^1,2Tuan H. Vu1, Robert Hodyss, ^1,2Mathieu Choukroun, ^1,2Paul V. Johnson
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²NASA Astrobiology Institute

The composition of Europa’s subsurface ocean is a critical determinant of its habitability. However, our current understanding of the ocean composition is limited to its expression on the surface. This work investigates experimentally the composition of mixed sodium–magnesium–sulfate–chloride solutions when frozen to 100 K, simulating conditions that likely occur as ocean fluids are emplaced onto Europa’s surface. Micro-Raman spectroscopy is used to characterize phase composition of the frozen brines at 100 K. Our results show that solutions containing Na+, Cl−, Mg2+, and ${{\mathrm{SO}}_{4}}^{2-}$ preferentially crystallize into Na2SO4 and MgCl2 hydrated minerals upon freezing, even at elevated [Mg2+]/[Na+] ratios. The detection of epsomite (MgSO4•7H2O) on Europa’s surface, if confirmed, may thus imply a relatively sodium-poor ocean composition or a radiolytic process that converts MgCl2 to MgSO4 as suggested by Brown & Hand. The formation of NaCl on the surface, while dependent upon a number of factors such as freezing rate, may indicate an ocean significantly more concentrated in sodium than in magnesium.

Reference
Vu TH, Hodyss R, Choukroun M, Johnson PV (2016) CHEMISTRY OF FROZEN SODIUM–MAGNESIUM–SULFATE–CHLORIDE BRINES: IMPLICATIONS FOR SURFACE EXPRESSION OF EUROPA’S OCEAN COMPOSITION. The Astrophysical Journal (Letters) 816, L26
Link to Article [http://dx.doi.org/10.3847/2041-8205/816/2/L26]

¹S. Xu, ²M. Jura, ³P. Dufour, ²B. Zuckerman
¹European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching, Germany
²Department of Physics and Astronomy, University of California, Los Angeles CA 90095-1562, USA
³Institut de Recherche sur les Exoplanètes (iREx), Université de Montréal, Montréal, QC H3C 3J7, Canada

We report high-resolution spectroscopic observations of WD 1145+017—a white dwarf that was recently found to be transitted by multiple asteroid-sized objects within its tidal radius. We discovered numerous circumstellar absorption lines with linewidths of ~300 km s−1 from Mg, Ca, Ti, Cr, Mn, Fe, and Ni, possibly from several gas streams produced by collisions among the actively disintegrating objects. The atmosphere of WD 1145+017 is polluted with 11 heavy elements, including O, Mg, Al, Si, Ca, Ti, V:, Cr, Mn, Fe, and Ni. Evidently, we are witnessing the active disintegration and subsequent accretion of an extrasolar asteroid.

Reference
Xu S, Jura M, Dufour P, Zuckerman B (2016) EVIDENCE FOR GAS FROM A DISINTEGRATING EXTRASOLAR ASTEROID. The Astrophysical journal (Letters), 816 L22
Link to Article [http://dx.doi.org/10.3847/2041-8205/816/2/L22]

¹Laurence E. Nyquist, ²Chi-Yu Shih, ^1,3Francis M. McCubbin, ^3,4Alison R. Santos, ^3,4Charles K. Shearer, ²Zhan X. Peng, ^3,4Paul V. Burger,³Carl B. Agee
¹NASA Johnson Space Center, Mailcode XI, Houston, Texas, USA
²Jacobs, NASA Johnson Space Center, Houston, Texas, USA
³Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA
⁴Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA

