¹Zongcheng Ling,²Alian Wang
¹Shandong Provincial Key Laboratory of Optical Astronomy & Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, China
²Department of Earth & Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO, United States

Miller Range (MIL) 03346 is a nakhlite meteorite that has been extensively studied due to its unique complex secondary mineral phases and their potential implications for the hydrologic history of Mars. We conducted a set of Raman spectroscopic and Raman imaging studies of MIL 03346,168, focusing on the secondary mineral phases and their spatial distributions, with a goal to better understand the possible processes by which they were generated on Mars. This study revealed three types of calcium sulfates, a solid solutions of (K, Na)-jarosite and two groups of hydrated species with low crystallinity (HSLC) in the veins and/or mesostasis areas of the meteorite. The most abundant Ca-sulfate is bassanite that suggests two possible paths for its direct precipitation from a Ca-S-H2O brine, either having low water activity or with incomplete development (producing bassanite with gypsum microcrystals) on Mars. The second most abundant Ca-sulfate is soluble γ-CaSO4 which raises a new question on the origins of this phase in the martian meteorite, since γ-CaSO4 readily hydrates in the laboratory but is apparently stable in Atacama Desert. The close spatial relationship of (K, Na)-jarosite solid solutions with rasvumite (KFe2S3), magnetite, HSLC, and fine-grained low crystallinity alkali feldspar in mesostasis suggests a potential in situ formation of mesostasis jarosite from these Fe-K,Na-S-O-H2O species.

Reference
Ling Z, Wang A (2015) Spatial distributions of secondary minerals in the Martian meteorite MIL 03346,168 determined by Raman spectroscopic Imaging. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004805]

Published by Arrangement with John Wiley&Sons

^1,2Steven M. Chemtob,¹Ryan D. Nickerson,³Richard V. Morris,⁴David G. Agresti,^1,2Jeffrey G. Catalano
¹Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri, USA
²McDonnell Center for the Space Sciences, Washington University, St. Louis, Missouri, USA
³EIS Directorate, NASA Johnson Space Center, Houston, Texas, USA
⁴Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama, USA

Widespread detections of phyllosilicates in Noachian terrains on Mars imply a history of near-surface fluid-rock interaction. Ferrous trioctahedral smectites are thermodynamically predicted products of basalt weathering on early Mars, but to date only Fe3+-bearing dioctahedral smectites have been identified from orbital observations. In general, the physicochemical properties of ferrous smectites are poorly studied because they are susceptible to air oxidation. In this study, eight Fe2+-bearing smectites were synthesized from Fe2+-Mg-Al-silicate gels at 200° C under anoxic conditions. Samples were characterized by inductively-coupled plasma optical emission spectrometry, powder X-ray diffraction, Fe K-edge X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy, and visible/near-infrared (VNIR) reflectance spectroscopy. The range of redox states was Fe3+/ΣFe = 0 to 0.06±0.01 as determined by both XAS and, for short integration times, Mössbauer. The smectites have 060 distances (d(060)) between 1.53 and 1.56 Å, indicating a trioctahedral structure. d(060) and XAS-derived interatomic Fe-(Fe,Mg,Al) distance scaled with Fe content. Smectite VNIR spectra feature OH/H2O absorption bands at 1.4 and 1.9 µm, (Fe2+,Mg,Al)3-OH stretching bands near 1.4 µm, and Fe2+Fe2+Fe2+-OH, MgMgMg-OH, AlAl(Mg,Fe2+)-OH, and AlAl-OH combination bands at 2.36 µm, 2.32 µm 2.25 µm, and 2.20 µm, respectively. The spectra for ferrrous saponites are distinct from those for dioctahedral ferric smectites, permitting their differentiation from orbital observations. XRD patterns for synthetic high-Mg ferrosaponite and high-Mg ferrian saponite are both consistent with the Sheepbed saponite detected by the CheMin instrument at Gale Crater, Mars, suggesting that anoxic basalt alteration was a viable pathway for clay mineral formation on early Mars.

