^1,2,3H. Downes, ⁴F. A. J. Abernethy, ³C. L. Smith, ^2,3,5A. J. Ross, ⁴A. B. Verchovsky, ⁴M. M. Grady, ⁶P. Jenniskens,⁷M. H. Shaddad
¹Department of Earth and Planetary Sciences, Birkbeck University of London, London, UK
²UCL/Birkbeck Centre for Planetary Sciences, UCL, London, UK
³Department of Earth Sciences, Natural History Museum, London, UK
⁴Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, UK
⁵Department of Earth Sciences, University College London, London, UK
⁶SETI Institute, Carl Sagan Centre, Mountain View, California, USA
⁷Department of Physics, University of Khartoum, Khartoum, Sudan

This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon-rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ13C from −7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ13C = ca −25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ15N values of −53 to −94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen-rich and isotopically heavier. Thus, although the parent asteroid 2008TC3 was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ13C and δ15N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.

Reference
Downes H, Abernethy FAJ, Smith CL, Ross AJ, Verchovsky AB, Grady MM, Jenniskens P, Shaddad MH (2015) Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta Meteorite. Meteoritics and Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12413]
Published by arrangement with John Wiley&Sons

¹James F. J. Bryson, ¹Claire I. O. Nichols,^2,3Julia Herrero-Albillos,⁴Florian Kronast,⁵Takeshi Kasama,⁵Hossein Alimadadi,⁶Gerrit van der Laan,⁷Francis Nimmo¹Richard J. Harrison
¹Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
²Centro Universitario de la Defensa, Carretera de Huesca s/n, E-50090 Zaragoza, Spain
³Instituto de Ciencia de Materiales de Aragón, CSIC—Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain
⁴Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, 12489 Berlin, Germany
⁵Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
⁶Diamond Light Source, Chilton, Didcot, Oxfordshire OX11 0DE, UK
⁷Department of Earth and Planetary Sciences, University of California, Santa Cruz, California 95064, USA

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

Bryson JFJ, Nichols CIO, Herrero-Albillos J, Kronast F, Kasama T, Alimadadi H, van der Laan G, Nimmo F, Harrison RJ (2015) Long-lived magnetism from solidification-driven convection on the pallasite parent Body. Nature 517, 472–475
Link to Article [doi:10.1038/nature14114]

¹M. Millot et al. (>10)*
¹Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
*Find the extensive, full author and affiliation list on the publishers Website

Deep inside planets, extreme density, pressure, and temperature strongly modify the properties of the constituent materials. In particular, how much heat solids can sustain before melting under pressure is key to determining a planet’s internal structure and evolution. We report laser-driven shock experiments on fused silica, α-quartz, and stishovite yielding equation-of-state and electronic conductivity data at unprecedented conditions and showing that the melting temperature of SiO2 rises to 8300 K at a pressure of 500 gigapascals, comparable to the core-mantle boundary conditions for a 5–Earth mass super-Earth. We show that mantle silicates and core metal have comparable melting temperatures above 500 to 700 gigapascals, which could favor long-lived magma oceans for large terrestrial planets with implications for planetary magnetic-field generation in silicate magma layers deep inside such planets.

Reference
Millot M (2015) Shock compression of stishovite and melting of silica at planetary interior conditions. Science 347, 6220, 418-420
Link to Article [DOI: 10.1126/science.1261507]

Published with permission from AAAS

¹P. R. Mahaffy et al. (>10)*
¹Planetary Environments Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
*Find the extensive, full author and affiliation list on the publishers Website

The deuterium-to-hydrogen (D/H) ratio in strongly bound water or hydroxyl groups in ancient martian clays retains the imprint of the water of formation of these minerals. Curiosity’s Sample Analysis at Mars (SAM) experiment measured thermally evolved water and hydrogen gas released between 550° and 950°C from samples of Hesperian-era Gale crater smectite to determine this isotope ratio. The D/H value is 3.0 (±0.2) times the ratio in standard mean ocean water. The D/H ratio in this ~3-billion-year-old mudstone, which is half that of the present martian atmosphere but substantially higher than that expected in very early Mars, indicates an extended history of hydrogen escape and desiccation of the planet.

