^1,2Derek W.G. Sears, ^1,2Hazel Sears, ^1,2Daniel R. Ostrowski, ^1,2Kathryne L. Bryson,
¹Jessie Dotson, ³Megan Syal, ³Damian C Swift
¹NASA Ames Research Center, Mountain View, CA 94035, USA
²Bay Area Environmental Research Institute, NASA Ames Research Center, Mountain View, CA 94035, USA
³Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA

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Reference
Sears DWG, Sears H, Ostrowski DR, Bryson KL, Dotson J, Syal M, Swift DC (2016) A meteorite perspective on asteroid hazard mitigation. Planetary and Space Science (in Press)
Link to Article [doi:10.1016/j.pss.2016.01.016]

¹Yang Liu, ²Jeffrey G. Catalano
¹Southwest Research Institute, San Antonio, Texas, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri, USA

Using visible and near infrared data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we identified a sequence of hydrated sulfates and Fe/Mg-smectites in southwest Melas Chasma. Specifically, these hydrated sulfate and semectite deposits are interbedded and have been highly deformed. Equilibrium thermodynamic calculations of coupled basalt weathering and fluid evaporation predict that sequential formation of Fe/Mg-smectites and sulfate evaporites in similar quantities, as observed in the interbedded smectite-sulfate sequences, is chemically plausible. The Fe/Mg-smectite-sulfate deposits may have thus formed through in situ basalt weathering and fluid evaporation, although an origin involving repeated cycles of transport and deposition of detrital clays by a neutral fluid containing Mg and SO4 and subsequent evaporation cannot be ruled out. If the Fe/Mg-smectites are authigenic, the Hesperian (or younger) age Melas Chasma would have experienced prolonged periods with near-surface water, providing potentially habitable conditions.

Reference
Liu Y, Catalano JG (2016) Implications for the Aqueous History of Southwest Melas Chasma, Mars as Revealed by Interbedded Hydrated Sulfate and Fe/Mg-Smectite Deposits. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.02.015]
Copyright Elsevier

¹Tsuyoshi Iizuka, ²Yi-Jen Lai, ^2,3Waheed Akram, ⁴Yuri Amelin, ^2,3Maria Schönbächler
¹Department of Earth and Planetary Science, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
²Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
³School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁴Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia

Niobium-92 is an extinct proton-rich nuclide, which decays to 92Zr with a half-life of 37 Ma. This radionuclide potentially offers a unique opportunity to determine the timescales of early Solar System processes and the site(s) of nucleosynthesis for p-nuclei, once its initial abundance and distribution in the Solar System are well established. Here we present internal Nb–Zr isochrons for three basaltic achondrites with known U–Pb ages: the angrite NWA 4590, the eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the relative Nb–Zr isochron ages of the three meteorites are consistent with the time intervals obtained from the Pb–Pb chronometer for pyroxene and plagioclase, indicating that 92Nb was homogeneously distributed among their source regions. The Nb–Zr and Pb–Pb data for NWA 4590 yield the most reliable and precise reference point for anchoring the Nb–Zr chronometer to the absolute timescale: an initial 92Nb/93Nb ratio of (1.4±0.5)×10−5(1.4±0.5)×10−5 at 4557.93±0.36 Ma4557.93±0.36 Ma, which corresponds to a 92Nb/93Nb ratio of (1.7±0.6)×10−5(1.7±0.6)×10−5 at the time of the Solar System formation. On the basis of this new initial ratio, we demonstrate the capability of the Nb–Zr chronometer to date early Solar System objects including troilite and rutile, such as iron and stony-iron meteorites. Furthermore, we estimate a nucleosynthetic production ratio of 92Nb to the p-nucleus 92Mo between 0.0015 and 0.035. This production ratio, together with the solar abundances of other p-nuclei with similar masses, can be best explained if these light p-nuclei were primarily synthesized by photodisintegration reactions in Type Ia supernovae.

