¹Jeffrey R. Johnson et al. (>10)*
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005187]
¹Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
*Find the extensive, full author and affiliation list on the publishers website
Published by arrangement with John Wiley & Sons

As part of the Bagnold Dune campaign conducted by Mars Science Laboratory rover Curiosity, visible/near-infrared reflectance spectra of dune sands were acquired using Mast Camera (Mastcam) multispectral imaging (445-1013 nm) and Chemistry and Camera (ChemCam) passive point spectroscopy (400-840 nm). By comparing spectra from pristine and rover-disturbed ripple crests and troughs within the dune field, and through analysis of sieved grain size fractions, constraints on mineral segregation from grain sorting could be determined. In general, the dune areas exhibited low relative reflectance, a weak ~530 nm absorption band, an absorption band near 620 nm, and a spectral downturn after ~685 nm consistent with olivine-bearing sands. The finest grain size fractions occurred within ripple troughs and in the subsurface, and typically exhibited the strongest ~530 nm bands, highest relative reflectances, and weakest red/near-infrared ratios, consistent with a combination of crystalline and amorphous ferric materials. Coarser-grained samples were the darkest and bluest, and exhibited weaker ~530 nm bands, lower relative reflectances, and stronger downturns in the near-infrared, consistent with greater proportions of mafic minerals such as olivine and pyroxene. These grains were typically segregated along ripple crests and among the upper surfaces of grain flows in disturbed sands. Sieved dune sands exhibited progressive decreases in reflectance with increasing grain size, as observed in laboratory spectra of olivine size separates. The continuum of spectral features observed between the coarse- and fine-grained dune sands suggests that mafic grains, ferric materials, and airfall dust mix in variable proportions depending on aeolian activity and grain sorting.

¹C. D. O’Connell-Cooper,¹J. G. Spray,¹L. M. Thompson,²R. Gellert,³J. A. Berger,²N. I. Boyd,²E. D. Desouza,⁴G. M. Perrett,⁵M. Schmidt,²S. J. VanBommel
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005268]
¹Planetary and Space Science Centre, University of New Brunswick, Fredericton, NB, Canada
²Guelph-Waterloo Physics Institute, University of Guelph, Guelph, ON, Canada
³Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, London, ON, Canada
⁴Department of Astronomy, Cornell University, Ithaca, NY, USA
⁵Department of Earth Sciences, Brock University, St. Catharines, ON, Canada
Published by arrangement with John Wiley & Sons

We present APXS data for the active Bagnold dune field within the Gale impact crater (MSL mission). We derive an APXS-based Average Basaltic Soil (ABS) composition for Mars based on past and recent data from the MSL and MER missions. This represents an update to the Taylor and McLennan (2009) average martian soil, and facilitates comparison across martian datasets.

The active Bagnold dune field is compositionally distinct from the ABS, with elevated Mg, Ni and Fe, suggesting mafic mineral enrichment, and uniformly low levels of S, Cl and Zn, indicating only a minimal dust component. A relationship between decreasing grain size and increasing felsic content is revealed. The Bagnold Sands possess the lowest S/Cl of all martian unconsolidated materials..

Gale soils exhibit relatively uniform major element compositions, similar to Meridiani Planum and Gusev Crater basaltic soils (MER missions). However, they show minor enrichments in K, Cr, Mn and Fe, which may signify a local contribution.

The lithified eolian Stimson Formation within the Gale impact crater is compositionally similar to the ABS and Bagnold sands, which provide a modern analogue for these ancient eolian deposits. Compilation of APXS-derived soil data reveals a generally homogenous global composition for martian soils, but one that can be locally modified due to past or extant geologic processes that are limited in both space and time.

