
Chambers J (2014) Solar System: Ring in the new. Nature 508:48.
[doi:10.1038/nature13218]

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Chambers J (2014) Planetary science: A chronometer for Earth’s age. Nature 508:51.
[doi:10.1038/508051a]

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F. Braga-Ribas et al. (>10)*
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Reference
Braga-Ribas et al. (2014) A ring system detected around the Centaur (10199) Chariklo. Nature 508:72
[doi:10.1038/nature13155]

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John Chambers

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Reference
Chambers J (2014) Planetary science: A chronometer for Earth’s age. Nature 508:51.
[doi:10.1038/508051a]

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Seth A. Jacobson, Alessandro Morbidelli, Sean N. Raymond, David P. O’Brien, Kevin J. Walsh and David C. Rubie

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Reference
Jacobson SA, Morbidelli A, Raymond SN, O’Brien DP, Walsh KJ and Rubie DC (2014) Highly siderophile elements in Earth’s mantle as a clock for the Moon-forming impact. Nature 508:84.
[doi:10.1038/nature13172]

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