Reference
Chemtob SM, Nickerson RD, Morris RV, Agresti DG, Catalano JG (2015) Synthesis and structural characterization of ferrous trioctahedral smectites: Implications for clay mineral genesis and detectability on Mars. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004763]

Published by arrangement with John Wiley&Sons

¹Toru Matsumoto, ²Akira Tsuchiyama, ²Akira Miyake, ³Takaaki Noguchi, ⁴Michihiko Nakamura, ⁵Kentaro Uesugi, ⁵Akihisa Takeuchi, ⁵Yoshio Suzuki, ⁶Tsukasa Nakano
¹Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
²Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto-shi, 606-8502, Japan
³Faculty of Arts and Science, Kyushu University, 2-1-1 Bunkyo. Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁴Department of Earth Science, Tohoku University, 6-3, AramakiAoba, Aoba-ku, Sendai-shi, Miyagi 980-8578, Japan
⁵Japan Synchrotron Radiation Research Institute/ SPring-8, Sayo, Hyogo 679-5198, Japan
⁶Geological Survey of Japan/National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8567, Japan

Surface morphologies of a regolith particle retrieved from asteroid 25143 Itokawa were observed using field-emission scanning electron microscopy (FE-SEM). The images were compared with the internal structures of the space-weathered rim of the same particle observed by transmission electron and scanning transmission electron microscopies (TEM/STEM) to investigate whether there is a direct link between the surface morphology and internal structure. FE-SEM observation showed that most of the particle surface is covered by convex spots less than 100 nm in size. TEM/STEM observation revealed that this particle has a space-weathered rim composed of partially amorphous structures with nano-Fe particles and vesicles. The vesicles swell the surface and form blisters that correspond to the spotted structures observed by FE-SEM. These observations indicate that a space-weathered rim with blisters can be observed by FE-SEM without using destructive methods. The observation of the space-weathered rim by FE-SEM also enabled us to obtain the distribution of the space-weathered rim on the particle surfaces. The existence of space-weathered rims on the opposing surfaces of the particle shows that most of the surfaces were directly exposed to the space environment by movement on the Itokawa surface. The depths of the blister locations and the chemical composition of the space-weathered rim indicate that the observed space-weathered rim with blisters was formed mainly by solar wind irradiation. The space-weathered rim analyzed in this study is thicker than those of Itokawa particles previously examined, indicating that the rim may has experienced longer solar wind exposure than those previously observed.

Reference
Matsumoto T, Tsuchiyama A, Miyake A, Noguchi T, Nakamura M, Uesugi K, Takeuchi A, Suzuki Y, Nakano T (2015)
Surface and internal structures of a space-weathered rim of an Itokawa regolith particle. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.05.001]

Copyright Elsevier

¹Driss Takir, ¹Joshua P. Emery, ¹Harry Y. McSween Jr.
¹Department of Earth and Planetary Sciences and Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996, United States

Proposed mineralogical linkages between CM/CI carbonaceous chondrites and outer Main Belt asteroids remain uncertain due to a dearth of diagnostic absorptions in visible and near-infrared (∼0.4 to 2.5 μm) spectra of the two sets of objects. Absorptions near 3 μm in both sets hold promise for illuminating the potential linkages. Spectral comparisons of meteorites and asteroids have been challenging because meteorite spectra have usually been acquired in ambient terrestrial environments, and hence were contaminated by atmospheric water. In this study, we compare near-infrared spectra of chondrites measured in the laboratory under asteroid-like conditions (Takir et al. 2013) and spectra of asteroids measured with the long-wavelength cross-dispersed (LXD: 1.9-4.2-μm) mode of the SpeX spectrograph/imager at the NASA Infrared Telescope Facility (IRTF) (Takir and Emery 2012). Using the 3-μm band shape, we find that spectral Group 2 CM and CI (Ivuna) chondrites are possible meteorite analogs for asteroids with the sharp 3-μm features, which are predominately located in the 2.5 < a < 3.3 AU region. Spectral Group 2 CM chondrites contain phyllosilicate phases intermediate between endmembers Fe-serpentine and Mg-serpentine, with a petrological subtype ranging from 2.2 to 2.1 (Takir et al. 2013). No meteorite match was found for asteroids showing a rounded 3-μm feature, which tend to be located farther from the Sun (3.0 < a < 4.0 AU), or for asteroids with distinctive spectra like 1 Ceres or 52 Europa. The study of the 3-μm band in meteorites and asteroids has implications for the understanding of phyllosilicate mineralogy and its distribution in the outer Main Belt region.

Reference
Takir D, EmeryJP, McSween Jr. HY (2015) Toward an Understanding of Phyllosilicate Mineralogy in the Outer Main Asteroid Belt. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.04.042]
Copyright Elsevier