¹Pavol Matlovič, ¹Juraj Tóth, ²Regina Rudawska, ¹Leonard Kornoš
Planetary and Space Science (in Press) Link to Article [http://dx.doi.org/10.1016/j.pss.2017.02.007]
¹Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia
²ESA European Space Research and Technology Centre, Noordwijk, The Netherlands

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¹Andrew C. Schuerger, ²Doug W. Ming, ³D.C. Golden
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2017.02.023]
¹Dept. of Plant Pathology, University of Florida, 505 Odyssey Way, Exploration Park, Merritt Island, FL, 32953
²Astromaterials Research and Exploration Science Office, Mail Code XI, NASA Johnson Space Center, Houston, TX 77058
³ESCG, Mail Code: JE 23, Houston, TX, 77058
Copyright Elsevier

The search for an extant microbiota on Mars depends on exploring sites that contain transient or permanent liquid water near the surface. Examples of possible sites for liquid water may be active recurring slope lineae (RSL) and fluid inclusions in ice or salt deposits. The presence of saline fluids on Mars will act to depress the freezing points of liquid water to as low as ‒60 °C, potentially permitting the metabolism and growth of halophilic microorganisms to temperatures significantly below the freezing point of pure water at 0 °C. In order to predict the potential risks of forward contamination by Earth microorganisms to subsurface sites on Mars with liquid brines, experiments were designed to characterize the short-term survival of two bacteria in aqueous soil solutions from six analog soils. The term ‘‘soil’’ is used here to denote any loose, unconsolidated matrix with no implications for the presence or absence of organics or biology. The analog soils were previously described (Schuerger et al., 2012, Planetary Space Sci., 72, 91-101), and represented crushed Basalt (benign control), Salt, Acid, Alkaline, Aeolian, and Phoenix analogs on Mars. The survival rates of spores of Bacillus subtilis and vegetative cells of Enterococcus faecalis were tested in soil solutions from each analog at 24, 0, or ‒70 °C for time periods up to 28 d. Survival of dormant spores of B. subtilis were mostly unaffected by incubation in the aqueous extracts of all six Mars analogs. In contrast, survival rates of E. faecalis cells were suppressed by all soil solutions when incubated at 24 °C but improved at 0 and ‒70 °C, except for assays in the Salt and Acid soil solutions in which most cells were killed. Results suggest that Earth microorganisms that form spores may persist in liquid brines on Mars better than non-spore forming species, and thus, spore-forming species may pose a potential forward contamination risk to sites with liquid brines.

¹K. Righter, ²B.M. Go, ³K.A. Pando, ³L. Danielson, ³D.K. Ross, ⁴Z. Rahman, ⁵L.P. Keller
Earth and Planetary Science Letters (in Press) Link to Article [http://dx.doi.org/10.1016/j.epsl.2017.02.003]
¹Mailcode XI2, NASA Johnson Space Center, 2101 NASA Pkwy, Houston, TX 77058, United States
²University of Chicago, Dept. Geophys. Sci., 5801 S. Ellis Ave., Chicago, IL 60637, United States
³Jacobs JETS, NASA Johnson Space Center, 2101 NASA Pkwy, Houston, TX 77058, United States
⁴Jacobs, NASA Johnson Space Center, Houston, TX 77058, United States
⁵Mailcode XI3, NASA Johnson Space Center, 2101 NASA Pkwy, Houston, TX 77058, United States
Copyright Elsevier