Reference
Iizuka I, Lai Y-J, Akram W, Amelin Y, Schönbächler M (2016) The initial abundance and distribution of 92Nb in the Solar System. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2016.02.005]
Copyright Elsevier

¹G. W. Evatt, ¹M. J. Coughlan, ²K. H. Joy, ³A. R. D. Smedley, ³P. J. Connolly,
¹I. D. Abrahams
¹UK School of Mathematics, University of Manchester, Manchester M13 9PL, UK
²The School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK
³Centre for Atmospheric Science, The School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK

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Reference
Evatt GW, Coughlan MJ, Joy KH, Smedley ARD, Connolly PJ, Abrahams ID (2016) A potential hidden layer of meteorites below the ice surface of Antarctica. Nature Communications 7, Article number: 10679
Link to Article [doi:10.1038/ncomms10679]

¹Michael A. Velbel
¹Division of Meteorites, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th and Constitution Avenues NW, Washington, DC 20560-0119

Several nakhlites (clinopyroxenite meteorites from Mars) contain olivine phenocrysts with corrosion features identical in size, shape and distribution to the smaller etch-pits of well-characterized weathered terrestrial olivine. Miller Range (MIL) 03346 is an Antarctic nakhlite find, recovered after long exposure to Antarctic conditions. The distribution of discrete olivine etch-pits almost exclusively within a few hundred microns of allocation MIL 03346,171’s documentably exposed surface suggests that they formed by terrestrial weathering in Antarctica. The small size of olivine etch-pits in MIL 03346,171 relative to commonly much larger etch-pits in even incipiently weathered terrestrial examples suggests that the duration of its exposure to weathering conditions was short, or the weathering conditions to which it was exposed did not favor olivine corrosion (in the form of etch-pit formation), or both. Time-scales for the formation of etch-pits, estimated from experimentally determined dissolution rates of olivine over a range of pHs, are comparable to the measured terrestrial age of the meteorite and short relative to the time available for possible similar corrosion on Mars. Etch-pits of the observed size on MIL 03346 olivine phenocrysts would be relatively easy to form supraglacially under brief episodic acidic Antarctic conditions, but the terrestrial age of MIL 03346 is long enough that its olivine might have been weathered to the observed state by englacial films of alkaline Antarctic water. The paucity of similar etch-pits in olivine from the interior of MIL 03346 suggests that olivine in this Mars meteorite was exposed to even less aqueous alteration after iddingsitization during its 1.3 billion years on Mars than its exterior was subjected to during its Pleistocene-Holocene exposure to Antarctic weathering conditions.

Reference
Velbel MA (2016) Aqueous Corrosion of Olivine in the Mars Meteorite Miller Range (MIL) 03346 During Antarctic Weathering: Implications for Water on Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.036]
Copyright Elsevier

¹Agnese Fazio, ¹Massimo D’Orazio, ^2,3Carole Cordier, ¹Luigi Folco
¹Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy
²Université de Grenoble Alpes, ISTerre, BP 53, F-38041 Grenoble CEDEX 9, France
³CNRS, ISTerre, BP 53, F-38041 Grenoble CEDEX 9, France

In small meteorite impacts, the projectile may survive through fragmentation; in addition, it may melt, and chemically and physically interact with both shocked and melted target rocks. However, the mixing/mingling between projectile and target melts is a process still not completely understood. Kamil Crater (45 m in diameter; Egypt), generated by the hypervelocity impact of the Gebel Kamil Ni-rich ataxite on sandstone target, allows to study the target-projectile interaction in a simple and fresh geological setting. We conducted a petrographic and geochemical study of macroscopic impact melt lapilli and bombs ejected from the crater, which were collected during our geophysical campaign in February 2010. Two types of glasses constitute the impact melt lapilli and bombs: a white glass and a dark glass. The white glass is mostly made of SiO2 and it is devoid of inclusions. Its negligible Ni and Co contents suggest derivation from the target rocks without interaction with the projectile (<0.1 wt% of projectile contamination). The dark glass is a silicate melt with variable contents of Al2O3 (0.84-18.7 wt%), FeOT (1.83-61.5 wt%), and NiO (<0.01-10.2 wt%). The dark glass typically includes fragments (from few μm to several mm in size) of shocked sandstone, diaplectic glass, lechatelierite, and Ni-Fe metal blebs. The metal blebs are enriched in Ni compared to the Gebel Kamil meteorite. The dark glass is thus a mixture of target and projectile melts (11-12 wt% of projectile contamination). Based on recently proposed models for target-projectile interaction and for impact glass formation, we suggest a scenario for the glass formation at Kamil. During the transition from the contact and compression stage and the excavation stage, projectile and target liquids formed at their interface and chemically interact in a restricted zone. Projectile contamination affected only a shallow portion of the target rocks. The SiO2 melt that eventually solidified as white glass behaved as an immiscible liquid and did not interact with the projectile. During the excavation stage dark glass melt engulfed and coated the white glass melt, target fragments, and got stuck to iron meteorite shrapnel fragments. This model could also explain the common formation of white and dark glasses in small impact craters generated by iron bodies (e.g., Wabar).

