¹Bruce Hapke, ²Hiroyuki Sato, ³Mark Robinson
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.10.001]
¹Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh. PA 15260.
²Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodal, Chuo-Ku, Sagamihara, Kanagawa, 252-5210, JAPAN.
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287.
Copyright Elsevier

A new algorithm is proposed for estimating TiO2 abundance on the moon using lunar reflectance values measured by the Wide Angle Camera on the Lunar Reconnaissance Orbiter spacecraft. The algorithm provides useful values for mature regoliths on the entire lunar surface including highlands and low titanium maria. However, it underestimates the abundances of immature regoliths, so that the algorithm returns a lower limit for such features as young craters and rays.

¹Rebecca J. Smith, ²Elizabeth B. Rampe, ¹Briony H. N. Horgan, ³Erwin Dehouck
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005612]
¹Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN
²NASA/Johnson Space Center, Houston, TX
³Laboratoire de Géologie de Lyon – Terre, Planètes, Environnement, UMR 5276, CNRS, Université Lyon 1, ENS Lyon, Villeurbanne, France
Published by arrangement with John Wiley & Sons

X‐ray amorphous materials have been detected in all samples measured by the CheMin X‐ray diffractometer (XRD) onboard the Mars Science Laboratory rover in Gale Crater, Mars. The origin (s) of these materials are poorly understood, and there are significant uncertainties on their estimated abundances and compositions. Three methods are used to estimate the bulk amorphous component abundance and composition of martian samples using XRD and bulk chemical data: (1) Rietveld refinements, (2) FULLPAT analyses, and (3) mass balance calculations (MBCs). We tested these methods against a quantitative XRD (internal standard) method commonly used in terrestrial laboratories. Additionally, we tested for instrumentation effects by measuring our samples on a laboratory XRD instrument (PANalytical X’Pert Pro) and the CheMin test‐bed instrument (CheMin IV). We used three natural samples known to contain amorphous materials: glacial sediment, Hawaiian soil, and a paleosol. Our methods resulted in nine amorphous abundances and four amorphous compositions for each sample. For a single sample, amorphous abundance estimates and amorphous compositions are relatively similar across all estimation methods. CheMin analog measurements perform well in our tests, with amorphous abundances and compositions comparable to laboratory QXRD measurements, though slightly underestimated. This suggests that previous amorphous component estimates for martian samples are relatively accurate. This study highlights the usefulness of the MBC method for characterizing amorphous materials in terrestrial samples, providing important supplemental information to destructive and time‐consuming size‐separation and dissolution procedures.

^1,2K. Fiege, ²M. Guglielmino, ³N. Altobelli, ²M. Trieloff, ⁴R. Srama, ¹T. M. Orlando
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005564]
¹Georgia Institute of Technology, Atlanta, GA, USA
²Universität Heidelberg, Klaus–Tschira–Labor für Kosmochemie, Institut für Geowissenschaften, Heidelberg, Germany
³ESA, European Space Agency, Madrid, Spain
⁴Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, Germany
Published by arrangement with John Wiley & Sons

The role of micrometeorite bombardment in space weathering on asteroid surfaces was studied using a 2 MV Van–De–Graaff accelerator. About 90000–100000 micron– to sub–micron sized copper particles with a mass– and velocity distribution similar to the interplanetary dust population, were fired onto the surfaces of polished Allende CV3 chondrite and eucrite NWA 6966 samples at speeds between km s−1. We find a clear relationship between micro–particle bombardment, infrared reflectance decrease, and overall spectral reddening. Differences in impact effects due to variable particle speed, size and structure are observed. Some Cu–particles form large clusters that break up upon impact and disperse. Other impactors leave imprints on the surface, implant or generate typical craters with rims and spallation features. Very small, fast particles generate small craters without spallation or significant crater rim. Mid–IR–spectra (bulk– and microscopic measurements of individual components), 3D–Laser microscopic images and XRD–spectra from the processed and unprocessed samples were collected. Mid–IR–spectra (700–6000 cm−1) over the entire sample surface, show overall darkening of features. Microscopic IR–spectra show the damage seen as reflectance decrease and spectral reddening, which is variable in the μ–range, depending on impact density and target properties (mineralogic composition). The fine–grained Allende matrix with predominantly Fe–rich olivine seems less affected than coarse–grained chondrules with Mg–rich silicates, where darkening can reach 60%. XRD–analysis also suggests chemical and crystallographical differences in the bombarded sample, due to impact shock.

