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.