Richard A. Kerr

Orbiting Saturn, Cassini had “tasted” the plumes of salty water that spew from Enceladus’s south polar region, so scientists had growing confidence that the plumes tap briny liquid water tens of kilometers beneath the surface. Now, analyses of Cassini measurements of undulations in Enceladus’s gravity field point to a 10-kilometer-thick layer of water beneath the south polar region, if not the entire moon.

Reference
Kerr RA (2014) Cassini Plumbs the Depths of the Enceladus Sea. Science 344:17.
[doi:10.1126/science.344.6179.17]
Reprinted with permission from AAAS

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L. Iess¹ et al. (>10)*
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¹Dipartimento di Ingegneria Meccanica e Aerospaziale, Sapienza Università di Roma, via Eudossiana 18, 00184 Rome, Italy.

The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (10⁶J₂ = 5435.2 ± 34.9, 10⁶C₂₂ = 1549.8 ± 15.6, 1σ) and their ratio (J₂/C₂₂ = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR², where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.

Reference
Iess et al. (2014) The Gravity Field and Interior Structure of Enceladus. Science 344:78.
[doi:10.1126/science.1250551]
Reprinted with permission from AAAS

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Eric Quirico^a et al. (>10)*
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^aInstitut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes / CNRS-INSU, UMR 5274, Grenoble F-38041, France

Insoluble organic matter (IOM) extracted from primitive chondrites is a polyaromatic solid with a structure and composition resembling that of terrestrial kerogens. A survey of its composition and structure has been carried out on a series of 27 CR, CM, CI and ungrouped C2 carbonaceous chondrites (Tagish Lake, Bells, Essebi, Acfer 094) using infrared and multi-wavelength Raman micro-spectroscopy (244, 514 and 785 nm laser excitations). The results show that chondritic IOM from PCA 91008 (CM2), WIS 91600 (CM2), QUE 93005 (CM2), Tagish Lake (C2 ungrouped) and possibly Cold Bokkeveld (CM2) has been subjected to the past action of short duration thermal metamorphism, presumably triggered by impacts. The IOM in most of the CM chondrites that experienced moderate to heavy aqueous alteration may have been slightly modified by collision-induced heating. However, even IOM from chondrites that escaped significant thermal metamorphism displays Raman characteristics consistent with a formation by thermal processing, either in the protosolar disk or in the parent body. An alternative energetic process to thermal heating is ion irradiation. After thoroughly analyzing both these scenarii, no conclusion can be drawn as to which is the most plausible mechanism nor whether the heating process took place prior or after accretion. The results show for the first time that the width of the G band in spectra collected with a 514 nm excitation correlates with the O/C atomic ratio, suggesting a major role of oxygen in the cross-linking of polyaromatic units.

Reference
Quirico et al. (in press) Origin of insoluble organic matter in type 1 and 2 chondrites: new clues, new questions. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.03.025]
Copyright Elsevier

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Jon M. Friedrich^a,b, Michael K. Weisberg^b,c and Mark L. Rivers^d

^aDepartment of Chemistry, Fordham University, Bronx, NY 10458, USA
^bDepartment of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
^cDepartment of Physical Sciences, Kingsborough College and Graduate School of the City University of New York, Brooklyn, NY 11235, USA
^dConsortium for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439, USA

The major geologic process that has shaped the asteroids and led to development of their regoliths is impact. Petrofabrics in ordinary chondrites are undoubtedly the result of impact events on their asteroidal parent bodies and the foliation present in a chondrite serves as an enduring record of the magnitude of the most intense compacting event experienced by the material. An overwhelming majority of chondrites have an internally consistent petrofabric contained within the spatial dimensions of the entire rock, including across clasts or different petrographic domains. This indicates that the magnitude of the most recent impact to have affected the assembled chondrite was significant enough to impart a foliation across all lithologies. Information of any previous impacts is largely lost because of the consistent, realigned foliations. We present X-ray microtomography derived 3D petrofabric intensity and orientation data for three lithologies in the NWA 7298 breccia. The internally inconsistent petrofabrics among differing lithologies indicate that the magnitude of the final impact event was smaller than previous ones. This latter case preserves fabric information recorded during previous impacts and allows a more complete interpretation of the impact history of a local region of the asteroidal parent. We used our data to infer the sequence and intensity of distinct impact events affecting the NWA 7298 parent asteroid. We suggest a near-surface impact debris zone on the H chondrite parent asteroid as an origin for NWA 7298. These observations yield new opportunities for investigating and interpreting the dynamic collisional evolution of asteroids.

Reference
Friedrich JM, Weisberg MK and Rivers ML (2014) Multiple impact events recorded in the NWA 7298 H chondrite breccia and the dynamical evolution of an ordinary chondrite asteroid. Earth and Planetary Science Letters 394:13.
[doi:10.1016/j.epsl.2014.03.016]
Copyright Elsevier

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Sohsuke Ohno¹ et al. (>10)*
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We currently do not have a copyright agreement with this publisher and cannot display the abstract here.

Reference
Ohno S (2014) Production of sulphate-rich vapour during the Chicxulub impact and implications for ocean acidification. Nature Geoscience  7:279.
[doi:10.1038/ngeo2095]

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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

We currently do not have a copyright agreement with this publisher and cannot display the abstract here.

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

We currently do not have a copyright agreement with this publisher and cannot display the abstract here.

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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