¹Kathleen E. Vander Kaaden,²Francis M. McCubbin,^1,3Amber A. Turner,^1,4D. Kent Ross
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006239]
¹Jacobs, NASA Johnson Space Center, Houston, TX, USA
²NASA Johnson Space Center, Houston, TX, USA
³Department of Geoscience, University of Las Vegas, Las Vegas, NV, USA
⁴University of Texas at El Paso‐CASSMAR, El Paso, TX, USA
Published by arrangement with John Wiley & Sons

The composition of a planet’s core has important implications for the thermal and magmatic evolution of that planet. Here, we conducted carbon (C) solubility experiments on iron‐silicon (Fe‐Si) metal mixtures (up to 35 wt% [~52 atom%] Si) at 1 GPa and 800–1800°C to determine the carbon concentration at graphite saturation (CCGS) in metallic melt and crystalline metal with varying proportions of Fe and Si to constrain the C content of Mercury’s core. Our results, combined with those in the literature, show that composition is the major controlling factor for carbon solubility in Fe‐rich metal with minimal effects from temperature and pressure. Moreover, there is a strong anticorrelation between the abundances of carbon and silicon in iron‐rich metallic systems. Based on the previous estimates of <1–25 wt% Si in Mercury’s core, our results indicate that a carbon‐saturated Mercurian core has 0.5–6.4 wt% C, with 6.4 wt% C corresponding to an Si‐free, Fe core and 0.5 wt% C corresponding to an Fe‐rich core with 25 wt% Si. The upper end of estimated FeO abundances in the mantle (up to 2.2 wt%) are consistent with a core that has <1 wt% Si and up to 6.4 wt% C, which would imply that bulk Mercury has a superchondritic Fe/Si ratio. However, the lower end of estimated FeO (≤0.05 wt%) supports CB chondrite‐like bulk compositions of Mercury with core Si abundances in the range of 5–18.5 wt% and C abundances in the range of 0.8–4.0 wt%.

¹B. S. Prince,¹M. P. Magnuson,²L. C. Chaves,²M. S. Thompson,^3,4M. J. Loeffler
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006242]
¹Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
³Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA⁴Center for Materials Interfaces in Research and Applications, Northern Arizona University, Flagstaff, AZ, USA
Published by arrangement with John Wiley & Sons

Here we present results from pulsed laser irradiation of troilite samples in an effort to simulate space weathering on airless bodies via micrometeorite impacts. We find that the spectral trends observed in directly irradiated samples and samples with a vapor‐deposited coating are different than those found in silicate minerals previously studied. For instance, direct laser irradiation causes our troilite samples to initially brighten, but continued irradiation causes darkening and a decrease in spectral slope. In contrast, our samples with a vapor‐deposited coating show a continuous increase in spectral slope and overall albedo as the deposit thickness increases. Observation using both digital imaging and electron microscopy of our directly irradiated samples leads us to conclude that topography effects likely become important after a relatively high number of laser pulses in our directly irradiated samples, causing the apparent darkening and decrease in spectral slope. Thus, we conclude that the spectral changes observed relevant to space weathering via micrometeorite impacts are an increase in spectral slope and an increase in the albedo of troilite. Future studies will investigate whether these trends are generally representative of other sulfide‐bearing minerals and of weathering trends in other components found in the asteroid regolith.

¹Roger R. Fu,^1,2Pauli Kehayias,¹Benjamin P. Weiss,³Devin L. Schrader,⁴Xue‐Ning Bai,^5,6,7Jacob B. Simon
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2019JE006260]
¹Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
²Sandia National Laboratories, Albuquerque, NM, USA
³Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
⁴Institute for Advanced Study, Tsinghua University, Beijing, China
⁵Department of Physics and Astronomy, Iowa State University of Science and Technology, Ames, IA, USA
⁶JILA, University of Colorado Boulder and NIST, Boulder, CO, USA
⁷Department of Space Studies, Southwest Research Institute, Boulder, CO, USA
Published by arrangement with John Wiley & Sons