Reference
Fazio A, D’Orazio M, Cordier C, Folco L (2016) Target-projectile interaction during impact melting at Kamil Crater, Egypt. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.02.003]
Copyright Elsevier

^1,2Ann N. Nguyen, ¹Lindsay P. Keller, ¹Scott Messenger
¹Robert M. Walker Laboratory for Space Science, EISD Directorate, Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA
²JETS/Jacobs Technology, NASA JSC, Houston, TX 77058, USA

We report the chemical and structural analysis of nine presolar silicate grains and one presolar oxide grain from the ungrouped chondrite Acfer 094 and the CR chondrite Queen Alexandra Range 99177. Oxygen isotopic analyses indicate that five of these grains condensed in the outflows of asymptotic giant branch (AGB) stars, four have supernova (SN) origins, and one grain likely has a nova origin. Transmission electron microscopy studies show that most of the grains are amorphous with widely varying non-stoichiometric chemical compositions. Three crystalline AGB grains were identified: a clinoenstatite-containing grain assemblage, a Fe-rich olivine grain, and a nanocrystalline enstatite grain encased in an amorphous silicate shell. An amorphous stoichiometric enstatite (MgSiO3) SN grain likely condensed as a crystal and was later rendered amorphous. We do not observe a systematic difference in the chemistries and mineralogies of presolar silicates from different stellar sources, suggesting that the grains formed under a similar range of conditions.

Reference
Nguyen AN, Keller LP, Messenger S (2016) MINERALOGY OF PRESOLAR SILICATE AND OXIDE GRAINS OF DIVERSE STELLAR ORIGINS. The Astrophysical Journal 818, 51
Link to Article [http://dx.doi.org/10.3847/0004-637X/818/1/51]

¹Hiroshi Hidaka, ¹Takuya Higuchi, ²Shigekazu Yoneda
¹Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
²Department of Science and Engineering, National Museum of Nature and Science, Tsukuba 305-0005, Japan

It is known that the Sayama meteorite (CM2) shows an extensive signature for aqueous alteration on the meteorite parent body, and that most of the primary minerals in the chondrules are replaced with phyllosilicates as the result of the aqueous alteration. In this paper, it is confirmed from the observation of two-dimensional Raman spectra that a part of olivine in a chondrule collected from the Sayama chondrite is serperntinized. Ion microprobe analysis of the chondrule showed that alkaline elements such as Rb and Cs are heterogeneously redistributed in the chondrule. The result of higher Rb and Cs contents in serpentinized phases in the chondrule rather than in other parts suggested the selective adsorption of alkaline elements into the serpentine in association with early aqueous activity on the meteorite parent body. Furthermore Ba isotopic analysis provided variations of 135Ba/138Ba and 137Ba/138Ba in the chondrule. This result was consistent with our previous isotopic data suggesting isotopic evidence for the existence of the presently extinct nuclide 135Cs in the Sayama meteorite, but the abundance of 135Cs in the solar system remains unclear because of large analytical uncertainties.

Reference
Hidaka H, Higuchi T, Yoneda S (2015) REDISTRIBUTION OF ALKALINE ELEMENTS IN ASSOCIATION WITH AQUEOUS ACTIVITY IN THE EARLY SOLAR SYSTEM. The Astrophysical Journal 815, 76
Link to Article [http://dx.doi.org/10.1088/0004-637X/815/1/76]

¹N. G. Rudraswami, ¹M. Shyam Prasad, ^1,2S. Dey, ³J. M. C. Plane, ³W. Feng, ⁴S. Taylor
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India
²Indian Institute of Technology, Roorkee, Uttarakhand 247667, India
³School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
⁴Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, NH 03755-1290, USA