Multiple lines of geochemical and geophysical evidence suggest the Moon has a small metallic core, yet the composition of the core is poorly constrained. The physical state of the core (now or in the past) depends on detailed knowledge of its composition, and unfortunately, there is little available data on relevant multicomponent systems (i.e., Fe–Ni–S–C) at lunar interior conditions. In particular, there is a dearth of phase equilibrium data to elucidate whether a specific core composition could help to explain an early lunar geodynamo and magnetic field intensities, or current solid inner core/liquid outer core states. We utilize geochemical information to estimate the Ni, S and C contents of the lunar core, and then carry out phase equilibria experiments on several possible core compositions at the pressure and temperature conditions relevant to the lunar interior. The first composition is 0.5 wt% S and 0.375 wt% C, based on S and C contents of Apollo glasses. A second composition contains 1 wt% each of S and C, and assumes that the lunar mantle experienced degassing of up to 50% of its S and C. Finally a third composition contains C as the dominant light element. Phase equilibrium experiments were completed at 1, 3 and 5 GPa, using piston cylinder and multi-anvil techniques. The first composition has a liquidus near 1550 °C and solidus near 1250 °C. The second composition has a narrower liquidus and solidus temperatures of 1400 and 1270 °C, respectively, while the third composition is molten down to 1150 °C. As the composition crystallizes, the residual liquid becomes enriched in S and C, but S enrichment is greater due to the incorporation of C (but not S) into solid metallic FeNi. Comparison of these results to thermal models for the Moon allow an evaluation of which composition is consistent with the geophysical data of an early dynamo and a currently solid inner and liquid outer core. Composition 1 has a high enough liquidus to start crystallizing early in lunar history (4.3 Ga), consistent with the possible core dynamo initiated by crystallization of a solid inner core. Composition 1 also stays partially molten throughout lunar history, and could easily explain the seismic data. Composition 2, on the other hand, can satisfy one or the other set of geophysical data, but not both and thus seems like a poor candidate for a lunar core composition. Composition 3 remains molten to temperatures that are lower than current estimates for the lunar core, thus ruling out the possibility of a C-rich (and S-poor) lunar core. The S- and C-poor core composition studied here (composition 1) is consistent with all available geochemical and geophysical data and provides a simple heat source and mechanism for a lunar core dynamo (core crystallization) that would obviate the need for other primary mechanisms such as impacts, core–mantle coupling, or unusual thermal histories.

¹Ann N. Nguyen, ¹Larry R. Nittler, ¹Conel M.O’D. Alexander, ²Peter Hoppe
Geochimica et Cosmochmica Acta(in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.02.026]
¹Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington DC, USA
²Max Planck Institute for Chemistry, Mainz, Germany
Copyright Elsevier

We report the Ti isotopic compositions of 8 mainstream, 22 Y, 9 Z, and 26 AB presolar SiC grains from two SiC-rich residues of the Murchison CM2 meteorite together with Si, C and Mg-Al isotopic data for the same grains. Mainstream, Y and Z grains are believed to originate in asymptotic giant branch (AGB) stars of varying metallicities, but the stellar sources of AB grains are poorly understood. We find that the 46,47,49Ti/48Ti ratios are correlated with 29Si/28Si for all of the grain types, indicating these ratios are mainly dominated by Galactic chemical evolution (GCE). The mainstream, Y and Z grains all show enrichments in 50Ti from neutron capture nucleosynthesis. However, AGB models predict smaller excesses in 50Ti (and 49Ti) than are observed in these grains. For Z grains and especially for Y grains, the enhancement of 50Ti is greater than the enhancement in 30Si, indicating that the 13C neutron source produced a greater total fluence of neutrons than the 22Ne source in the low metallicity parent AGB stars. The Z grains plot below the mainstream correlation lines at more 48Ti- and 28Si-rich compositions in plots of 46,47,49Ti/48Ti vs. 29Si/28Si. On the other hand, the Y grains plot close to the mainstream correlation line. This could imply that the Ti isotopes evolved non-linearly at metallicities below ∼1/3 solar. The AB grains in this study have Ti isotopic compositions that fall along correlation lines defined by the mainstream grains, suggesting origins in close to solar metallicity stars. However, these grains fall below the mainstream correlation lines in plots of 46,49,50Ti/48Ti vs. 29Si/28Si and do not show enhancements in 50Ti, indicating that their parent stars did not undergo significant s-process nucleosynthesis. These data support origins of AB grains in J-type C stars rather than born-again AGB stars that undergo s-process nucleosynthesis. AB grains that do not have 50Ti excesses may provide the best measure of Si and Ti isotope GCE since their parent stars were less affected by s-process nucleosynthesis than the mainstream grains.