The bulk matrix domain of the Martian breccia NWA 7034 was examined petrographically and isotopically to better understand the provenance and age of the source material that make up the breccia. Both 147Sm-143Nd and 146Sm-142Nd age results for mineral separates from the bulk matrix portion of breccia NWA 7034 suggest that various lithological components in the breccia probably formed contemporaneously ~4.44 Ga ago. This old age is in excellent agreement with the upper intersection ages (4.35–4.45 Ga) for U-Pb discordia and also concordia defined by zircon and baddeleyite grains in matrix and igneous-textured clasts. Consequently, we confirm an ancient age for the igneous components that make up the NWA 7034 breccia. Substantial disturbance in the Rb-Sr system was detected, and no age significance could be gleaned from our Rb-Sr data. The disturbance to the Rb-Sr system may be due to a thermal event recorded by bulk-rock K-Ar ages of 1.56 Ga and U-Pb ages of phosphates at about 1.35–1.5 Ga, which suggest partial resetting from an unknown thermal event(s), possibly accompanying breccia formation. The NWA 7034 bulk rock is LREE enriched and similar to KREEP-rich lunar rocks, which indicates that the earliest Martian crust was geochemically enriched. This enrichment supports the idea that the crust is one of the enriched geochemical reservoirs on Mars that have been detected in studies of other Martian meteorites.

Reference
Nyquist E, Shih C-Y,McCubbin, FM,Santos AR, Shearer CK, Peng ZX, Burger PV, Agee CB (2016) Rb-Sr and Sm-Nd isotopic and REE studies of igneous components in the bulk matrix domain of Martian breccia Northwest Africa 7034. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12606]
Published by arrangement wit John Wiley & Sons

^1,2Kathryn A. Dyl et al.(>10*)
¹Department of Applied Geology, Curtin University, Perth, WA, Australia
²CSIRO Earth Sciences and Resource Engineering, Perth, WA, Australia
*Find the extensive, full author and affiliation list on the publishers website

Mason Gully, the second meteorite recovered using the Desert Fireball Network (DFN), is characterized using petrography, mineralogy, oxygen isotopes, bulk chemistry, and physical properties. Geochemical data are consistent with its classification as an H5 ordinary chondrite. Several properties distinguish it from most other H chondrites. Its 10.7% porosity is predominantly macroscopic, present as intergranular void spaces rather than microscopic cracks. Modal mineralogy (determined via PS-XRD, element mapping via energy dispersive spectroscopy [EDS], and X-ray tomography [for sulfide, metal, and porosity volume fractions]) consistently gives an unusually low olivine/orthopyroxene ratio (0.67−0.76 for Mason Gully versus ~1.3 for typical H5 ordinary chondrites). Widespread “silicate darkening” is observed. In addition, it contains a bright green crystalline object at the surface of the recovered stone (diameter ≈ 1.5 mm), which has a tridymite core with minor α-quartz and a rim of both low- and high-Ca pyroxene. The mineralogy allows the calculation of the temperatures and ƒ(O2) characterizing thermal metamorphism on the parent body using both the two-pyroxene and the olivine-chromite geo-oxybarometers. These indicate that MG experienced a peak metamorphic temperature of ~900 °C and had a similar ƒ(O2) to Kernouvé (H6) that was buffered by the reaction between olivine, metal, and pyroxene. There is no evidence for shock, consistent with the observed porosity structure. Thus, while Mason Gully has some unique properties, its geochemistry indicates a similar thermal evolution to other H chondrites. The presence of tridymite, while rare, is seen in other OCs and likely exogenous; however, the green object itself may result from metamorphism.

Reference
Dyl KA et al. (2016) Characterization of Mason Gully (H5): The second recovered fall from the Desert Fireball Network. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12605]
Published by arrangement with John Wiley & Sons

^1,2McCanta, M. C., ¹Beckett, J. R., ¹Stolper, E. M.
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Department of Earth and Ocean Sciences, Tufts University, Medford, Massachusetts, USA