¹Nicolas Altobelli, ^2,3Katherina Fiege, ⁴Benoit Carry, ³Rachel Soja, ²Massimo Guglielmino, ²Mario Trieloff, ⁵Thomas Michael Orlando, ²Ralf Srama
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005563]
¹ESA, European Space Agency, Madrid, Spain
²Klaus–Tschira–Labor für Kosmochemie, Institut für Geowissenschaften, Universität Heidelberg, Germany
³Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, Germany
⁴Université Côte d’ Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁵Georgia Institute of Technology, Atlanta, GA, USA
Published by arrangement with John Wiley & Sons

The processes of alteration of airless bodies exposed to the space environment are referred to be as ‘space weathering’. Multiple agents contribute generally to space weathering, to an extent that depends on the specific location of the surface within the Solar System. Typical space weathering agents encountered in the Solar System are: solar radiation, solar wind and cosmic rays, magnetospheric plasma (for example, at Jupiter or Saturn), and cosmic dust. The effect of space weathering is generally assessed by measuring the surfaces optical properties, for example by near‐infrared (IR) spectroscopy. The alteration of the surfaces is due to a cumulative effect over time of all agents. We investigate in this paper the contribution of micro‐meteoroid (dust) bombardment on different asteroids, by using the Micrometeoroid Environment Model (IMEM) for the interplanetary dust populations (IDPs), and a simplified model of Interstellar Dust (ISD) dynamics. We quantify, for different representative asteroids (Main Belt and NEOs), the particle cumulative flux, mass flux, impact velocity and the kinetic impact energy deposited. This work is primarily intended to support laboratory work investigating the effect of energy deposition onto sample surfaces, as well as astronomical observations of optical properties of asteroid surfaces.

¹Pablo Núñez Demarcoa, ²Gonzalo Tancredi, ³Maria Elizabeth Zucolotto, ⁵Loiva Lizia Antonello, ³José María Monzón, ⁴Valentina Pezano, ⁵Amanda Tosi, ¹Caio Villaça
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.09.007]
¹Instituto de Ciencias Geológicas, Facultad de Ciencias, UdelaR, Uruguay
²Departamento de Astronomía, Instituto de Física, Facultad de Ciencias, UdelaR, Uruguay
³LABET/MN/UFRJ, Laboratório Extraterrestre, Departamento de Geologia e Paleontologia, Museu Nacional, Universidade Federal do Rio de Janeiro, Brazil
⁴Centro Universitario Regional Este, UdelaR, Uruguay
⁵LABSONDA/IGEO/UFRJ, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Brazil

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

¹David L.Cook, ²Thomas Smith, ²Ingo Leya, ³Connor D.Hilton, ³Richard J.Walker, ¹Maria Schönbächler
Earth and Planetary Science Letters 503, 29-36 Link to Article [https://doi.org/10.1016/j.epsl.2018.09.021]
¹Institut für Geochemie und Petrologie, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
²Space Research and Planetology, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
³Department of Geology, University of Maryland, 8000 Regents Dr., College Park, MD 20742, USA
Copyright Elsevier

The origin of 180W excesses in iron meteorites has been a recently debated topic. Here, a suite of IIAB iron meteorites was studied in order to accurately determine the contribution from galactic cosmic rays (GCR) and from potential decay of 184Os to measured excesses in the minor isotope 180W. In addition to W isotopes, trace element concentrations (Re, Os, Ir, Pt, W) were determined on the same samples, as well as their cosmic ray exposure ages, using 36Cl–36Ar systematics. These data were used in combination with an improved model of GCR effects on W isotopes to correct effects resulting from neutron capture and spallation reactions. After these corrections, the residual 180W excesses correlate with Os/W ratios and indicate a clear contribution from 184Os decay. A newly derived decay constant is equivalent to a half-life for 184Os of (3.38 ± 2.13) × 1013 a. Furthermore, when the data are plotted on an Os–W isochron diagram, the intercept (ε180Wi = 0.63 ± 0.35) reveals that the IIAB parent body was characterized by a small initial nucleosynthetic excess in 180W upon which radiogenic and GCR effects were superimposed. This is the first cogent evidence for p-process variability in W isotopes in early Solar System material.