^1,2B.L.Ehlmann et al. (>10)*
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005267]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
*Find the extensive, full author and affiliation list on the publishers website
Published by arrangement with John Wiley & Sons

The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine- to medium- sized (~45-500 µm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust-covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt-sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising >90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet, Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si-enriched relative to other soils at Gale crater, and H2O, S, and Cl are lower relative to all previously measured martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse-sieved fraction of Bagnold sands, corroborated by VNIR spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in martian soils: (1) amorphous components in the sand-sized fraction (represented by Bagnold) that are Si-enriched, hydroxylated alteration products and/or impact or volcanic glasses; and (2) amorphous components in the fine fraction (<40 µm; represented by Rocknest and other bright soils) that are Fe-, S-, and Cl-enriched with low Si and adsorbed and structural H2O.

¹C.N.Achilles et al. (>10)*
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005262]
¹Department of Geosciences, The University of Arizona, Tucson, AZ, USA
*Find the extensive, full author and affiliation list on the publishers website
Published by arrangement with John Wiley & Sons

The Mars Science Laboratory rover, Curiosity, is using a comprehensive scientific payload to explore rocks and soils in Gale crater, Mars. Recent investigations of the Bagnold Dune Field provided the first in situ assessment of an active dune on Mars. The CheMin X-ray diffraction instrument on Curiosity performed quantitative mineralogical analyses of the <150 μm size fraction of the Namib dune at a location called Gobabeb. Gobabeb is dominated by basaltic minerals. Plagioclase, Fo56 olivine, and two Ca-Mg-Fe pyroxenes account for the majority of crystalline phases along with minor magnetite, quartz, hematite, and anhydrite. In addition to the crystalline phases, a minimum ~42 wt% of the Gobabeb sample is X-ray amorphous. Mineralogical analysis of the Gobabeb dataset provides insights into the origin(s) and geologic history of the dune material and offers an important opportunity for ground truth of orbital observations. CheMin’s analysis of the mineralogy and phase chemistry of modern and ancient Gale crater dune fields, together with other measurements by Curiosity’s science payload, provides new insights into present and past eolian processes on Mars.

¹Cousin Agnes et al. (>10)*
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005261]
¹Institut de Recherche en Astrophysique et Planétologie, CNRS-Université Toulouse, Toulouse, France
*Find the extensive, full author and affiliation list on the publishers website
Published by arrangement with John Wiley & Sons

The Curiosity rover conducted the first field investigation of an active extraterrestrial dune. This study of the Bagnold dunes focuses on the ChemCam chemical results and also presents findings on the grain size distributions based on the ChemCam RMI and MAHLI images. These active dunes are composed of grains that are mostly <250 μm. Their composition is overall similar to that of the aeolian deposits analyzed all along the traverse (“Aeolis Palus soils”). Nevertheless, the dunes contain less volatiles (Cl, H, S) than the Aeolis Palus soils, which appears to be due to a lower content of volatile-rich fine-grained particles (<100 μm), or a lower content of volatile-rich amorphous component, possibly as a result of: 1) a lower level of chemical alteration; 2) the removal of an alteration rind at the surface of the grains during transport; 3) a lower degree of interaction with volcanic gases/aerosols; or 4) physical sorting that removed the smallest and most altered grains. Analyses of the >150 μm grain-size dump piles have shown that coarser grains (150-250 μm) are enriched in the mafic elements Fe and Mn, suggesting a larger content in olivine compared to smaller grains (<150 μm) of the Bagnold dunes. Moreover, the chemistry of soils analyzed in the vicinity of the dunes indicates that they are similar to the dune material. All these observations suggest that the olivine content determined by X-ray diffraction of the <150 μm grain-size sample should be considered as a lower limit for the Bagnold dunes.