¹Stephen M. Elardo, ¹Anat Shahar
Nature Geoscience (in Press) Link to Article [doi:10.1038/ngeo2896]
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA

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¹M. Bose, ²R. A. Root, ³S. Pizzarello
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12811]
¹School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
²Department of Soil, Water & Environmental Science, University of Arizona, Tucson, Arizona, USA
³School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
Published by arrangement with John Wiley & Sons

Insoluble organic matter (IOM) and hydrothermally treated IOM extracted from two carbonaceous chondrites, Murchison and Allende, was studied using sulfur K-edge XANES (X-ray absorption near edge structure) and μ-Raman spectroscopy, with the aim to understand their IOM’s sulfur speciation and structural order, and how aqueous alteration or thermal metamorphism may have transformed these materials. We found that the sulfur-functional group chemistry of both the Murchison IOM and hydrothermally treated IOM samples have a large chemical variability ranging from oxidation states of S−2 to S+6, and exhibit a transformation in their oxidation state after the hydrothermal treatment (HT) to produce thiophenes and thiol compounds. Sulfoxide and sulfite peaks are also present in Murchison. Sulfates considered intrinsic to Murchison are most likely preaccretionary in nature, and not a result of reactions with water at high temperatures on the asteroid parent body. We argue that the reduced sulfides may have formed in the CM parent body, while the thiophenes and thiol compounds are a result of the HT. Micro-Raman spectra show the presence of aliphatic and aromatic moieties in Murchison’s material as observed previously, which exhibits no change after HT. Because the Murchison IOM was modified, as seen by XANES analysis, absence of a change observed using micro-Raman indicated that although the alkyl carbons of IOM were cleaved, the aromatic network was not largely modified after HT. By contrast, Allende IOM contains primarily disulfide and elemental sulfur, no organic sulfur, and shows no transformation after HT. This nontransformation of Allende IOM after HT would indicate that parent body alteration of sulfide to sulfate is not feasible up to temperatures of 300°C. The reduced sulfur products indicate extreme secondary chemical processing from the precursor compounds in its parent body at temperatures as high as 624°C, as estimated from μ-Raman D band parameters. The Raman parameters in Allende IOM that was interpreted in terms of amorphous carbon with regions of large clusters of benzene rings, was transformed after the HT to those with fewer benzene rings.

¹Anke Wohlers, ¹Bernard J. Wood
Geochimica et Cosmochimica acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.01.050]
¹Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
Copyright Elsevier

We have performed experiments at 1.5 GPa over the temperature range 1400-2100°C to determine the partitioning of lithophile elements (U, Th, Eu, Sm, Nd, Zr, La, Ce, Yb) between sulfide liquid, low-S metals and silicate melt. The data demonstrate pronounced increases in partitioning of all the lithophile elements into sulfide at very low FeO contents (10 in some cases. Similarly DSm may be > 2 under the same conditions of low silicate FeO. This strong partitioning behaviour is found only be important in S-rich metals, however because the observed effect of low FeO on partitioning is uniquely confined to metallic melts close to stoichiometric FeS in composition.
The results and the effects of FeS content of the metal and FeO content (or activity) of the silicate may be understood in terms of exchange reactions such as:

UO2+2FeS=2FeO+US2UO2+2FeS=2FeO+US2
silicate sulfide silicate sulfide

High concentrations of FeS (in metal) and low FeO contents of the silicate melts drive the reaction to the right, favouring high US2 in the sulfide and hence high DU. The effect is, we find, enhanced by the high solubility of S in the silicate (up to 11 wt%) at low FeO contents. This S content greatly reduces the activity coefficient of FeO in the silicate melt, enhancing the displacement of the reaction to the right.
For sulfide-silicate partitioning at 1.5GPa and 1400°C we obtain DNd/DSm of about 1.4 and DTh ∼ 0.1DU. With increasing temperature the differences between these geochemically similar element pairs decreases such that, at 2100°C DNd/DSm is 1.0 and DTh/DU is about 0.3. We used these results, together with DU and DSm to model addition of a putative Mercury-like component (with FeS core) to early Earth. We find that the 1400o results could lead to a significant (∼11ppm) 142Nd anomaly in silicate Earth and add >8 ppb U to the core, but lead to an unreasonably high Th/U of silicate Earth (4.54). Based on the 2100°C results the 142Nd anomaly would be 0 but addition of the sulfur-rich body could add up to 10 ppb of U to the core, generating, when the accompanying 21 ppb Th is also considered, ∼3 TW of the energy required for the geodynamo. In this case, the Th/U ratio of silicate Earth would approximate 4.3, within the range of some estimates.