¹Rosalind M.G.Armytage, ¹Vinciane Debaille, ²Alan D.Brandon, ³Carl B.Agee
Earth and Planetary Science Letters 502, 274-283 Link to Article [https://doi.org/10.1016/j.epsl.2018.08.013]
¹Laboratoire G-Time, CP 160/02, Université Libre de Bruxelles, Av. F. Roosevelt 50, 1050 Bruxelles, Belgium
²Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, 77204, USA
³Institute of Meteoritics, University of New Mexico, Albuquerque, NM, 87131, USA
Copyright Elsevier

Resolving the possible mantle and crustal sources for shergottite meteorites is crucial for understanding the formation and early differentiation of Mars. Orbiter and rover characterization of the martian surface reveal that the major element composition of most of its surface does not match the shergottites (McSween et al., 2009) leaving the relationship between them poorly understood. The identification of the meteorite NWA 7034 and its pairs as a Mars surface rock (Cartwright et al., 2014) provides access to a representative sample of Mars’ crust (Agee et al., 2013, Humayun et al., 2013). Utilizing the short-lived 146Sm–142Nd, and long-lived 147Sm–143Nd and 176Lu–176Hf chronometers, which are sensitive to silicate differentiation, we analyzed three fragments of NWA 7034. The very negative mean isotopic compositions for this breccia,μ142NdJNdi-1=−45±5
(2SD), ε143NdCHUR=−16.7±0.4(2SD) and ε176HfCHUR=−61±9(2SD) point to an ancient origin for this martian crust. However, modeling of the data shows that the crust sampled by NWA 7034 possesses a Hf/Nd ratio and coupled ε143Nd–μ142Nd
model age that are incompatible with this crustal reservoir being an end-member that generated the shergottite source mixing array. In addition, this crust is not juvenile, despite its rare earth element profile, but has had a multistage formation history. Therefore, early crustal extraction alone was not responsible for the creation of the reservoirs that produced the shergottites. Instead mantle reservoirs formed via other early differentiation processes such as in a Mars magma ocean must be responsible for the trace element and isotopic signatures present in shergottites.

¹Yuri Amelin, ¹Piers Koefoed, ²Tsuyoshi Iizuka, ^3,4,5Vera Assis Fernandes, ⁶Magdalena H.Huyskens, ⁶Qing-Zhu Yin, ⁷Anthony J.Irving
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.09.021]
¹Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia
²Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
³Museum für Naturkunde, Leibniz-Institut für Evolutions und Biodiversi-tätsforschung, Berlin, Germany
⁴School of Earth and Environmental Sciences, University of Manchester, M13 9PL Manchester, UK
⁵Instituto Dom Luiz, University of Lisbon, 1749-016 Lisbon, Portugal
⁶Department of Earth and Planetary Sciences, University of California at Davis, Davis, California, 95616, USA
⁷Department of Earth & Space Sciences, University of Washington, Seattle, WA 98195, USA
Copyright Elsevier

We report U-Pb, 87Rb-87Sr, 40Ar-39Ar , and 238U/235U isotopic data for paired ungrouped achondrites NWA 6704 and NWA 6693 that were derived from a highly oxidised parent body with broadly chondritic composition (Warren et al., 2013, Hibiya et al., 2018). Pb-isotopic ages derived from isochrons for multiple acid-leached pyroxene fractions are 4562.76+0.22/-0.30 Ma for NWA 6704 and 4562.63+0.29/-0.21 Ma for NWA 6693, calculated using 238U/235U ratio of 137.7784±0.0097 measured in NWA 6704. The Rb-Sr mineral isochron age of 4543±46 Ma (initial 87Sr/86Sr=0.699013±0.000055) is consistent with the Pb-isotopic age. Together with 187Re-187Os isochron age of 4576±250 Ma for NWA 6704 (Hibiya et al. 2018), and 26Al-26Mg and 53Mn-53Cr ages calculated using the rapidly crystallized angrite D’Orbigny as a time anchor are also consistent with the Pb-isotopic age (Sanborn et al. 2018), these data indicate that the parent rocks of NWA 6693 and NWA 6704 remained closed to migration of both lithophile and siderophile elements since crystallisation and initial cooling. The whole rock 40Ar-39Ar age of 4199±32 Ma suggests a complete resetting of the K-Ar system approximately 360 Ma after crystallisation. A later event at ≤2.12 Ga partially reset the K-Ar system as shown by the low temperature heating steps. Both meteorites have high 87Rb/86Sr ratios (up to 7.0 in NWA 6693 pyroxene) and very radiogenic 87Sr/86Sr up to 1.15. Together with the absence of secondary disturbance in the Rb-Sr and U-Pb systems, this makes them suitable for cross-calibration of the isotopic chronometers. These meteorites are also promising candidates to serve as age reference samples for the early Solar System chronology, as an alternative or complement to angrites of the early generation (D’Orbigny, Sahara 99555) that are currently used for this purpose. Plagioclase in NWA 6704 has a sufficiently low Rb/Sr ratio to define precise initial 87Sr/86Sr of 0.698997±0.000027, which corresponds to the time of separation of the parent body precursor material from the solar nebula of 1.5±2.1 Ma. This value suggests that the parent asteroid accreted within 3.6 Ma after CAI formation, or before 4563.7 Ma using the CAI age of 4567.3 Ma (Connelly et al. 2012).