¹H. Hijazi,¹M. E. Bannister,²H. M. Meyer III,³C. M. Rouleau,¹F. W. Meyer
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005300]
¹Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
²Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
³Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA
Published by arrangement with John Wiley & Sons

In this paper, we present measurements of He+ and He+2 ion-induced sputtering of an anorthite-like thin film at a fixed solar-wind-relevant impact energy of ~0.5 keV/amu using a quartz crystal microbalance approach (QCM) for determination of total absolute sputtering yields. He+2 ions are the most abundant multicharged ions in the solar wind and increased sputtering by these ions in comparison to equi-velocity He+ ions is expected to have the biggest effect on the overall sputtering efficiency of solar wind impact on the moon. Our measurements indicate an almost doubling of the sputtering yield for doubly charged incident He ions compared to same velocity He+ impact. Using a selective sputtering model, the new QCM results presented here, together with previously published results for Ar+q ions and SRIM results for the relevant kinetic sputtering yields, the effect due to multicharged solar-wind ion impact on local near-surface modification of lunar anorthite-like soil is explored. It is shown that the multicharged solar wind component leads to a more pronounced and significant differentiation of depleted and enriched surface elements as well as a shortening of the timescale over which such surface compositional modifications might occur in astrophysical settings. In addition, to validate previous and future determinations of multicharged-ion-induced sputtering enhancement for those cases where the QCM approach can’t be used, relative quadrupole-mass-spectrometry (QMS) based measurements are presented for the same anorthite-like thin film as were investigated by QCM, and their suitability and limitations for charge-state-enhanced yield measurements are discussed.

^1,2Jasmeet K. Dhaliwal, ¹James M.D. Day, ¹Christopher A. Corder, ³Kim T. Tait, ¹Kurt Marti, ⁴Nelly Assayag, ⁴Pierre Cartigny, ⁵Doug Rumble III, ⁶Lawrence A. Taylor
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.042]
¹Geosciences Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA
²Department of Geosciences, Penn State University, State College, PA 16803, USA
³Department of Natural History, Royal Ontario Museum, Toronto, Canada, M5S 2C6, Canada
⁴Institut de Physique du Globe de Paris, Université Paris Diderot, Institut Universitaire de France, Paris, France
⁵Geophysical Laboratory, Carnegie Institution for Science, Washington DC 20015, USA
⁶Planetary Geosciences Institute, Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, Tennessee, 37996, USA
Copyright Elsevier

In order to establish the role and expression of silicate-metal fractionation in early planetesimal bodies, we have conducted a highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance and 187Re-187Os study of acapulcoite-lodranite meteorites. These data are reported with new petrography, mineral chemistry, bulk-rock major and trace element geochemistry, and oxygen isotopes for Acapulco, Allan Hills (ALHA) 81187, Meteorite Hills (MET) 01195, Northwest Africa (NWA) 2871, NWA 4833, NWA 4875, NWA 7474 and two examples of transitional acapulcoite-lodranites, Elephant Moraine (EET) 84302 and Graves Nunataks (GRA) 95209. These data support previous studies that indicated these meteorites are linked to the same parent body and exhibit limited degrees (<2 to 7%) of silicate melt removal. New HSE and osmium isotope data demonstrate broadly chondritic relative and absolute abundances of these elements in acapulcoites, lower absolute abundances in lodranites and elevated (>2 × CI chondrite) HSE abundances in transitional acapulcoite-lodranite meteorites (EET 84302, GRA 95209). All of the meteorites have chondritic Re/Os with measured 187Os/188Os ratios of 0.1271 ± 0.0040 (2 St. Dev.). These geochemical characteristics imply that the precursor material of the acapulcoites and lodranites was broadly chondritic in composition, and were then heated and subject to melting of metal and sulfide in the Fe-Ni-S system. This resulted in metallic melt removal and accumulation to form lodranites and transitional acapulcoite-lodranites. There is considerable variation in the absolute abundances of the HSE, both among samples and between aliquots of the same sample, consistent with both inhomogeneous distribution of HSE-rich metal, and of heterogeneous melting and incomplete mixing of silicate material within the acapulcoite-lodranite parent body. Oxygen isotope data for acapulcoite-lodranites are also consistent with inhomogeneous melting and mixing of accreted components from different nebular sources, and do not form a well-defined mass-dependent fractionation line. Modeling of HSE inter-element fractionation suggests a continuum of melting in the Fe-Ni-S system and partitioning between solid metal and sulfur-bearing mineral melt, where lower S contents in the melt resulted in lower Pt/Os and Pd/Os ratios, as observed in lodranites. The transitional meteorites, EET 84302 and GRA 95209, exhibit the most elevated HSE abundances and do not follow modelled Pt/Os and Pd/Os solid metal-liquid metal partitioning trends. We interpret this to reflect metal melt pooling into domains that were sampled by these meteorites, suggesting that they may originate from deeper within the acapulcoite-lodranite parent body, perhaps close to a pooled metallic ‘core’ region. Petrographic examination of transitional samples reveals the most extensive melting, pooling and networking of metal among the acapulcoite-lodranite meteorites. Overall, our results show that solid metal-liquid metal partitioning in the Fe-Ni-S system in primitive achondrites follows a predictable sequence of limited partial melting and metal melt pooling that can lead to significant HSE inter-element fractionation effects in proto-planetary materials.

