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

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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).