¹Yogita Kadlag, ¹Harry Becker
Chemie der Erde (in Press) Link to Article [http://dx.doi.org/10.1016/j.chemer.2017.01.004]
¹Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany
Copyright Elsevier

Osmium isotopic compositions, abundances of highly siderophile elements (HSE: platinum group elements, Re and Au), the chalcogen elements S, Se and Te and major and minor elements were analysed in physically separated size fractions and components of the ordinary chondrites WSG 95300 (H3.3, meteorite find) and Parnallee (LL3.6, meteorite fall). Fine grained magnetic fractions are 268-65 times enriched in HSE compared to the non-magnetic fractions. A significant deviation of some fractions of WSG 95300 from the 4.568 Ga 187Re-187Os isochron was caused by redistribution of Re due to weathering of metal. HSE abundance patterns show that at least four different types of HSE carriers are present in WSG 95300 and Parnallee. The HSE carriers display (i) CI chondritic HSE ratios, (ii) variable Re/Os ratios, (iii) lower than CI chondritic Pd/Ir and Au/Ir and (iv) higher Pt/Ir and Pt/Ru than in CI chondrites. These differences between components clearly indicate the loss of refractory HSE carrier phases before accretion of the components. Tellurium abundances correlate with Pd and are decoupled from S, suggesting that most Te partitioned into metal during the last high-temperature event. Tellurium is depleted in all fractions compared to CI chondrite normalized Se abundances. The depletion of Te is likely associated with the high temperature history of the metal precursors of H and LL chondrites and occurred independent of the metal loss event that depleted LL chondrites in siderophile elements. Most non-magnetic and slightly magnetic fractions have S/Se close to CI chondrites. In contrast, the decoupling of Te and Se from S in magnetic fractions suggests the influence of volatility and metal-silicate partitioning on the abundances of the chalcogen elements. The influence of terrestrial weathering on chalcogen element systematics of these meteorites appears to be negligible.

¹C.M.O’D. Alexander, ²G.D. Cody, ³B.T. De Gregorio, ¹L.R. Nittler, ³R.M. Stroud
Chemie der Erde (in Press) Link to Article [http://dx.doi.org/10.1016/j.chemer.2017.01.007]
¹Dept. Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA
²Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Washington, DC 20015, USA
³Materials Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
Copyright Elsevier

All chondrites accreted ∼3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth’s volatiles and a potential source of complex prebiotic molecules.

Large enrichments in D and 15N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion.

Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.

¹Romy D. Hanna, ¹Richard A. Ketcham
Chemie der Erde (in Press) Link to Article [http://dx.doi.org/10.1016/j.chemer.2017.01.006]
¹Jackson School of Geosciences, University of Texas, 2275 Speedway Stop C9000, Austin, TX 78712, USA
Copyright Elsevier

X-ray computed tomography (XCT) is a powerful 3D imaging technique that has been used to investigate meteorites, mission-returned samples, and other planetary materials of all scales from dust particles to large rocks. With this technique, a 3D volume representing the X-ray attenuation (which is sensitive to composition and density) of the materials within an object is produced, allowing various components and textures to be observed and quantified. As with any analytical technique, a thorough understanding of the underlying physical principles, system components, and data acquisition parameters provides a strong foundation for the optimal acquisition and interpretation of the data. Here we present a technical overview of the physics of XCT, describe the major components of a typical laboratory-based XCT instrument, and provide a guide for how to optimize data collection for planetary materials using such systems. We also discuss data processing, visualization and analysis, including a discussion of common data artifacts and how to minimize them. We review a variety of recent studies in which XCT has been used to study extraterrestrial materials and/or to address fundamental problems in planetary science. We conclude with a short discussion of anticipated future directions of XCT technology and application.