^1,3Elena S.Amador, ²Joshua L.Bandfield, ³Nancy H.Thomas
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.03.021]
¹University of Washington, Department of Earth and Space Sciences and Astrobiology, Seattle, WA
²Space Science Institute, Boulder, CO
³California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA
Copyright Elsevier

Sites associated with serpentinization processes, both on Earth and throughout the Solar System, are becoming increasingly compelling for the study of habitability and astrobiology. The co-occurrence of serpentine, Mg-carbonate, and talc/saponite on Mars is most like terrestrial sites where this mineral suite is produced in low-temperature serpentinizing environments, and where on Earth these reactions support biological activity. This study aims to understand the global distribution of minerals associated with serpentinization. We performed a comprehensive analysis of the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectral dataset using factor analysis and target transformation methods to efficiently parse through the large quantity of data. These methods allow for the rapid analysis of thousands of images and provide a quantitative means to determine the significant spectral constituents of an image. These methods were used to produce a global distribution map of CRISM images with a significant likelihood of containing the spectral types of interest. Previous detections of serpentine using traditional CRISM analysis techniques were typically corroborated and additional detections were identified in isolated locations across the martian southern highlands. Most serpentine across Mars is associated with another Fe/Mg-phyllosilicate phase like talc and/or saponite. Except for in the Nili Fossae region, serpentine shows no clear relationship with ultramafic bedrock or with the other mineral phases investigated (Mg-carbonate and talc/saponite). Most serpentine detections were found in isolated exposures, associated with crater ejecta, knobby terrain, or as part of discontinuous layers in crater or valley walls. Nili Fossae shows more pervasive and extensive detections of a serpentine + phyllosilicate endmember than previously recognized, particularly in the eastern portion of Nili Fossae where the highest concentration of olivine-rich basalts is located. These findings imply that large, regional-scale near surface serpentinizing systems were likely rare on Mars. However, low-concentration serpentine detections across the southern highlands do suggest more pervasive serpentinization early in Mars history, when the planet was more geologically active.

¹J. L’Haridon et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.01.028]
¹Laboratoire de Planétologie et de Géodynamique, Université de Nantes, Nantes, France
Copyright Elsevier

ChemCam has observed a wide range of diagenetic features along the Curiosity rover traverse including pervasive Ca-sulfate veins. Observations by multiple instruments on Curiosity indicate that these veins are hydrated, formed during diagenetic fluid event(s). In this study, we delve into the chemical variability in these Ca-sulfate bearing veins and have identified two subsets in the Murray formation with enrichments in Fe and Fe+Mg. These chemical trends do not reflect a sampling mixture with the surrounding host rock but likely indicates the presence of authigenic phases formed during the emplacement of these veins. Based on passive reflectance spectral analysis and correlation with other elements, Fe3+ oxides and/or sulfates are proposed to account for the Fe-rich observations in the vicinity of the Naukluft Plateau whereas the Fe+Mg trend is also observed in adjacent dark-toned features with elevated Mn and P near the Old Soaker outcrop. The specific localization of these observations in the Gale stratigraphy implies changing pH and redox conditions in the groundwater at the time of formation of these veins, from oxidizing and likely more acidic near the Naukluft Plateau to more reducing conditions in the upper part of the Murray formation.

¹Thomas B.McCord, ²Francesca Zambon
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.03.004]
¹The Bear Fight Institute, 22 Fiddler’s Road, Winthrop WA 98862
²Istituto di Astrofisica e Planetologia Spaziali, INAF-IAPS, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy
Copyright Elsevier