^1,2François L.H. Tissot, ¹Nicolas Dauphas, ²Timothy L. Grove
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.045]
¹Origins Laboratory, Department of the Geophysical Sciences, Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL, USA
²Department of the Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA
Copyright Elsevier

Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar System formation, when several short-lived nuclides (e.g., 26Al-26Mg, 53Mn-53Cr, 182Hf-182W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an “assumed” constant 238U/235U ratio (i.e., Pb-Pb ages published before 2009-2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D’Orbigny, and Sahara 99555) using multi-collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from -0.17 to -1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (53Mn/55Mn)0 ratio in the Early Solar system (ESS) is recalculated as being (7±1)×10-6 at the formation of the solar system (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions –CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs.

A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17 ‰. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The δ238U values are consistent with fractionation during magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction.

¹Caroline de Beule, ^1,2Joachim Landers, ^1,2Soma Salamon, ^1,2Heiko Wende, ¹Gerhard Wurm
The Astrophysical Journal 837, 59 Link to Article [https://doi.org/10.3847/1538-4357/837/1/59]
¹Faculty of Physics, University of Duisburg-Essen, Lotharstr. 1, D-47057 Duisburg, Germany
²Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Carl-Benz-Str. 199, D-47057 Duisburg, Germany

It is an open question how elevated temperatures in the inner parts of protoplanetary disks influence the formation of planetesimals. We approach this problem here by studying the tensile strength of granular beds with dust samples tempered at different temperatures. We find via laboratory experiments that tempering at increasing temperatures is correlated with an increase in cohesive forces. We studied dust samples of palagonite (JSC Mars-1a) which were tempered for up to 200 hr at temperatures between 600 and 1200 K, and measured the relative tensile strengths of highly porous dust layers once the samples cooled to room temperature. Tempering increases the tensile strength from 800 K upwards. This change is accompanied by mineral transformations, the formation of iron oxide crystallites as analyzed by Mössbauer spectroscopy, changes in the number size distribution, and the morphology of the surface visible as cracks in larger grains. These results suggest a difference in the collisional evolution toward larger bodies with increasing temperature as collisional growth is fundamentally based on cohesion. While high temperatures might also increase sticking (not studied here), compositional evolution will already enhance the cohesion and the possibility of growing larger aggregates on the way toward planetesimals. This might lead to a preferred in situ formation of inner planets and explain the observed presence of dense inner planetary systems.

^1,2Julie D. Stopar, ³Bradley L. Jolliff, ¹Emerson J. Speyerer, ¹Erik I. Asphaug, ¹Mark S. Robinson
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.05.010]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
²Lunar and Planetary Institute, Houston, TX, USA
³Department of Earth and Planetary Sciences, Washington University in St. Louis, MO, USA

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