Holmbeck^1,2 et al. (>10)
Astrophysical Journal Letters 859, L24 Link to Article [DOI: 10.3847/2041-8213/aac722]
¹Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

We report the discovery of a new actinide-boost star, 2MASS J09544277+5246414, originally identified as a very bright (V = 10.1), extremely metal-poor ([Fe/H] = −2.99) K giant in the LAMOST survey, and found to be highly r-process-enhanced (r-II; [Eu/Fe] = +1.28]), during the snapshot phase of the R-Process Alliance (RPA). Based on a high signal-to-noise ratio (S/N), high-resolution spectrum obtained with the Harlan J. Smith 2.7 m telescope, this star is the first confirmed actinide-boost star found by RPA efforts. With an enhancement of [Th/Eu] = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhanced r-II star yet discovered, and only the sixth metal-poor star with a measured uranium abundance ([U/Fe] = +1.40). Using the Th/U chronometer, we estimate an age of 13.0 ± 4.7 Gyr for this star. The unambiguous actinide-boost signature of this extremely metal-poor star, combined with additional r-process-enhanced and actinide-boost stars identified by the RPA, will provide strong constraints on the nature and origin of the r-process at early times.

^1,2Shuai Li, ¹Paul G. Lucey, ²Ralph E. Milliken, ³Paul O. Hayne, ²Elizabeth Fisher, ⁴Jean-Pierre Williams, ⁵Dana M. Hurley, ⁶Richard C. Elphic
Processdings of the National Academy of Sciences of the United States of America (PNAS) 115, 8907-8912 Link to Article [https://doi.org/10.1073/pnas.1802345115]
¹Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822
²Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912
³Department of Astrophysical & Planetary Sciences, University of Colorado Boulder, Boulder, CO 80309
⁴Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095
⁵Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723
⁶Ames Research Center, NASA, Mountain View, CA 94035

Water ice may be allowed to accumulate in permanently shaded regions on airless bodies in the inner solar system such as Mercury, the Moon, and Ceres [Watson K, et al. (1961) J Geophys Res 66:3033–3045]. Unlike Mercury and Ceres, direct evidence for water ice exposed at the lunar surface has remained elusive. We utilize indirect lighting in regions of permanent shadow to report the detection of diagnostic near-infrared absorption features of water ice in reflectance spectra acquired by the Moon Mineralogy Mapper [M (3)] instrument. Several thousand M (3) pixels (∼280 × 280 m) with signatures of water ice at the optical surface (depth of less than a few millimeters) are identified within 20° latitude of both poles, including locations where independent measurements have suggested that water ice may be present. Most ice locations detected in M (3) data also exhibit lunar orbiter laser altimeter reflectance values and Lyman Alpha Mapping Project instrument UV ratio values consistent with the presence of water ice and also exhibit annual maximum temperatures below 110 K. However, only ∼3.5% of cold traps exhibit ice exposures. Spectral modeling shows that some ice-bearing pixels may contain ∼30 wt % ice that is intimately mixed with dry regolith. The patchy distribution and low abundance of lunar surface-exposed water ice might be associated with the true polar wander and impact gardening. The observation of spectral features of H2O confirms that water ice is trapped and accumulates in permanently shadowed regions of the Moon, and in some locations, it is exposed at the modern optical surface.