Theoretical investigations suggest that magnetic fields may have played an important role in driving rapid stellar accretion rates and efficient planet formation in protoplanetary disks. Experimental constraints on magnetic field strengths throughout the solar nebula can test the occurrence of magnetically driven disk accretion and the effect of magnetic fields on planetary accretion. Here we conduct paleomagnetic experiments on chondrule samples from primitive CR (Renazzo type) chondrites GRA 95229 and LAP 02342, which likely originated in the outer solar system between 3 and 7 AU approximately 3.7 million years after calcium aluminum‐rich inclusion formation. By extracting and analyzing 18 chondrule subsamples that contain primary, igneous ferromagnetic minerals, we show that CR chondrules carry internally non‐unidirectional magnetization that requires formation in a nebular magnetic field of ≤8.0 ± 4.3 μT (2σ ). These weak magnetic fields may be due to the secular decay of nebular magnetic fields by 3.7 million years after calcium aluminum‐rich inclusions, spatial heterogeneities in the nebular magnetic field, or a combination of both effects. The possible inferred existence of spatial variations in the nebular magnetic field would be consistent with a prominent role for disk magnetism in the formation of density structures leading to gaps and planet formation.

^1,2Lidia Pittarello,^3,4,5Luke Daly,³Annemarie E. Pickersgil,¹Ludovic Ferrière,³Martin R. Lee
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13494]
¹Department of Mineralogy and Petrography, Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
²Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A‐1090 Vienna, Austria
³School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow, G12 8QQ UK
⁴Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, GPO Box U 1987, Perth, Western Australia, 6845 Australia
⁵Australian Centre for Microscopy and Microanalysis, University of Sydney, Sydney, 2006 New South Wales, Australia
Published by arrangement with John Wiley & Sons

Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals.

¹David A. Kring et al. (>10)
Science Advances 22, eaaz3053 Link to Article [DOI: https://doi.org/10.1126/sciadv.aaz3053]
¹Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, TX 77058, USA.

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^1,2Lee F. White,^1,2Kimberly T. Tait,³Brian Langelier,⁴Elizabeth A. Lymer,^1,2Ana Černok,^1,2 Tanya V. Kizovski,⁵Chi Ma,⁶Oliver Tschauner,¹Richard I. Nicklin
Proceedings of the National Academy of Sciences of the United States of America (in Press) Link to Article [DOI: https://doi.org/10.1073/pnas.2003276117]
¹Department of Natural History, Royal Ontario Museum, Toronto, ON M5S 2C6, Canada;
²Department of Earth Sciences, University of Toronto, Toronto, ON M5S 3B1, Canada;
³Canadian Centre for Electron Microscopy, McMaster University, Hamilton, ON L8S 4M1, Canada;
⁴Lassonde School of Engineering, York University, Toronto, ON M3J 1P3, Canada;
⁵Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125;
⁶Department of Geoscience, University of Nevada, Las Vegas, NV 89154

Understanding the timing and mechanisms of amino acid synthesis and racemization on asteroidal parent bodies is key to demonstrating how amino acids evolved to be mostly left-handed in living organisms on Earth. It has been postulated that racemization can occur rapidly dependent on several factors, including the pH of the aqueous solution. Here, we conduct nanoscale geochemical analysis of a framboidal magnetite grain within the Tagish Lake carbonaceous chondrite to demonstrate that the interlocking crystal arrangement formed within a sodium-rich, alkaline fluid environment. Notably, we report on the discovery of Na-enriched subgrain boundaries and nanometer-scale Ca and Mg layers surrounding individual framboids. These interstitial coatings would yield a surface charge state of zero in more-alkaline fluids and prevent assimilation of the individual framboids into a single grain. This basic solution would support rapid synthesis and racemization rates on the order of years, suggesting that the low abundances of amino acids in Tagish Lake cannot be ascribed to fluid chemistry.

