¹Gian Paolo Sighinolfi, ^2,3Emanuela Sibilia, ¹Gabriele Contini,²Marco Martini
¹Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio E., Modena, Italy
²Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, Milano, Italy
³INFN, Sezione di Milano Bicocca, Milano, Italy

Thermoluminescence (TL) dating has been used to determine the age of the meteorite impact crater at Gebel Kamil (Egyptian Sahara). Previous studies suggested that the 45 m diameter structure was produced by a fall in recent times (less than 5000 years ago) of an iron meteorite impactor into quartz-arenites and siltstones belonging to the Lower Cretaceous Gilf Kebir Formation. The impact caused the complete fragmentation of the impactor, and the formation of a variety of impactites (e.g., partially vitrified dark and light materials) present as ejecta within the crater and in the surrounding area. After a series of tests to evaluate the TL properties of different materials including shocked intra-crater target rocks and different types of ejecta, we selected a suite of light-colored ejecta that showed evidence of strong thermal shock effects (e.g., partial vitrification and the presence of high-temperature and -pressure silica phases). The abundance of quartz in the target rocks, including the vitrified impactites, allowed TL dating to be undertaken. The variability of radioactivity of the intracrateric target rocks and the lack of direct in situ dosimetric evaluations prevented precise dating; it was, however, possible to constrain the impact in the 2000 BC–500 AD range. If, as we believe, the radioactivity measured in the fallback deposits is a reliable estimate of the mean radioactivity of the site, the narrower range 1600–400 BC (at the 2σ confidence level) can be realistically proposed.

Reference
Sighinolfi GP, Sibilia E, Contini G, Martini M (2015) Thermoluminescence dating of the Kamil impact crater (Egypt). Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12417]

Published by arrangement with John Wiley&Sons

¹Pavel P. Povinec et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website
¹Department of Nuclear Physics and Biophysics, Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia

On February 15, 2013, after the observation of a brilliant fireball and a spectacular airburst over the southern Ural region (Russia), thousands of stones fell and were rapidly recovered, bringing some extremely fresh material for scientific investigations. We undertook a multidisciplinary study of a dozen stones of the Chelyabinsk meteorite, including petrographic and microprobe investigations to unravel intrinsic characteristics of this meteorite. We also study the short and long-lived cosmogenic radionuclides to characterize the initial meteoroid size and exposure age. Petrographic observations, as well as the mineral compositions obtained by electron microprobe analyses, allow us to confirm the classification of the Chelyabinsk meteorite as an LL5 chondrite. The fragments studied, a few of which are impact melt rocks, contain abundant shock melt veins and melt pockets. It is likely that the catastrophic explosion and fragmentation of the Chelyabinsk meteoroid into thousands of stones was in part determined by the initial state of the meteoroid. The radionuclide results obtained show a wide range of concentrations of 14C, 22Na, 26Al, 54Mn, 57Co, 58Co, and 60Co, which indicate that the pre-atmospheric object had a radius >5 m, consistent with other size estimates based on the magnitude of the airburst caused by the atmospheric entry and breakup of the Chelyabinsk meteoroid. Considering the observed 26Al activities of the investigated samples, Monte Carlo simulations, and taking into account the 26Al half-life (0.717 Myr), the cosmic-ray exposure age of the Chelyabinsk meteorite is estimated to be 1.2 ± 0.2 Myr. In contrast to the other radionuclides, 14C showed a very large range only consistent with most samples having been exposed to anthropogenic sources of 14C, which we associate with radioactive contamination of the Chelyabinsk region by past nuclear accidents and waste disposal, which has also been confirmed by elevated levels of anthropogenic 137Cs and primordial 40K in some of the Chelyabinsk fragments.

Reference
Povinec PP et al. (2015) Cosmogenic radionuclides and mineralogical properties of the Chelyabinsk (LL5) meteorite: What do we learn about the meteoroid? Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12419]

Published by arrangement with John Wiley&Sons

^1,2A. Calzada-Diaz, ³K. H. Joy, ^1,2I. A. Crawford, ^2,4T. A. Nordheim
¹Department of Earth and Planetary Sciences, Birkbeck College, London, UK
²Centre for Planetary Sciences UCL/Birkbeck, London, UK
³School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
⁴Mullard Space Science Laboratory, University College London, Dorking, UK

Lunar meteorites provide important new samples of the Moon remote from regions visited by the Apollo and Luna sample return missions. Petrologic and geochemical analysis of these meteorites, combined with orbital remote sensing measurements, have enabled additional discoveries about the composition and age of the lunar surface on a global scale. However, the interpretation of these samples is limited by the fact that we do not know the source region of any individual lunar meteorite. Here, we investigate the link between meteorite and source region on the Moon using the Lunar Prospector gamma ray spectrometer remote sensing data set for the elements Fe, Ti, and Th. The approach has been validated using Apollo and Luna bulk regolith samples, and we have applied it to 48 meteorites excluding paired stones. Our approach is able broadly to differentiate the best compositional matches as potential regions of origin for the various classes of lunar meteorites. Basaltic and intermediate Fe regolith breccia meteorites are found to have the best constrained potential launch sites, with some impact breccias and pristine mare basalts also having reasonably well-defined potential source regions. Launch areas for highland feldspathic meteorites are much less well constrained and the addition of another element, such as Mg, will probably be required to identify potential source regions for these.

Reference
Calzada-Diaz A, Joy KH, Crawford IA, Nordheim TA (2015) Constraining the source regions of lunar meteorites using orbital geochemical data. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12412]
Published by arrangement with John Wiley&Sons

^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]