Ceres’ surface composition is of special interest because it is a window into the interior state and the past evolution of this dwarf planet. Disk-integrated telescopic spectral observations indicated that Ceres’ surface is hydroxylated, similar to but not exactly the same as some of the carbonaceous chondrite classes of meteorites. Furthermore, Ceres’ bulk density is low, suggesting significant water content. The Dawn mission in orbit around Ceres provided a new and much larger set of observations on the mineralogy, molecular and elemental composition, and their distributions in association with surface features and geology. The set of articles contained in this special issue is the first treatment of the entire surface composition of Ceres using the complete High Altitude Mapping Orbit (HAMO) Dawn Ceres data set and the calibrations from all the Dawn instruments. Most articles here treat the different geologic quadrangles of Ceres within the context of the entire body. There also are articles that treat global or technical topics. As a whole, these articles provide a current and comprehensive view of Ceres’ surface composition. Ceres’ surface composition shows a fairly uniform and widespread distribution of NH4- and Mg-phyllosilicates and carbonates, mixed with a dark component and with some exposures of salts and water-ice on Ceres’ surface, all indicative of the presence of aqueous alteration processes that involved the entire dwarf planet. There is also likely some contamination by low velocity infall, as seen on Vesta, but it is more difficult to distinguish this infall from native Ceres material, unlike for the Vesta case. This article introduces and provides the context for the following papers, presents a summary of the various findings, and integrates them into some general conclusions.

¹A.Longobardo et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.02.022]
¹INAF-IAPS, via Fosso del Cavaliere 100, Rome I-00133, Italy
Copyright Elsevier

Spectral parameters of Ceres measured by the Dawn/VIR imaging spectrometer are studied as a function of illumination angles, by applying a semi-empirical method based on a statistical analysis of the VIR dataset acquired up to September 2016. The study also focuses on the photometry of the Occator faculae, i.e. the brightest spots of the Ceres surface, showing an albedo up to eight times the Ceres average. The considered semi-empirical approach takes into account the small extension (and hence small dataset) of this region and lays the groundwork to apply scattering models even on such a limited area.

The behavior of Ceres visible and infrared reflectance with phase angle is similar to other asteroids belonging to its same spectral class, i.e. C-type. The depth of the bands at 2.7 µm (phyllosilicates), 3.1 µm (ammonium), 3.4 µm (magnesium carbonates) and the infrared spectral slope linearly increase with phase angle, showing analogies with other asteroids and occurrence of phase reddening. The different behavior of the 3.9 µm band depth (also due to Mg carbonates), independent of illumination angles, could indicate that other carriers contribute to the 3.4 µm band and play a more important role in photometry outside the carbonate deposits.

The phase function of the Occator faculae is much steeper than expected from its high albedo. Mixture of bright and dark material and larger roughness can be at the basis of this result. The phyllosilicate bands show a steeper increase with phase angle with respect to the Ceres average, due to the lower presence of dark materials, and/or again larger roughness. The absence of trends with phase angles of the two carbonate bands and of the spectral slope suggests that carbonates do not produce phase reddening.

¹Yang Liu, ¹Yang Chen, ²Yunbin Guan, ²Chi Ma, ²George R. Rossman, ²John M. Eiler, ³Youxue Zhang
Earth and Planetary Science Letters 490, 206-215 Link to Article [https://doi.org/10.1016/j.epsl.2018.03.019]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA
Copyright Elsevier

We report volatile concentrations and hydrogen isotope compositions of impact melts and minerals in EETA 79001. We observed chemical changes in pyroxene, maskelynite (or feldspathic glass), and merrillite in contact with or inside impact melts. All pyroxene grains analyzed here are inside or close to impact melt pockets and contain 10–41 ppm H2O and enriched in D (δD=+1729 to +3707‰), with the highest values found in a grain enclosed in an impact melt pocket. Maskelynite or feldspathic glass contains 6.3 to 98 ppm H2O with δ D values of +1604 to +3938‰. The high H2O and δD values were obtained in those enclosed inside or in contact with the impact melts, whereas low H2O content (4 ppm) and terrestrial-like D/H value (δ D of −90±82‰) were found in one maskelynite grain away from impact melts contains. Rims of ∼5 μm thickness of merrillite grains next to impact melts display Na-depletion by ∼0.9 wt%, and the sides in contact with impact melts show Mg-enrichment by ∼0.5 wt%. However, the H2O and δD values of merrillite interiors (39–242 ppm H2O and δ D of +1682 to +3884‰) do not show correlation with their proximity to the impact melts. Rather, δD and 1/H2O of merrillite form a negative trend different from that of impact melt pockets and maskelynite, suggesting post-crystallization or late-crystallization interactions with the crustal fluids.