Phosphorus zoning is observed in olivines in high-FeO (type IIA) chondrules in H chondrites over the entire range of petrologic grades: H3.1–H6. Features in P concentrations such as oscillatory and sector zoning, and high P cores are present in olivines that are otherwise unzoned in the divalent cations. Aluminum concentrations are low and not significantly associated with P zoning in chondrule olivines. In highly unequilibrated H chondrites, phosphorus zoning is generally positively correlated with Cr. Atomic Cr:P in olivine is roughly 1:1 (3:1 for one zone in one olivine in RC 075), consistent with Cr3+ charge-balancing P5+ substituting for Si4+. Normal igneous zonation involving the dominant chrome species Cr2+ was observed only in the LL3.0 chondrite Semarkona. In more equilibrated chondrites (H3.5–H3.8), Cr spatially correlated with P is occasionally observed but it is diffuse relative to the P zones. In H4–H6 chondrites, P-correlated Cr is absent. One signature of higher metamorphic grades (≥H3.8) is the presence of near matrix olivines that are devoid of P oscillatory zoning. The restriction to relatively high metamorphic grade and to grains near the chondrule–matrix interface suggests that this is a response to metasomatic processes. We also observed P-enriched halos near the chondrule–matrix interface in H3.3–H3.8 chondrites, likely reflecting the loss of P and Ca from mesostasis and precipitation of Ca phosphate near the chondrule surface. These halos are absent in equilibrated chondrites due to coarsening of the phosphate and in unequilibrated chondrites due to low degrees of metasomatism. Olivines in type IA chondrules show none of the P-zoning ubiquitous in type IIA chondrules or terrestrial igneous olivines, likely reflecting sequestration of P in reduced form within metallic alloys and sulfides during melting of type IA chondrules.

Reference
McCanta MC, Beckett JR, Stolper EM (2016) Correlations and zoning patterns of phosphorus and chromium in olivine from H chondrites and the LL chondrite Semarkona. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12604]
Published by arrangement with John Wiley & Sons

¹Frans J.M. Rietmeijer
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA

In Stardust tracks C2044,0,38, C2044,0,39, and C2044,0,42 (Brennan et al. 2007) and Stardust track 10 (this work) gold is present in excess of its cosmochemical abundance. Ultra-thin sections of allocation FC6,0,10,0,26 (track 10) show a somewhat wavy, compressed silica aerogel/silica glass interface which challenges exact location identification, i.e., silica glass, compressed silica aerogel, or areas of overlap. In addition to domains of pure silica ranging from SiO2 to SiO3 glass, there is MgO-rich silica glass with a deep metastable composition, MgO = 14 ± 6 wt%, due to assimilation of Wild 2 Mg-silicate matter in silica melt. This magnesiosilica composition formed when temperatures during hypervelocity capture reached >2000 °C followed by ultrafast quenching of the magnesiosilica melt when it came into contact with compressed aerogel at ~155 °C. The compressed silica aerogel in track 10 has a continuous Au background as result of the melting point depression of gold particles

Reference
Rietmeijer FJM (2016) At the interface of silica glass and compressed silica aerogel in Stardust track 10: Comet Wild 2 is not a goldmine. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12608]
Published by arrangement with John Wiley & Sons

¹Aurore Hutzler et al.(>10)*
¹ Aix-Marseille Université, CNRS, CEREGE UM34, Technopôle de l’Environnement Arbois-Méditerranée, Aix-en-Provence, France
*Find the extensive, full author and affiliation list on the publishers website

We describe the geological, morphological, and climatic settings of two new meteorite collections from Atacama (Chile). The “El Médano collection” was recovered by systematic on-foot search in El Médano and Caleta el Cobre dense collection areas and is composed of 213 meteorites before pairing, 142 after pairing. The “private collection” has been recovered by car by three private hunters and consists of 213 meteorites. Similar to other hot desert finds, and contrary to the falls and Antarctica finds, both collections show an overabundance of H chondrites. A recovery density can be calculated only for the El Médano collection and gives 251 and 168 meteorites larger than 10 g km−2, before and after pairing, respectively. It is by far the densest collection area described in hot deserts. The Atacama Desert is known to have been hyperarid for a long period of time and, based on cosmic-ray exposure ages on the order of 1–10 Ma, to have been stable over a period of time of several million years. Such a high meteorite concentration might be explained invoking either a yet unclear concentration mechanism (possibly related to downslope creeping) or a previously underestimated meteorite flux in previous studies or an average terrestrial age over 2 Myr. This last hypothesis is supported by the high weathering grade of meteorites and by the common terrestrial fragmentation (with fragments scattered over a few meters) of recovered meteorites.

