^1,2A. R. Poppe,^2,3W. M. Farrell,^2,4J. S. Halekas
Journal of Geophysical Research, Planets Link to Article [DOI: 10.1002/2017JE005426]
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA, USA
²Solar System Exploration Research Virtual Institute, NASA Ames Research Center, Moffett Field, CA, USA
³NASA Goddard Space Flight Center, Greenbelt, MD, USA
⁴Dept. of Physics and Astronomy, University of Iowa, Iowa City, IA, USA
Published by arrangement with John Wiley & Sons

The weathering of airless bodies exposed to space is a fundamental process in the formation and evolution of planetary surfaces. At the Moon, space weathering induces a variety of physical, chemical, and optical changes including the formation of nanometer sized amorphous rims on individual lunar grains. These rims are formed by vapor redeposition from micrometeoroid impacts and ion irradiation-induced amorphization of the crystalline matrix. For ion irradiation-induced rims, however, laboratory experiments of the depth and formation timescales of these rims stand in stark disagreement with observations of lunar soil grains. We use observations by the ARTEMIS spacecraft in orbit around the Moon to compute the mean ion flux to the lunar surface between 10 eV and 5 MeV and convolve this flux with ion irradiation-induced vacancy production rates as a function of depth calculated using the Stopping Range of Ions in Matter (SRIM) model. By combining these results with laboratory measurements of the critical fluence for charged-particle amorphization in olivine, we can predict the formation timescale of amorphous rims as a function of depth in olivinic grains. This analysis resolves two outstanding issues: (1) the provenance of >100 nm amorphous rims on lunar grains and (2) the nature of the depth-age relationship for amorphous rims on lunar grains.

¹P.E.Martin et al. (>10)
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005445]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
Published by arrangement with John Wiley & Sons

Following K-Ar dating of a mudstone and a sandstone, a third sample has been dated by the Curiosity rover exploring Gale Crater. The Mojave 2 mudstone, which contains relatively abundant jarosite, yielded a young K-Ar bulk age of 2.57 ± 0.39 Ga (1σ precision). A two-step heating experiment was implemented in an effort to resolve the K-Ar ages of primary and secondary mineralogical components within the sample. This technique involves measurement of 40Ar released in low (500oC) and high (930oC) temperature steps, and a model of the potassium distribution within the mineralogical components of the sample. Using this method, the high-temperature step yields a K-Ar model age of 4.07 ± 0.63 Ga associated with detrital plagioclase, compatible with the age obtained on the Cumberland mudstone by Curiosity. The low-temperature step, associated with jarosite mixed with K-bearing evaporites and/or phyllosilicates, gave a youthful K-Ar model age of 2.12 ± 0.36 Ga. The interpretation of this result is complicated by the potential for argon loss after mineral formation. Comparison with the results on Cumberland and previously published constraints on argon retentivity of the individual phases likely to be present suggests that the formation age of the secondary materials, correcting for plausible extents of argon loss, is still less than 3 Ga, suggesting post-3 Ga aqueous processes occurred in the sediments in Gale Crater. Such a result is inconsistent with K-bearing mineral formation in Gale Lake, and instead suggests post-depositional fluid flow at a time after surface fluvial activity on Mars is thought to have largely ceased.

¹D.S. Vogt, ¹K. Rammelkamp, ¹S. Schröder, ^1,2H.W. Hübers
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.12.006]
¹Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institute of Optical Sensor Systems, Berlin, Germany
²Humboldt-Universität zu Berlin, Department of Physics, Berlin, Germany
Copyright Elsevier

The intensity of the molecular CaCl emission in LIBS spectra is examined in order to evaluate its suitability for the detection of chlorine in a Martian environment. Various mixtures resembling Martian targets with varying Cl content are investigated under simulated Martian conditions. The reactions leading to the formation of CaCl are modeled based on reaction kinetics and are used to fit the measured CaCl band intensities. MgCl bands are also investigated as potential alternatives to CaCl, but no MgCl bands can be identified in samples containing both Mg and Cl. The study confirms that CaCl is well suited for the indirect detection of chlorine, but finds a strong dependence on the concentrations of Ca and Cl in the sample. Spectra from samples with a high chlorine concentration can have low-intensity CaCl emission due to a deficiency of Ca. A qualitative estimate of the sample composition is possible based on the ratio of the band intensity of CaCl to the intensity of Ca emission lines. Time-resolved measurements show that the CaCl concentration in the plasma is highest after about 1 µs.

