¹McCollom Thomas M., ¹Donaldson Christopher.
Astrobiology June 2016, 16(6): 389-406. Link to Article [doi:10.1089/ast.2015.1382]
¹Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, CO 80309

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¹A.B. Sanina et al. (>10)*
Icarus (in Press)    Link to Article [doi:10.1016/j.icarus.2016.06.002]
¹Institute for Space Research of Russian Academy of Sciences, Moscow 117997, Russian Federation
*Find the extensive, full author and affiliation list on the publishers website

We present a method of conversion of the lunar neutron counting rate measured by the Lunar Reconnaissance Orbiter (LRO) Lunar Exploration Neutron Detector (LEND) instrument collimated neutron detectors, to water equivalent hydrogen (WEH) in the top ∼1 meter layer of lunar regolith. Polar maps of the Moon’s inferred hydrogen abundance are presented and discussed.

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¹Hasnaa Chennaoui Aoudjehane, ¹Houda El Kerni,^2,3Wolf Uwe Reimold, ^4,5David Baratoux, ^6,7Christian Koeberl, ⁸Sylvain Bouley,⁹Mohamed Aoudjehane
Meteoritics & Planetary Science (in Press)  Link to Article [DOI: 10.1111/maps.12661]
¹Hassan II University Casablanca, Faculty of Sciences Ain Chock, GAIA Laboratory, Casablanca, Morocco
²Museum für Naturkunde Berlin—Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
³Humboldt-Universität zu Berlin, Berlin, Germany
⁴Géosciences-Environnement-Toulouse, Université Paul Sabatier CNRS & IRD UMR 5563, Toulouse, France
⁵Institut Fondamental d’Afrique Noire Cheikh Anta Diop, Dakar, Senegal
⁶Natural History Museum, Vienna, Austria
⁷Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁸GEOPS—Géosciences Paris Sud—Université Paris Sud—Bât, Orsay Cedex, France
⁹Casablanca, Morocco
Published by arrangement with John Wiley & Sons

Associations between impact structures and meteorite occurrences are rare and restricted to very young structures. Meteorite fragments are often disrupted in the atmosphere, and in most cases, meteorite falls that have been decelerated by atmospheric drag do not form a crater. Furthermore, meteorites are rapidly weathered. In this context, the finding of shatter cones in Jurassic marly limestone in the same location as a recent (105 ± 40 ka) iron meteorite fall near the village of Agoudal (High Atlas Mountains, Morocco) is enigmatic. The shatter cones are the only piece of evidence of a meteorite impact in the area.

The overlap of a meteorite strewn field with the area of occurrence of shatter cones led previous researchers to consider that the meteorite fall was responsible for the formation of shatter cones in the context of formation of one or several small (<100 m) impact craters that had since been eroded. Shatter cones are generally not reported in association with subkilometer-diameter impact craters. Here, we present new field observations and an analysis of the distribution and characteristics of shatter cones, breccia, and meteorites in the Agoudal area. Evidence for local deformation not related to the structural High Atlas tectonics has been observed, such as a vertical to overturned stratum trending N150-N160. New outcrops with exposures of shatter cones are reported and extend the previously known area of occurrence. The area of in situ shatter cones (~0.15 km²) and the strewn field of meteorites are distinct, although they show some overlap. The alleged impact breccia is revealed as calcrete formations. No evidence for a genetic relationship between the shatter cones and the meteorites can be inferred from field observations. The extent of the area where in situ shatter cones and macrodeformation not corresponding to Atlas tectonic deformation are observed suggest that the original diameter of an impact structure could have been between at least 1–3 km. For typical erosion rates in the Atlas region (~0.08 cm yr⁻¹), the period of time required for the erosion of such a structure (1.25–3.75 Ma) is much larger than the age of the meteorite fall. This line of reasoning excludes a genetic link between the shatter cones and the meteorite fall and indicates that the observed shatter cones belong to an ancient impact structure that has been almost entirely eroded.

¹Patrick N. Peplowski,¹Andrew W. Beck,¹David J. Lawrence
Journal of Geophysical Research Planets (in Press)  Link to Article [DOI: 10.1002/2015JE004950]
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
Published by arrangement with John Wiley & Sons

Thermal neutron emissions from the lunar surface provide a direct measure of bulk elemental composition that can be used to constrain the chemical properties of near-surface (depth <1 m) lunar materials. We present a new calibration of the Lunar Prospector thermal neutron map, providing a direct link between measured count rates and bulk elemental composition. The data are used to examine the chemical and mineralogical composition of the lunar surface, with an emphasis on constraining the plagioclase concentration across the highlands. We observe that the regions of lowest neutron absorption, which correspond to estimated plagioclase concentrations of >85%, are generally associated with large impact basins and are colocated with clusters of nearly pure plagioclase identified with spectral reflectance data.