Reference
Hutzler A et al. (2016) Description of a very dense meteorite collection area in western Atacama: Insight into the long-term composition of the meteorite flux to Earth. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12607]
Published by arrangement with John Wiley & Sons

¹Mary Sue Bell
¹Jacobs@NASA/Johnson Space Center, Houston, Texas, USA

A study of pure, single crystal calcite shocked to pressures from 9.0 to 60.8 GPa was conducted to address contradictory data for carbonate shock behavior. The recovered materials were analyzed optically and by transmission electron microscopy (TEM), as well as by thermogravimetry (TGA), X-ray diffraction (XRD), and Raman-spectroscopy. In thin section, progressive comminution of calcite is observed although grains remain birefringent to at least 60.8 GPa. TGA analysis reveals a positive correlation between percent of mass loss due to shock and increasing shock pressure (R = 0.77) and suggests that shock loading leads to the modest removal of structural volatiles in this pressure range. XRD patterns of shocked Iceland spar samples produce peaks that are qualitatively and quantitatively less intense, more diffuse, and shift to lower o2θ. However, the regularity observed in these shocked powder patterns suggests that structures with very uniform unit cell separations persist to shock pressures as high as 60.8 GPa. Raman spectral analyses indicate no band asymmetry and no systematic peak shifting or broadening. TEM micrographs display progressively diminishing crystallite domain sizes. Selected area electron diffraction (SAED) patterns reveal no signatures of amorphous material. These data show that essentially intact calcite is recovered at shock pressures up to 60.8 GPa with only slight mass loss (~7%). This work suggests that the amount of CO2 gas derived from shock devolatilization of carbonate by large meteorite impacts into carbonate targets has been (substantially) overestimated.

Reference
Bell MS (2016) CO2 release due to impact devolatilization of carbonate: Results of shock experiments. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12613]
Published by arrangement with John Wiley & Sons

^1,2Christian M. Schrader, ¹Barbara A. Cohen, ³John J. Donovan, ⁴Edward P. Vicenzi
¹Marshall Space Flight Center, NASA, Huntsville, Alabama, USA
²Geology Department, Bowdoin College, Brunswick, Maine, USA
³Department of Chemistry, University of Oregon, Eugene, Oregon, USA
⁴Smithsonian Institution, Museum Conservation Institute, Suitland, Maryland, USA

Martian meteorite Elephant Moraine A79001 (EET 79001) has received considerable attention for the unusual composition of its shock melt glass, particularly its enrichment in sulfur relative to the host shergottite. It has been hypothesized that Martian regolith was incorporated into the melt or, conversely, that the S-enrichment stems from preferential melting of sulfide minerals in the host rock during shock. We present results from an electron microprobe study of EET 79001 including robust measurements of major and trace elements in the shock melt glass (S, Cl, Ni, Co, V, and Sc) and minerals in the host rock (Ni, Co, and V). We find that both S and major element abundances can be reconciled with previous hypotheses of regolith incorporation and/or excess sulfide melt. However, trace element characteristics of the shock melt glass, particularly Ni and Cl abundances relative to S, cannot be explained either by the incorporation of regolith or sulfide minerals. We therefore propose an alternative hypothesis whereby, prior to shock melting, portions of EET 79001 experienced acid-sulfate leaching of the mesostasis, possibly groundmass feldspar, and olivine, producing Al-sulfates that were later incorporated into the shock melt, which then quenched to glass. Such activity in the Martian near-surface is supported by observations from the Mars Exploration Rovers and laboratory experiments. Our preimpact alteration model, accompanied by the preferential survival of olivine and excess melting of feldspar during impact, explains the measured trace element abundances better than either the regolith incorporation or excess sulfide melting hypothesis does.

Reference
Schrader CM, Cohen BA, Donovan JJ, Vicenzi EP (2016) Ni/S/Cl systematics and the origin of impact-melt glasses in Martian meteorite Elephant Moraine 79001. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12612]
Published by arrangement with John Wiley & Sons

¹E. Dobrică, ²R. C. Ogliore
¹Department of Earth and Planetary Sciences MSC03-2040, 1 University of New Mexico, Albuquerque 87131-0001, NM, USA
²Department of Physics, Washington University in St. Louis, St. Louis 63117, MO, USA

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

Reference
Dobrică E, Ogliore RC (2016) Adhering grains and surface features on two Itokawa particles. Earth, Planets and Space 68:21
Link to Article [doi:10.1186/s40623-016-0391-7]