¹Zoe A. Landsman, ²Joshua P. Emery, ¹Humberto Campins, ³Josef Hanuš, ⁴Lucy F. Lim, ⁵Dale P. Cruikshank
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.11.035]
¹Department of Physics, University of Central Florida, 4111 Libra Drive, PS 430, Orlando, FL, 32826, United States
²Earth and Planetary Science Department, Planetary Geosciences Institute, University of Tennessee, 306 EPS Building, 1412 Circle Dr, Knoxville, TN, 37996, United States
³Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, Prague, 18000, Czech Republic
⁴NASA Goddard Space Flight Center, Mail Code 691, Greenbelt, MD, 20771, United States
⁵NASA Ames Research Center, Mail Stop 245-6, Moffett Field, CA, 94035, United States
Copyright Elsevier

Asteroid (16) Psyche is a unique, metal-rich object belonging to the “M” taxonomic class. It may be a remnant protoplanet that has been stripped of most silicates by a hit-and-run collision. Because Psyche offers insight into the planetary formation process, it is the target of NASA’s Psyche mission, set to launch in 2023. In order to constrain Psyche’s surface properties, we have carried out a mid-infrared (5–14 μm) spectroscopic study using data collected with the Spitzer Space Telescope’s Infrared Spectrograph. Our study includes two observations covering different rotational phases. Using thermophysical modeling, we find that Psyche’s surface is smooth and likely has a thermal inertia Γ = 5–25 J/m2/K/s1/2 and bolometric emissivity ϵ= 0.9, although a scenario with ϵ=0.7 and thermal inertia up to 95 J/m2/K/s1/2 is possible if Psyche is somewhat larger than previously determined. The smooth surface is consistent with the presence of a metallic bedrock, which would be more ductile than silicate bedrock, and thus may not readily form boulders upon impact events. From comparisons with laboratory spectra of silicate and meteorite powders, Psyche’s 7–14μm emissivity spectrum is consistent with the presence of fine-grained ( < 75μm) silicates on Psyche’s surface. We conclude that Psyche is likely covered in a fine silicate regolith, which may also contain iron grains, overlying an iron-rich bedrock.

¹Christopher W. Haberle, ^1,2Laurence A.J. Garvie
The American Mineralogist 102,2415-2421 Link to Article [DOI: https://doi.org/10.2138/am-2017-6180]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
¹Center for Meteorite Studies, Arizona State University, Arizona 85287-6004, U.S.A
Copyright: The Mineralogical Society of America

The CM and CI carbonaceous chondrites are typically dominated by phyllosilicates with variable proportions of tochilinite, anhydrous silicates, carbonates, sulfides, sulfates, oxides, and organic compounds. During thermal metamorphism the phyllosilicates dehydrate and decompose yielding water and olivine/enstatite. The thermal transformation of carbonate is less well understood, especially in the presence of volatile decomposition products, such as CO, CO2, SO2, H2S, and H2O. Here is described the mineralogical transformation of calcite (CaCO3) to oldhamite (CaS) and portlandite [Ca(OH)2] during extraterrestrial thermal metamorphism on the Sutter’s Mill parent body. Sutter’s Mill is a regolith breccia consisting of at least two lithologic components: phyllosilicate-calcite-bearing and anhydrous olivine-rich. Evidence suggests that the anhydrous stones were derived from extraterrestrial heating of the phyllosilicate-calcite-bearing material. One of only three Sutter’s Mill stones (SM3) collected prior to heavy rainfall over the recovery site is the focus of this study. Its powder X-ray diffraction patterns are dominated by olivine, with lesser enstatite, Fe-sulfides, magnetite, and oldhamite. Oldhamite is absent in the rained-on stones reflecting its water sensitivity and the pristine nature of SM3. Optical micrographs show whitish to bluish grains of oldhamite and portlandite embedded in dark, fine-grained matrix. The presence of abundant olivine and absence of phyllosilicates, tochilinite, and carbonate indicates that SM3 underwent heating to ~750 °C. At this temperature, calcite would have decomposed to lime (CaO). Volatilization experiments show that CO, CO2, SO2, and H2S evolve from CM and CI chondrites heated above 600 °C. Lime that formed through calcite decomposition would have reacted with these gases forming oldhamite under reducing conditions. Residual lime not converted to oldhamite, would have readily hydrated to portlandite, possibly through retrograde reactions during cooling on the parent body. These reactions have parallels to those in coal-fired electricity generating plants and provide an analogous system to draw comparison. Furthermore, the identification of these minerals, which are sensitive to terrestrial alteration, and determination of their formation is enabled only by the rapid collection of samples from an observed fall and their subsequent curation.