¹Dominik Loroch, ^1,2Alexander Deutsch, ¹Jasper Berndt,³André Bornemann
Meteoritics & Planetary Science (in Press)    Link to Article [DOI: 10.1111/maps.12667]
¹Institut für Mineralogie, Westfälische Wilhelms-Universität Münster (WWU), Muenster, Germany
²Institut für Planetologie, WWU Münster, Muenster, Germany
³Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Hannover, Germany
Published by arrangement with John Wiley & Sons

We present results of an in-situ geochemical study using laser-ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses along a ~4.3 cm long section across the K-Pg event bed, drilled during IODP Expedition 342 at J Anomaly Ridge south of St. John’s, Newfoundland. This section comprises the Maastrichtian with a sharp boundary to the graded, between 1.5 and 1.8 cm thick ejecta layer with totally altered impact glass spherules, which in turn is topped by Danian sediments. The porous and clayey material required elaborate preparation in order to yield reliable data. The ejecta bed shows a highly variable depletion in rare earth elements that even results in strongly subchondritic concentrations. The Ce/Ce* varies strongly (0.81–34), Ni/Cr ranges from 0.38 to 2.79. The maximum platinum group elements (PGE) concentrations are located in one LA-spot exactly at the basis of the ejecta layer; they amount (in μg g−1) to 0.35 (Rh), 1.64 (Pd), 2.79 (Pt), and 0.86 (Au). The Nb/Ta ratio increases in the Ma from ~10 to 35.9 toward the ejecta horizon, which itself has higher Nb, Ta, Zr, and Hf concentrations than the background sedimentation, combined with low Nb/Ta (~5–10), and low Zr/Hf (~20–30). The overall result is that alteration processes changed totally the original geochemical characteristics of this K-Pg spherule bed. To explain the exorbitant element mobility at distances of hundreds of μm, we discuss a combination of mostly reducing redox processes and interaction with organic compounds. This study demonstrates the high potential of in-situ analyses with high spatial resolution at complex geological materials. Moreover, our results indicate that some caution is necessary in determining the projectile type in impactites via PGE ratios.

¹Joseph A. Nuth,²Natasha M. Johnson,^2,3Frank T. Ferguson,²Alicia Carayon
Meteoritics & Planetary Science (in Press)   Link to Article [DOI: 10.1111/maps.12666]
¹Solar System Exploration Division, Code 690, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
²Astrochemistry Laboratory, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
³Chemistry Department, The Catholic University of America, Washington, D.C. 20064, USA
⁴International Space University, Strasbourg Central Campus, France
Published by arrangement with John Wiley & Sons

We report the ratio of the initial carbon available as CO that forms gas-phase compounds compared to the fraction that deposits as a carbonaceous solid (the gas/solid branching ratio) as a function of time and temperature for iron, magnetite, and amorphous iron silicate smoke catalysts during surface-mediated reactions in an excess of hydrogen and in the presence of N2. This fraction varies from more than 99% for an amorphous iron silicate smoke at 673 K to less than 40% for a magnetite catalyst at 873 K. The CO not converted into solids primarily forms methane, ethane, water, and CO2, as well as a very wide range of organic molecules at very low concentration. Carbon deposits do not form continuous coatings on the catalytic surfaces, but instead form extremely high surface area per unit volume “filamentous” structures. While these structures will likely form more slowly but over much longer times in protostellar nebulae than in our experiments due to the much lower partial pressure of CO, such fluffy coatings on the surfaces of chondrules or calcium aluminum inclusions could promote grain–grain sticking during low-velocity collisions.

¹Hu, J., ¹Sharp, T. G
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA

Northwest Africa 757 is unique in the LL chondrite group because of its abundant shock-induced melt and high-pressure minerals. Olivine fragments entrained in the melt transform partially and completely into ringwoodite. Plagioclase and Ca-phosphate transform to maskelynite, lingunite, and tuite. Two distinct shock-melt crystallization assemblages were studied by FIB-TEM analysis. The first melt assemblage, which includes majoritic garnet, ringwoodite plus magnetite-magnesiowüstite, crystallized at pressures of 20–25 GPa. The other melt assemblage, which consists of clinopyroxene and wadsleyite, solidified at ~15 GPa, suggesting a second veining event under lower pressure conditions. These shock features are similar to those in S6 L chondrites and indicate that NWA 757 experienced an intense impact event, comparable to the impact event that disrupted the L chondrite parent body at 470 Ma.

