¹T.J.Bowling,² B.C.Johnson,³ S.E.Wiggins,⁴E.L.Walton,²H.J.Melosh,⁵T.G.Sharp
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113689]
¹Department of Geophysical Sciences, University of Chicago, United States of America
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, United States of America
³Department of Earth, Environmental, and Planetary Sciences, Brown University, United States of America
⁴Department of Physical Sciences, MacEwan University, Canada
⁵School of Earth and Space Exploration, Arizona State University, United States of America
Copyright Elsevier

Martian meteorites are currently the only rock samples from Mars available for direct study in terrestrial laboratories. Linking individual specimens back to their source terrains is a major scientific priority, and constraining the size of the impact craters from which each sample was ejected is a critical step in achieving this goal. During ejection from the surface of Mars by hypervelocity impacts, these meteorites were briefly compressed to high temperatures and pressures. The period of time that these meteorites spent at high pressure during ejection, or the ‘dwell time’, has been used to infer the size of the crater from which they were ejected. This inference requires assumptions that relate shock duration to impactor size, and the relation used by many authors is neither physically motivated nor accurate. Using the iSALE2D shock physics code we simulate vertical impacts at high resolution to investigate the dwell time that basaltic rocks from Mars (shergottites) spend at high pressure and temperature during ejection. Future simulation of oblique impacts will lead to more accurate dwell time estimates. Ultimately, we find that dwell time is insensitive to changes in impact velocity but for a given impact, dwell times are longer for material originating from greater depth and material that experiences higher shock pressures. Using our results, we provide scaling laws for estimating impactor size. During the formation of craters 1.9, 14, and 104 km in diameter, material capable of escaping Mars will have mean dwell times of 1, 10, and 100 ms, respectively.

^1,2Hendrix, A.R.,^1,2Vilas, F.
Geophysical Research Letters 46, 14307-14317 Link to Article [DOI: 10.1029/2019GL085883]
¹Planetary Science Institute, Tucson, AZ, United States
²SSERVI/TREX, Planetary Science Institute, Tucson, AZ, United States

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

^1,2,3Wakita, S.,¹Genda, H.,⁴Kurosawa, K.,⁵Davison, T.M.
Geophysical Research Letters 46, 13678-13686 Link to Article [DOI: 10.1029/2019GL085174]
¹Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Japan
²Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, United States
³Now at Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, United States
⁴Planetary Exploration Research Center, Chiba Institute of Technology, Narashino, Japan
⁵Department of Earth Science and Engineering, Imperial College London, London, United Kingdom

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^1,2Stefanik, M.,¹Cesnek, M.,¹Sklenka, L.,³ Kmjec, T.,^1,4Miglierini, M.
Radiation Physics and Chemistry 171, 108675 Link to Article [DOI: 10.1016/j.radphyschem.2019.108675]
¹Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Brehova 7, Prague, 115 19, Czech Republic
²Nuclear Physics Institute of The Czech Academy of Sciences, P.r.i., Rez 130, Rez, 250 68, Czech Republic
³Charles University, Faculty of Mathematics and Physics, Ke Karlovu 3, Prague, 120 00, Czech Republic
⁴Slovak University of Technology in Bratislava, Faculty of Nuclear Engineering and Information Technology, Ilkovicova 3, Bratislava, 812 19, Slovakia

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¹Richard C.Greenwood,²Thomas H.Burbine,¹Ian A.Franchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.02.004]
¹Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
²Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075, USA
Copyright Elsevier

Meteorites provide a unique insight into early Solar System processes. However, to fully interpret this record requires that these meteorites are related back to their source asteroids and ultimately to the original planetesimal population that formed early in Solar System history. As a first step in this process an assessment has been undertaken of the likely number of distinct source asteroids sampled by meteorites and related extraterrestrial materials. The results of this survey indicate that there are between 95 and 148 parent bodies represented in our sample collections. This number has been steadily increasing as new “anomalous” meteorites are characterized. Attempts to link these parent bodies to identified asteroidal sources has so far been of limited success, due to the non-unique reflectance spectra of almost all known asteroids. Asteroid (4) Vesta and the HEDs (howardites, eucrite, diogenite) meteorites is the best example of a relatively non-disputed asteroid-meteorite linkage.

As part of this study the “parent body” concept has been examined and it is found to be a widely, but loosely, used term in the literature to designate “a body that supplies meteorites to Earth.” This concept could be rendered more meaningful by discriminating between primary and secondary parent bodies. A primary parent body is the source asteroid from which the meteorite is ultimately derived, and a secondary parent body is an asteroid derived through impact or break-up of the primary body. A clear example of this usage is provided by (4) Vesta, with the main asteroid being the primary parent body and the Vestoids representing secondary parent bodies. The concept of primary vs. secondary parent bodies may have important implications for early Solar System evolution. Chondritic parent bodies are known to have accreted between 1 and 4 Myr after CAIs. This timing difference may reflect the fact that their source asteroids, particularly those of the carbonaceous chondrites, are secondary bodies, with the original CAI-bearing primary bodies destroyed during early collisional processing.