We evaluate the heating of extraterrestrial particles entering the atmosphere using the comprehensive chemical ablation model (CABMOD). This model predicts the ablation rates of individual elements in a particle with a defined size, composition, entry velocity, and entry angle with respect to the zenith (ZA). In the present study, bulk chemical analyses of 1133 Antarctica micrometeorites (collected from the south pole water well) are interpreted using CABMOD. The marked spread in Fe/Si values in unmelted, partially melted, and melted micrometeorites is explained by the loss of relatively volatile Fe during atmospheric entry. The combined theoretical modeling and elemental composition of the micrometeorites (Mg/Si ratios) suggest that ~85% of particles have a provenance of carbonaceous chondrites, the remaining ~15% are either ordinary or enstatite chondrites. About 65% of the micrometeorites have undergone <20% ablation, while a further 20% have lost between 20% and 60% of their original mass. This has implications for understanding the micrometeorite flux that reaches the Earth’s surface, as well as estimating the pre-atmospheric size of the particles. Our work shows that the unmelted particles that contribute ~50% to the total micrometeorite collection on Earth’s surface have a small entry zone: ZA = 60°–90° if the entry velocity is ~11 km s−1, and ZA = 80°–90° for >11–21 km s−1.

Reference
Rudraswami NG, Shyam Prasad M, Dey S, Plane JMC, Feng W, Taylor S (2015) EVALUATING CHANGES IN THE ELEMENTAL COMPOSITION OF MICROMETEORITES DURING ENTRY INTO THE EARTH’S ATMOSPHERE. The Astrophysicla Journal 814, 78
Link to Article [http://dx.doi.org/10.1088/0004-637X/814/1/78]

¹M. Lemelin, ¹P.G. Lucey, ²G.A. Neumann, ²E.M. Mazarico, ³M.K. Barker, ¹A. Kakazu, ¹D. Trang, ⁴D.E. Smith, ⁴M.T. Zuber
¹Hawaii Institute of Geophysics and Planetology, Dept. of Geophysics and Geology, University of Hawaii at Manoa, 1680 East-West Rd, Honolulu HI
²Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20771
³Sigma Space Corporation, 4600 Forbes Blvd., Lanham MD 20706
⁴Dept. of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Ave. Cambridge, MA 02139

The Lunar Orbiter Laser Altimeter (LOLA) experiment on Lunar Reconnaissance Orbiter (LRO) is a laser altimeter that also measures the strength of the return pulse from the lunar surface. These data have been used to estimate the reflectance of the lunar surface, including regions lacking direct solar illumination. A new calibration of these data are presented that features lower uncertainties overall and more consistent results in the polar regions. We use these data, along with newly available maps of the distribution of lunar maria, also derived from LRO instrument data, to investigate a newly discovered dependence of the albedo of the lunar maria on latitude (Hemingway et al. 2015). We confirm that there is an increase in albedo with latitude in the lunar maria, and confirm that this variation is not an artifact arising from the distribution of compositions within the lunar maria, using data from the Lunar Prospector neutron spectrometer. Radiative transfer modeling of the albedo dependence within the lunar maria is consistent with the very weak to absent dependence of albedo on latitude in the lunar highlands; the lower abundance of the iron source for space weathering products in the lunar highlands weakens the latitude dependence to the extent that it is only weakly detectable in current data. In addition, photometric models and normalization may take into account the fact that the lunar albedo is latitude dependent, but this dependence can cause errors in normalized reflectance of at most 2 % for the majority of near-nadir geometries. We also investigate whether the latitude dependent albedo may have obscured detection of small mare deposits at high latitudes. We find that small regions at high latitudes with low roughness similar to the lunar maria are not mare deposits that may have been misclassified owing to high albedos imposed by the latitude dependence. Finally, we suggest that the only modest correlations among space weathering indicators defined for the lunar samples may be due to mixing of soils from distinct latitudes.

Reference
Lemelin M, Lucey PG, Neumann GA, Mazarico EM, Barker MK, Kakazu A, Trang D, Smith DE, Zuber MT (2016) Improved calibration of reflectance data from the LRO Lunar Orbiter Laser Altimeter (LOLA) and implications for space weathering. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.02.006]
Coypright Elsevier