Reference
Hu J, Sharp TG (2016) High-pressure phases in shock-induced melt of the unique highly shocked LL6 chondrite Northwest Africa 757.
Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12672]
Published by arrangement with John Wiley & Sons

¹Rebecca J. Thomas, ^1,2Brian M. Hynek, ³David A. Rothery, ⁴Susan J. Conway
¹Laboratory for Atmospheric and Space Physics, University of Colorado, 3665 Discovery Drive, Boulder, CO 80303, USA
²Department of Geological Sciences, University of Colorado, 399 UCB, Boulder, CO 80309, USA
³Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁴Laboratoire de Planétologie et Géodynamique – UMR CNRS 6112, 2 rue de la Houssinière – BP 92208, 44322 Nantes Cedex 3, France

Unusually low reflectance material, within which depressions known as hollows appear to be actively forming by sublimation, is a major component of Mercury’s surface geology. The observation that this material is exhumed from depth by large impacts has the intriguing implication that the planet’s lower crust or upper mantle contains a significant volatile–rich, low–reflectance layer, the composition of which will be key for developing our understanding of Mercury’s geochemical evolution and bulk composition. Hollows provide a means by which the composition of both the volatile and non–volatile components of the low–reflectance material (LRM) can be constrained, as they result from the loss of the volatile component, and any remaining lag can be expected to be formed of the non–volatile component. However, previous work has approached this by investigating the spectral character of hollows as a whole, including that of bright deposits surrounding the hollows, a unit of uncertain character. Here we use high–resolution multispectral images, obtained as the MESSENGER spacecraft approached Mercury at lower altitudes in the latter part of its mission, to investigate reflectance spectra of inactive hollow floors where sublimation appears to have ceased, and compare this to those of the bright surrounding products and the parent material. This analysis reveals that the final lag after hollow–formation has a flatter spectral slope than that of any other unit on the planet and reflectance approaching that of more space–weathered parent material. This indicates firstly that the volatile material lost has a steeper spectral slope and higher reflectance than the parent material, consistent with (Ca,Mg) sulfides, and secondly, that the low–reflectance component of LRM is non–volatile and may be graphite.

Reference
Thomas RJ, Hynek BM, Rothery DA, Conway SJ (2016) Mercury’s Low-Reflectance Material: Constraints from Hollows. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.036]
Copyright Elsevier

¹Daniel M. Applin, ^1,2,3Matthew R.M. Izawa,¹Edward A. Cloutis
¹Hyperspectral Optical Sensing for Extraterrestrial Reconnaissance Laboratory, Dept. Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada R3B 2E9
²Dept. Earth Sciences, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, Canada L2S 3A1
³Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395

The diversity of oxalate formation mechanisms suggests that significant concentrations of oxalic acid and oxalate minerals could be widely distributed in the solar system. We have carried out a systematic study of the reflectance spectra of oxalate minerals and oxalic acid, covering the 0.2-16 µm wavelength region.. Our analyses show that oxalates exhibit unique spectral features that enable discrimination between oxalate phases and from other commonly occurring compounds, including carbonates, in all regions of the spectrum except for the visible. Using these spectral data, we consider the possible contribution of oxalate minerals to previously observed reflectance spectra of many objects throughout the solar system, including satellites, comets, and asteroids. We find that polycarboxylic acid dimers and their salts may explain the reflectance spectra of many carbonaceous asteroids in the 3 µm spectral region.. We suggest surface concentration of these compounds may be a type of space weathering from the photochemical and oxidative decomposition of the organic polymer found in carbonaceous chondrites. The stability and ubiquity of these minerals on Earth, in extraterrestrial materials, and in association with biological processes make them useful for many applications in Earth and planetary sciences.

Reference
Applin DM, Izawa MRM, Cloutis EA (2016) Reflectance spectroscopy of oxalate minerals and relevance to solar system carbon inventories. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.005]
Copyright Elsevier

¹Paolo A. Sossi, ^1,2Oliver Nebel, ^3,4Mahesh Anand, ⁵Franck Poitrasson
¹Research School of Earth Sciences, Australian National University, Canberra 2601, ACT, Australia
²School of Earth, Atmosphere and Environment, Monash University, Melbourne 3800, VIC, Australia
³Department of Physical Sciences, Open University, Milton Keynes, MK76AA, UK
⁴Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
⁵Laboratoire Géosciences Environnement Toulouse, CNRS UMR 5563 – UPS – IRD, 14-16, Avenue Edouard Belin, 31400, Toulouse, France

Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have View the MathML sourceδFe57≈0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, View the MathML sourceδFe57≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth’s mantle is View the MathML sourceδFe57=+0.04±0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier View the MathML sourceδFe57(+0.05±0.03‰), whereas MgO-rich rocks are lighter (View the MathML sourceδFe57≈−0.01±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the View the MathML sourceδFe57 SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites.

Reference
Sossi PA, Nebel O, Anand A, Poitrasson F (2016) On the iron isotope composition of Mars and volatile depletion in the terrestrial planets. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2016.05.030]
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