The number of primary parent bodies represented by meteorites (95 to 148) appears low when compared to the estimated number of asteroids in the main belt (> 100,000 with diameters exceeding ∼2 km). A range of potential reasons may explain this apparent mismatch: i) meteorites provide an unrepresentative sampling of the main belt, ii) the belt may only contain a limited number of primary parent bodies, iii) meteorites may be preferentially derived from the ∼120 identified asteroid families, iv) loosely consolidated types are filtered by Earth’s atmosphere, v) multiple, near-identical, “clone” parent bodies may be present in the belt. At present, it is not possible to determine which of these potential mechanisms are dominant and all may be operating to a greater or lesser extent.

Based on classical accretion models the meteorite record appears to be highly unrepresentative of the primordial asteroid population. In contrast, pebble accretion models suggest that these first-generation bodies may have been relatively large, in which case meteorites may provide a more unbiased record of early Solar System processes.

¹David L. Cook,²Ingo Leya,¹Maria Schönbächler
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13446]
¹Institute for Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
²Space Research and Planetology, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
Published by arrangement with John Wiley & Sons

We present model calculations for cosmogenic production rates in order to quantify the potential effects of spallation and neutron capture reactions on Fe and Ni isotopes in iron meteorites. We aim to determine whether the magnitude of any cosmogenic effects on the isotopic ratios of Fe and/or Ni may hinder the search for nucleosynthetic variations in these elements or in the application of the ⁶⁰Fe‐⁶⁰Ni chronometer. The model shows that neutron capture reactions are the dominant source of shifts in Fe and Ni isotopic ratios and that spallation reactions are mostly negligible. The effects on ⁶⁰Ni are sensitive to the Co/Ni ratio in the metal. The total galactic cosmic ray (GCR) effects on ⁶⁰Ni and ⁶⁴Ni can be minimized through the choice of normalizing isotopes (⁶¹Ni/⁵⁸Ni versus ⁶²Ni/⁵⁸Ni). In nearly all cases, the GCR effects (neutron capture and/or spallation) on Fe and Ni isotopic ratios are smaller than the current analytical resolution of the isotopic measurements. The model predictions are compared to the Fe and Ni isotopic compositions measured in a suite of six group IAB irons with a range of cosmic ray exposure histories. The experimental data are in good agreement with the model results. The minimal effects of GCRs on Fe and Ni isotopes should not hamper the search for nucleosynthetic variations in these two elements or the application of the ⁶⁰Fe‐⁶⁰Ni chronometer in iron meteorites or chondrites.

¹Pavel Spurný,¹Jiří Borovička,¹Lukáš Shrbený
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13444]
¹Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 25165 Ondřejov Observatory, Czech Republic
Published by arrangement with John Wiley & Sons

We report a comprehensive analysis of the instrumentally observed meteorite fall Žďár nad Sázavou, which occurred in the Czech Republic on December 9, 2014, at 16:16:45–54 UT. The original meteoroid with an estimated initial mass of 150 kg entered the atmosphere with a speed of 21.89 km s⁻¹ and began a luminous trajectory at an altitude of 98.06 km. At the maximum, it reached −15.26 absolute magnitude and terminated after a 9.16 s and 170.5 km long flight at an altitude of 24.71 km with a speed of 4.8 km/s. The average slope of the atmospheric trajectory to the Earth’s surface was only 25.66°. Before its collision with Earth, the initial meteoroid orbited the Sun on a moderately eccentric orbit with perihelion near Venus orbit, aphelion in the outer main belt, and low inclination. During the atmospheric entry, the meteoroid severely fragmented at a very low dynamic pressure 0.016 MPa and further multiple fragmentations occurred at 1.4–2.5 MPa. Based on our analysis, so far three small meteorites classified as L3.9 ordinary chondrites totaling 87 g have been found almost exactly in the locations predicted for a given mass. Because of very high quality of photographic and radiometric records, taken by the dedicated instruments of the Czech part of the European Fireball Network, Žďár nad Sázavou belongs to the most reliably, accurately, and thoroughly described meteorite falls in history.

¹S.Potin,²S.Manigand,^1,3P.Beck,¹ C.Wolters,¹B.Schmitt
Icarus (in Press) Link to Aricle [https://doi.org/10.1016/j.icarus.2020.113686]
¹Université Grenoble Alpes, CNRS, IPAG, 414 rue de la Piscine, 38400 Saint-Martin d’Hères, France
²Niels Bohr Institute & Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5–7, DK-1350 Copenhagen K., Denmark
³Institut Universitaire de France, Paris, France
Copyright Elsevier

We present here a new method to model the shape of the 3-μm absorption band in the reflectance spectra of meteorites and small bodies. The band is decomposed into several OH/H2O components using Exponentially Modified Gaussian (EMG) profiles, as well as possible organic components using Gaussian profiles when present. We compare this model to polynomial and multiple Gaussian profile fits and show that the EMGs model returns the best rendering of the shape of the band, with significantly lower residuals. We also propose as an example an algorithm to estimate the error on the band parameters using a bootstrap method. We then present an application of the model to two spectral analyses of smectites subjected to different H2O vapor pressures, and present the variations of the components with decreasing humidity. This example emphasizes the ability of this model to coherently retrieve weak bands that are hidden within much stronger ones.