^1,2,3Y. Li,^3,4A. E. Rubin,¹W. Hsu,⁴K. Ziegler
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2019JE006360]
¹Center for Excellence in Comparative Planetology, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
²Macau University of Science and Technology, Macau, China
³Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA
⁴Maine Mineral and Gem Museum, Bethel, ME, USA
⁵Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA
Published by arrangement with John Wiley & Sons

The NWA 11004 ordinary chondrite (OC) can provide insights into the complex petrogenetic processes of the early solar system. Although originally classified as an L7 chondrite, it is reclassified as LL based on kamacite Ni (4.9 ± 0.3 wt.%) and Co (3.6 ± 0.5 wt.%) and bulk O‐isotopic composition (δ¹⁷O = 3.76‰; δ¹⁸O = 5.39‰). NWA 11004 is characterized by (1) the occurrence of 3‐ to 5‐mm‐sized poikilitic pyroxene, (2) scattered low‐Ca pyroxene data in a TiO₂ versus Al₂O₃ diagram, (3) relatively magnesian olivine and low‐Ca pyroxene (Fa_25.4, Fs_21.3), (4) low abundances of high‐Ca pyroxene, plagioclase, troilite and Ca‐phosphate, and (5) low rare earth element contents in low‐Ca pyroxene. The geochemical features of olivine and low‐Ca pyroxene in NWA 11004 differ from literature data for grains that crystallized from a melt in an OC impact melt breccia. We suggest that in NWA 11004, a plagioclase‐phosphate high‐Ca pyroxene‐troilite melt migrated away during partial melting. Some high‐Ca pyroxene grains crystallized from the residual melt, as indicated by a positive linear trend in a TiO₂ versus Al₂O₃ diagram. Whereas poikilitic low‐Ca pyroxene in NWA 11004 exhibits undulose‐to‐weak mosaic extinction, the olivine chadacrysts exhibit sharp optical extinction; this implies that NWA 11004 experienced a late‐stage shock event (S4) followed by annealing. The Ca‐phosphate ²⁰⁷Pb/²⁰⁶Pb age of 4546 ± 34 Ma most likely dates this late‐stage shock event. We suggest that the presence of type 7 OC in the early solar system may be attributable to impacts on warm chondritic asteroids that were initially heated by the decay of ²⁶Al.

¹T. S. Peretyazhko,¹S. J. Ralston,¹B. Sutter,²D. W. Ming
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2019JE006220]
¹Jacobs, NASA Johnson Space Center, Houston, TX, USA
²NASA Johnson Space Center, Houston, TX, USA
Published by arrangement with John Wiley & Sons

Ancient aquatic environments in Yellowknife Bay, Gale crater, Mars, could create favorable conditions for adsorption of chlorine compounds (perchlorate and chloride) on Fe (III) (hydr)oxides present in the Sheepbed mudstone, such as akaganeite and ferrihydrite. In this work, 5.2 mM ClO4− and 1.7 to 12 mM Cl− were adsorbed onto ferrihydrite and 5.2 mM ClO4− was adsorbed onto akaganeite at ultraacidic (pH 2–2.5), acidic (pH 3.8–4.5), and near‐neutral (pH 6.2–7.7) pH. Samples were characterized by evolved gas analysis and compared to the data collected for the Cumberland sample from the Sheepbed mudstone. Evolved gas analysis showed that ferrihydrite with 0.5–1 wt.% ClO4− adsorbed under ultraacidic and acidic conditions had a well‐resolved O2 peak at 306 °C due to the thermal decomposition of adsorbed ClO4−. All akaganeite samples with 0.5 wt.% adsorbed ClO4− had a weak peak at 312 °C tentatively assigned to decomposing perchlorate. Evolved gas analysis demonstrated that 0.5–2 wt.% Cl− adsorbed on ferrihydrite at ultraacidic and acidic pH was the main contributor to HCl evolved at >470 °C. Comparison with martian observations indicated that the temperature of the O2 peak originating from ClO4− adsorbed on ferrihydrite matched well with the thermal evolution of O2 from the Cumberland. Evolved HCl originating from Cl− adsorbed on ferrihydrite was within the temperature range of the high‐temperature HCl release from Cumberland (~770 °C). These observations suggest that ferrihydrite containing adsorbed ClO4− and Cl− could exist in the mudstone. Experimental results are consistent with adsorption at acidic pH < 4 environments through postdepositional water‐rock interactions of ferrihydrite with acid‐sulfate groundwater containing dissolved chloride and perchlorate.