¹T. Hopp, ¹M. Fischer-Gödde, ¹T. Kleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.11.033]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

Mass-dependent Ru isotope variations (δ102/99Ru) and Ru concentrations were determined for 35 magmatic iron meteorites from the five major chemical groups (IIAB, IID, IIIAB, IVA, IVB). In addition, four equilibrated ordinary chondrites were analyzed. The IIAB, IIIAB and IVB iron meteorites display increasingly heavier Ru isotopic compositions with decreasing Ru content. Modeling demonstrates that the trends for these three iron groups can be reproduced by the incremental extraction of isotopically lighter Ru into solids, which leads to progressively heavier δ102/99Ru in the remaining melt. The modeling further shows that the Ru isotopic variations of the IIAB and IIIAB irons are consistent with derivation from parental melts with an ordinary chondrite-like δ102/99Ru, whereas the IVB irons more likely derive from a melt with heavier δ102/99Ru. This heavy Ru isotopic composition of the IVB parental melt probably results from high-temperature processing of the IVB precursor material. The Ru isotope systematics of the IID and IVA irons are more complex and show no correlation between δ102/99Ru and Ru content. Although most samples exhibit heavy Ru isotopic compositions, especially the late-crystallized irons of these groups deviate from the expected fractional crystallization trends. This deviation most likely results from mixing and re-equilibration of early-crystallized solids and late-stage liquids, followed by further fractional crystallization. The mixing might be related to the migration of liquids through a complex network of dendrites or to the overturn of a cumulate inner core, and bears testimony to the complex solidification history of at least some protoplanetary cores.

¹Zhou Zhang,¹Anne Pommier
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005390]
¹University of California San Diego, Scripps Institution of Oceanography, Institute of Geophysics and Planetary Physics, La Jolla, CA, USA
Published by arrangement with John Wiley & Sons

We report electrical conductivity measurements on metal–olivine systems at 5 and 7 GPa and up to 1675°C in order to investigate the electrical properties of Core-Mantle Boundary (CMB) systems. Electrical experiments were conducted in the multi-anvil apparatus using the impedance spectroscopy technique. The samples are composed of one metal layer (Fe, FeS, FeSi2, or Fe-Ni-S-Si) and one polycrystalline olivine layer, with the metal:olivine ratio ranging from 1:0.7 to 1:9.2. For all samples, we observe that the bulk electrical conductivity increases with temperature from 10-2.5 to 101.8 S/m, which is higher than the conductivity of polycrystalline olivine but lower than the conductivity of the pure metal phase at similar conditions. In some experiments, a conductivity jump is observed at the temperature corresponding to the melting temperature of the metallic phase. Both the metal:olivine ratio and the metal phase geometry control the electrical conductivity of the two-layer samples. By combining electrical results, textural analyses of the samples, and previous studies of the structure and composition of Mercury’s interior, we propose an electrical profile of the deep interior of the planet that accounts for a layered CMB-outer core structure. The electrical model agrees with existing conductivity estimates of Mercury’s lower mantle and CMB using magnetic observations and thermodynamic calculations, and thus, supports the hypothesis of a layered CMB-outermost core structure in the present-day interior of Mercury. We propose that the layered CMB-outer core structure is possibly electrically insulating, which may influence the planet’s structure and cooling history.

¹Bérengère Mougel, ^1,2Frédéric Moynier, ¹Christa Göpel
Earth and Planetary Science Letters 481, 1-8 Link to Article [https://doi.org/10.1016/j.epsl.2017.10.018]
¹Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS UMR 7154, Paris, France
²Institut Universitaire de France and Université Paris Diderot, Paris, France
Copyright Elsevier