The impact melt pockets in EETA 79001 contain 121–646 ppm H2O, 4.3–13 ppm F, 13–50 ppm Cl, 707–2702 ppm S, and the δ D values of +3368 to +4639‰. The correlations between H2O, F, Cl, P2O5, and δD values of impact melts and feldspathic glass are consistent with mixing between a volatile-rich and high δ D (+3000 to +5000‰) endmember and a volatile-poor and low δD endmember. The volatile-poor and low δD endmember is consistent with magmatic volatiles stored in silicates. The volatile-rich and high δD endmember represents pre-impact alteration materials by subsurface water. Alteration from the subsurface water, equilibrated with the present-day-like Martian atmosphere, occurred after the crystallization of the rock (∼170 Ma) and before impact launch (∼0.7 Ma). Our conclusion is different from the previous suggestion of an isotopically distinct subsurface water reservoir with a δ D value of +1000 to +2000‰ in EETA 79001. Although heterogeneous subsurface water on Mars is possible, the previous study was likely biased by a limited number of analyses (n=2) and possible terrestrial contamination. The δ D value of the subsurface source in EETA 79001 is ∼+4200‰, similar to those in the Tissint meteorite (crystallization at ∼600 Ma, impact launch at ∼0.7 Ma) and LAR 06319 (crystallization at ∼200 Ma, impact launch at ∼3 Ma), suggesting stable water chemistry for the subsurface environment in the last 600 Myrs.

¹Matthias M. M. Meier et al. (>10)
Earth and Planetary Science Letters 490, 122-131 Link to Article [https://doi.org/10.1016/j.epsl.2018.03.025]
¹Institute of Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland
Copyright Elsevier

The unique CV-type meteorite Khatyrka is the only natural sample in which “quasicrystals” and associated crystalline Cu, Al-alloys, including khatyrkite and cupalite, have been found. They are suspected to have formed in the early Solar System. To better understand the origin of these exotic phases, and the relationship of Khatyrka to other CV chondrites, we have measured He and Ne in six individual, ∼40–μm-sized olivine grains from Khatyrka. We find a cosmic-ray exposure age of about 2–4 Ma (if the meteoroid was < 3 m in diameter, more if it was larger). The U, Th–He ages of the olivine grains suggest that Khatyrka experienced a relatively recent (<600 Ma) shock event, which created pressure and temperature conditions sufficient to form both the quasicrystals and the high-pressure phases found in the meteorite. We propose that the parent body of Khatyrka is the large K-type asteroid 89 Julia, based on its peculiar, but matching reflectance spectrum, evidence for an impact/shock event within the last few 100 Ma (which formed the Julia family), and its location close to strong orbital resonances, so that the Khatyrka meteoroid could plausibly have reached Earth within its rather short cosmic-ray exposure age.

¹L. Florentin, ¹E. Deloule, ¹F. Faure, ²D. Mangin
Geochimica et Cosmochimica Acta (in Press) Links to Article [https://doi.org/10.1016/j.gca.2018.03.021]
¹CRPG, UMR CNRS 5873, (Université de Lorraine BP20, 54501, Vandœuvre les Nancy, FRANCE)
²IJL, UMR CNRS 7198 (Université de Lorraine, Parc de Saurupt, 54000 Nancy, FRANCE)
Copyright Elsevier

Natural glass inclusions – hosted in Mg-rich olivines from Allende (CV3) type I chondrules – and synthetic melt inclusions – trapped in forsterite crystallized from CMAS (CaO-MgO-Al2O3-SiO2) melts – were mapped by Secondary Ion Mass Spectrometry (SIMS) for CMAS major oxides. The first ever 3D chemical images of extra-terrestrial glass inclusions were obtained, along with chemical depth profiles for each oxide. Results show similar patterns for both synthetic glass inclusions (trapped in olivine formed by slow crystallization in a magmatic liquid) and natural inclusions from Allende’s olivines. No incompatible-rich boundary layer or diffusion pattern was observed in either case. The absence of an incompatible-rich boundary layer suggests that the olivine overgrowth surrounding glass inclusions in Allende’s olivines was formed during slow cooling of the host olivine and likely the surrounding chondrule. This provides new constraints on the cooling rates of type I chondrules.