¹N.H. Thomas,¹B.L. Ehlmann,¹W. Rapin,²F. Rivera‐Hernández,¹N.T. Stein,³J. Frydenvang,⁴T. Gabriel,⁵P.‐Y. Meslin,⁵S. Maurice,⁶R.C. Wiens
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006289]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Dartmouth College, Hanover, NH, USA
³Natural History Museum, University of Copenhagen, Denmark
⁴Arizona State University, Tempe, AZ, USA
⁵Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France
⁶Los Alamos National Laboratory, Los Alamos, NM, USA
Published by arrangement with John Wiley & Sons

The Mars Science Laboratory (MSL) Curiosity rover is exploring the Murray formation, a sequence of heterolithic mudstones and sandstones recording fluvial deltaic and lake deposits that comprise over 350 meters of sedimentary strata within Gale crater. We examine >4500 Murray formation bedrock points, employing recent laboratory calibrations for ChemCam laser‐induced breakdown spectroscopy H measurements at millimeter scale. Bedrock in the Murray formation has an interquartile range of 2.3‐3.1 wt. % H2O, similar to measurements using the DAN and SAM instruments. However, specific stratigraphic intervals include high H targets (6‐18 wt. % H2O) correlated with Si, Mg, Ca, Mn, or Fe, indicating units with opal, hydrated Mg‐sulfates, hydrated Ca‐sulfates, Mn‐enriched units, and akageneite or other iron oxyhydroxides, respectively. One stratigraphic interval with higher hydrogen is the Sutton Island unit and Blunts Point unit contact, where higher hydrogen is associated with Fe‐rich, Ca‐rich, and Mg‐rich points. A second interval with higher hydrogen occurs in the Vera Rubin ridge portion of the Murray formation, where higher hydrogen is associated with Fe‐rich, Ca‐rich, and Si‐rich points. We also observe trends in the H signal with grain size, separate from chemical variation, whereby coarser‐grained rocks have higher hydrogen. Variability in the hydrogen content of rocks points to a history of water‐rock interaction at Gale crater that included changes in lake water chemistry during Murray formation deposition and multiple subsequent groundwater episodes.

¹S.Taylor et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13483]
¹CRREL, 72 Lyme Road, Hanover, New Hampshire, 03755 USA
Published by arrangement with John Wiley & Sons

We built a collector to filter interplanetary dust particles (IDPs) larger than 5 μm from the clean air at the Amundsen Scott South Pole station. Our sampling strategy used long duration, continuous dry filtering of near‐surface air in place of short duration, high‐speed impact collection on flags flown in the stratosphere. We filtered ~107 m3 of clean Antarctic air through 20 cm diameter, 3 µm filters coupled to a suction blower of modest power consumption (5–6 kW). Our collector ran continuously for 2 years and yielded 41 filters for analyses. Based on stratospheric concentrations, we predicted that each month’s collection would provide 300–900 IDPs for analysis. We identified 19 extraterrestrial (ET) particles on the 66 cm2 of filter examined, which represented ~0.5% of the exposed filter surfaces. The 11 ET particles larger than 5 µm yield about a fifth of the expected flux based on >5 µm stratospheric ET particle flux. Of the 19 ET particles identified, four were chondritic porous IDPs, seven were FeNiS beads, two were FeNi grains, and six were chondritic material with FeNiS components. Most were <10 µm in diameter and none were cluster particles. Additionally, a carbon‐rich candidate particle was found to have a small 15N isotopic enrichment, supporting an ET origin. Many other candidate grains, including chondritic glasses and C‐rich particles with Mg and Si and FeS grains, require further analysis to determine if they are ET. The vast majority of exposed filter surfaces remain to be examined.