Among the elements exhibiting non-mass dependent isotopic variations in meteorites, chromium (Cr) has been central in arguing for an isotopic homogeneity between the Earth and the Moon, thus questioning physical models of Moon formation. However, the Cr isotopic composition of the Moon relies on two samples only, which define an average value that is slightly different from the terrestrial standard. Here, by determining the Cr isotopic composition of 17 lunar, 9 terrestrial and 5 enstatite chondrite samples, we re-assess the isotopic similarity between these different planetary bodies, and provide the first robust estimate for the Moon. In average, terrestrial and enstatite samples show similar ε54Cr. On the other hand, lunar samples show variables excesses of 53Cr and 54Cr compared to terrestrial and enstatite chondrites samples with correlated ε53Cr and ε54Cr (per 10,000 deviation of the 53Cr/52Cr and 54Cr/52Cr ratios normalized to the 50Cr/52Cr ratio from the NIST SRM 3112a Cr standard). Unlike previous suggestions, we show for the first time that cosmic irradiation can affect significantly the Cr isotopic composition of lunar materials. Moreover, we also suggest that rather than spallation reactions, neutron capture effects are the dominant process controlling the Cr isotope composition of lunar igneous rocks. This is supported by the correlation between ε53Cr and ε54Cr, and 150Sm/152Sm ratios. After correction of these effects, the average ε54Cr of the Moon is indistinguishable from the terrestrial and enstatite chondrite materials reinforcing the idea of an Earth–Moon–enstatite chondrite system homogeneity. This is compatible with the most recent scenarios of Moon formation suggesting an efficient physical homogenization after a high-energy impact on a fast spinning Earth, and/or with an impactor originating from the same reservoir in the inner proto-planetary disk as the Earth and enstatite chondrites and having similar composition.

¹K. Righter,²K. Pando,³N. Marin,^2,1,4D. K. Ross,⁵M. Righter,²L. Danielson,⁵T. J. Lapen,⁶C. Lee
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13005]
¹Mailcode XI2, NASA Johnson Space Center, Houston, Texas, 77058, USA
²Jacobs JETS, NASA Johnson Space Center, Houston, Texas, USA
³School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
⁴University of Texas El Paso, Houston, Texas, USA
⁵Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, USA
⁶Department of Earth Science, Rice University, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Volatile element concentrations in planets are controlled by many factors such as precursor material composition, core formation, differentiation, magma ocean and magmatic degassing, and late accretionary processes. To better constrain the role of core formation, we report new experiments defining the effect of temperature, and metallic S and C content on the metal-silicate partition coefficient (or D(i) metal/silicate) of the volatile siderophile elements (VSE) Bi, Cd, In, and Sn. Additionally, the effect of pressure on metal-silicate partitioning between 1 and 3 GPa, and olivine-melt partitioning at 1 GPa have been studied for Bi, Cd, In, Sn, As, Sb, and Ge. Temperature clearly causes a decrease in D(i) metal/silicate for all elements. Sulfur and C have a large influence on activity coefficients in metallic Fe liquids, with C causing a decrease in D(i) metal/silicate, and S causing an increase. Pressure has only a small effect on D(Cd), D(In), and D(Ge) metal/silicate. Depletions of Bi, Cd, In, and Sn in the terrestrial and Martian mantles are consistent with high PTcore formation and metal-silicate equilibrium at the high temperatures indicated by previous studies. A late Hadean matte would influence Bi the most, due to its high D(sulfide/silicate) ~2000, but segregation of a matte would only reduce the mantle Bi content by 50%; all other less chalcophile elements (e.g., Sn, In, and Cd) would be minimally affected. The lunar depletions of highly VSE require a combination of core formation and an additional depletion mechanism—most likely the Moon-forming giant impact, or lunar magma ocean degassing.

¹Seann J. McKibbin, ¹Hugh St. C. O’Neill
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13004]
¹Research School of Earth Sciences, Australian National University, Acton, Australian Capital Territory, Australia
Published by arrangement with John Wiley & Sons

Angrite meteorites are samples of early planetesimal magmatic rocks, distinguished from more typical “basaltic eucrites” by compositions that are silica undersaturated, relatively oxidized, and with high CaO/Al2O3. The latter is not expected from nebular, chondritic materials that might form a primitive mantle, such as a source enriched in refractory inclusions with fixed CaO/Al2O3 (e.g., CV chondrite). Here we present results of “reversal” crystallization experiments for two possible parental angrite compositions (approximating the D’Orbigny meteorite) to investigate the role of spinel as a sink for Al2O3. This mineral has previously been produced with angritic melts during “forward” melting of CV chondrite and may be abundant in the angrite source. At oxidizing conditions, we confirm that spinel is a liquidus phase and that angritic magmas form near the olivine-anorthite-spinel-liquid peritectic. A stability gap separates Al-rich liquidus spinels and lower temperature spinels, the latter of which are similar to those in basaltic eucrites. Al-rich spinel is likely more abundant in the angritic source than other Fe-rich core-forming components such as metal or sulfide, and a CV chondrite-like composition generates most features of angrite magmas by fractionation of observed olivine and liquidus spinel. Direct CaO excess, via carbonate addition, is therefore limited. In this model, discrepancies remain for Li, Sc, Cr(-Al), and Ba, which may record local accretion conditions or early processing. The possible role of spinel as a sink for 26Al may have strong influence on the thermal evolution of the angrite parent body.