Reference
Mahaffy PR et al. (2015) The imprint of atmospheric evolution in the D/H of Hesperian clay minerals on Mars.
Science 347, 6220, 412-414
Link to Article [DOI: 10.1126/science.1260291]

Reprinted with permission from AAAS

¹Alessandra Rotundi et al. (>10)*
¹Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica (INAF), Via Fosso del Cavaliere, 100, 0133 Rome, Italy.
*Find the extensive, full author and affiliation list on the publishers website

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10−10 to 10−7 kilograms, and 48 grains of mass 10−5 to 10−2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

Reference
Rotundi A. et al. (2015) Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun. Science 347, 6220
Link to Article [DOI: 10.1126/science.aaa3905]
Printed with permission from AAAS

¹F. Capaccioni et al. (>10)*
¹Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica (INAF), Rome, Italy.
*Find the extensive, full author and affiliation list on the publishers Website

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ−1), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

Reference
Capaccioni F. et al. (2015) The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta. Science 347, 6220
Link to Article [DOI: 10.1126/science.aaa0628]

Reprinted with permission of AAAS

¹Tsubasa Tobase, ¹Akira Yoshiasa, ¹Ling Wang, ¹Hidetomo Hongu, ¹Hiroshi Isobe, ²Ritsuro Miyawaki
¹Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan
²Department of Geology and Paleontology, National Museum of Science, 4-1-1, Amakubo, Tsukuba 305-0005, Japan

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Reference
Tobase T, Yoshiasa A, Wang L, Hongu H, Isobe H, Miyawaki R (2015) XAFS study on the Zr local structures in tektites and natural glasses. Journal of Mineralogical and Petrological Sciences (in Press)
Link to Article [http://dx.doi.org/10.2465/jmps.140317]

^1,2Dennis Harries, ³Peter Hoppe ¹Falko Langenhorst
¹Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
²Bayerisches Geoinstitut, Universität Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany
³Max-Planck-Institut für Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany

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Reference
Harries D, Hoppe P, Langenhorst F (2015) Reactive ammonia in the solar protoplanetary disk and the origin of Earth’s Nitrogen. Nature Geoscience (in Press)
Link to Article [doi:10.1038/ngeo2339]

¹Fara N. Lindsay, ¹Jeremy S. Delaney, ¹Gregory F. Herzog,²Brent D. Turrin, ^1,3Jisun Park, ²Carl C. Swisher III
¹Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854, USA
²Department of Earth and Planetary Science, Rutgers University, Piscataway, NJ 08854, USA
³Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA

We report 40Ar/39Ar ages for several lithological components of the brecciated howardite Kapoeta and compare the ages with results for asteroid 4 Vesta as observed by the Dawn mission. Our Kapoeta sample has an unusual, millimeter wide glass vein that intruded into a complex breccia. The plateau ages of three lithic clasts of basaltic composition that were remote from the glass vein range from 4.2 to 4.5 Ga. Such ages are typical of eucritic material; the oldest reflects early magmatic crystallization (∼4.5 Ga∼4.5 Ga), the younger (4.2–4.5 Ga) reflect magmatism associated with protracted cooling. Samples of the glass vein itself, which include relict grains, give apparent ages between 3.1 and 3.9 Ga as do chips from the matrix. We consider both glass and bulk matrix ages as mixing ages; not marking the time of a single event, but dating regolith activity (<3.1–4.1 Ga<3.1–4.1 Ga).
Eight feldspar grains close to the glass vein give markedly younger plateau ages averaging 1.4 Ga. The Ar release spectra for glass vein and breccia subsamples indicate a disturbance in the last 1.4 Ga. Taken together, these younger ages suggest a recent, major thermal event in the history of howardites that has been under-reported – perhaps the impact that formed the Rheasilvia basin on Vesta.

Reference
Lindsay FN, Delaney JS, Herzog GF, Turrin BD, Park J, Swisher III CC (2015) Rheasilvia provenance of the Kapoeta howardite inferred from ∼1 Ga 40Ar/39Ar feldspar Ages. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2014.12.049]

Copyright Elsevier

¹Kathleen E. Vander Kaaden,²Francis M. McCubbin
¹Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM, USA
²Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM, USA

The range in density and compressibility of mercurian melt compositions was determined to better understand the products of a possible mercurian magma ocean and subsequent volcanism. Our experiments indicate that the only mineral to remain buoyant with respect to melts of the mercurian mantle is graphite; consequently, it is the only candidate mineral to have composed a primary floatation crust during a global magma ocean. This exotic result is further supported by Mercury’s volatile-rich nature and inexplicably darkened surface. Additionally, our experiments illustrate that partial melts of the mercurian mantle that compose the secondary crust were buoyant over the entire mantle depth and could have come from as deep as the core-mantle boundary. Furthermore, Mercury could have erupted higher percentages of its partial melts compared to other terrestrial planets because magmas would not have stalled during ascent due to gravitational forces. These findings stem from the FeO-poor composition and shallow depth of Mercury’s mantle, which has resulted in both low melt density and a very limited range in melt density responsible for Mercury’s primary and secondary crusts. The enigmatically darkened, yet low-FeO surface, which is observed today can be explained by secondary volcanism and impact processes that have since mixed the primary and secondary crustal materials.

Reference
Vander Kaaden KE, McCubbin FM (2015) Exotic Crust Formation on Mercury: Consequences of a Shallow, FeO-poor Mantle. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004733]

Published by arrangement with John Wiley&Sons