¹Wataru Fujiya,¹Yuto Aoki,²Takayuki Ushikubo,¹Ko Hashizume,³Akira Yamaguchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.02.003]
¹Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, 310-8512 Ibaraki, Japan
²Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 200 Monobe-otsu, Nankoku, Kochi 783-8502, Japan
³National Institute for Polar Research, Midoricho10-3, Tachikawa, Tokyo 190-8518, Japan
Copyright Elsevier

We report the variability in carbon and oxygen isotopic compositions, chemical compositions, and cathodoluminescence intensities within calcite grains in the Yamato-791198 CM chondrite measured by secondary ion mass spectrometry. To understand the change in carbon isotopic compositions during calcite formation, the carbon isotope-analyses were performed on a series of crystal growth bands of each calcite grain. The crystal growth of calcite grains was inferred from comprehensive analyses of oxygen isotopes, chemical compositions, and cathodoluminescence characteristics.

The observed δ¹⁸O variations within individual grains are as large as 13‰. The oxygen-isotope data plot on a single straight line with a slope of 0.61 ± 0.06 (2σ) in an oxygen three-isotope diagram. This slope steeper than that of the terrestrial fractionation line indicates that the oxygen isotopic compositions of aqueous fluids evolved from higher δ¹⁸O and Δ¹⁷O to lower δ¹⁸O and Δ¹⁷O compositions due to the oxygen-isotope exchange between water and anhydrous silicates in the parent body. Thus, calcite crystals grew from higher Δ¹⁷O to lower Δ¹⁷O areas. The crystal growth inferred from oxygen isotopic compositions is corroborated by the morphology and cathodoluminescence characteristics of the calcite grains. The minor element concentrations of the calcite grains did not increase/decrease monotonically during calcite formation.

The δ¹³C variations within individual grains are no more than 4‰ except for one grain. The intra-grain δ¹³C variations observed here are much smaller than inter-grain δ¹³C variations of ∼80‰ previously reported. These observations indicate that the carbon isotopic compositions of dissolved carbon species did not change during calcite formation and that they were locally heterogeneous which reflects variable proportions of carbon reservoirs with different isotopic compositions.

¹Eniko R.Toth,¹Manuela A.Fehr,¹Matthias Friebel,¹Maria Schönbächler
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.059]
¹Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
Copyright Elsevier

Nucleosynthetic isotope variations are well documented for refractory elements in meteorites and the Earth, while moderately volatile elements generally display homogeneous compositions. Cadmium is a moderately volatile element with eight stable isotopes generated by a variety of nucleosynthetic processes. To address the extent of the nucleosynthetic variability in moderately volatile elements, new high precision Cd isotope data are presented for bulk samples of six carbonaceous and one enstatite chondrite. In addition, we report the first Cd isotope results of sequential acid leachates for the CM2 chondrite Jbilet Winselwan. Our new Cd data displays nucleosynthetic homogeneity for bulk chondrites and acid leachates within analytical uncertainties, in agreement with results for other moderately volatile elements. This implies that Cd isotopes were efficiently homogenised prior to incorporation into planetary bodies. We propose that Cd never significantly condensed into dust in stellar environments, or alternatively that such Cd-bearing dust was efficiently destroyed and recycled in the interstellar medium. Our leachate data provides evidence for further homogenisation during thermal processing in the protoplanetary disk including parent body processing. The data shows that Cd in carbonaceous chondrites mainly resides in the more easily dissolved phases, most likely sulphides that were affected by aqueous alteration. Less than 1% of the total Cd was recovered in the final leach fractions that employed HF and mainly dissolve silicates and refractory oxides.

Cadmium is susceptible to thermal neutron-capture effects due to the large neutron capture cross-section of ¹¹³Cd (∼20,000 barns). We report variations of up to −0.6 ± 0.3 for ε¹¹³Cd (internally normalised to ¹¹⁶Cd/¹¹¹Cd) in bulk chondrites, which renders Cd a potential thermal neutron-capture monitor. Most neutron dosimeters, such as Pt, Os and Hf, are sensitive to neutron capture in the epithermal energy range and have applications mainly limited to lunar samples or iron and stony-iron meteorites. The additional use of Cd, susceptible to neutron capture in the thermal energy range, therefore provides a new tool to determine the exposure histories of stony meteorites in more detail. Our study demonstrates that thermal neutron-capture effects in carbonaceous and enstatite chondrites can produce resolvable effects and require attention when assessing nucleosynthetic isotope variations.