A Paleozoic age for the Tunnunik impact structure

1Camille Lepaulard,1Jérôme Gattacceca,2Nicholas Swanson‐Hysell, 1Yoann Quesnel, 1François Demory, 3,4Gordon R. Osinski
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13239]
1Aix Marseille Univ, CNRS, IRD, Coll France, INRA, CEREGE, , Aix‐en‐Provence, France
2Department of Earth and Planetary Science, University of California, , Berkeley, California, 94720–4767 USA
3Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, , London, Ontario, N6A 5B7 Canada
4Department of Physics & Astronomy, University of Western Ontario, , London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

We report paleomagnetic directions from the target rocks of the Tunnunik impact structure, as well as from lithic impact breccia dikes that formed during the impact event. The target sedimentary rocks have been remagnetized after impact‐related tilting during a reverse polarity interval. Their magnetization is unblocked up to 350 °C. The diabase dikes intruding into these sediments retained their original magnetization which unblocks above 400 °C. The impact breccia records a paleomagnetic direction similar to that of the overprints in the target sedimentary rocks. The comparison of the resulting virtual geomagnetic pole for the Tunnunik impact structure with the apparent polar wander path for Laurentia combined with biostratigraphic constraints from the target sedimentary rocks is most consistent with an impact age in the Late Ordovician or Silurian, around 430–450 Ma, soon after the deposition of the youngest impacted sedimentary rocks. Our results from the overprinted sedimentary rocks and diabase dikes imply that the postimpact temperature of the studied rocks was about 350 °C.

Exploring the variability of argon loss in Apollo 17 impact melt rock 77135 using high‐spatial resolution 40Ar/39Ar geochronology

1M. Mercer, 1Kip V. Hodges, 2Bradley L. Jolliff, 1Matthijs C. Van Soest, 1,3Jo‐Anne Wartho, 1,4John R. Weirich
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13240]
1School of Earth and Space Exploration, Arizona State University, , Tempe, Arizona, 85287 USA
2Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, , St. Louis, Missouri, 63130 USA
3 Helmholtz Centre for Ocean Research Kiel, , D‐24148 Kiel, Germany
4 Science Institute, , Tucson, Arizona, 85719 USA
Published by arrangement with John Wiley & Sons

40Ar/39Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic 40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not 40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP) 40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on 40Ar/39Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published 40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally, UVLAMP analysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded 40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or 40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon’s evolution.

The M3 project: 2 – Global distributions of mafic mineral abundances on Mars

1L.Riu,1 F.Poulet,1J.-P.Bibring,1B.Gondet
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.002]
1IAS, Institut d’Astrophysique Spatiale, bâtiment 121Université Paris-Saclay, Orsay 91405, France
Copyright Elsevier

This paper is the first paper of a series that will present the derivation of the modal mineralogy of Mars (M3 project) at a global scale from the near-infrared dataset acquired by the imaging spectrometer OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) on board ESA/Mars Express. The objective is to create and provide a global 3-D image-cube of Mars at 32 px/° covering most of Mars surface. This product has several advantages. First, it can be used to instantaneously extract atmospheric- and aerosol-corrected near-infrared (NIR) spectra from any location on Mars. Second, several new data maps can be built as discussed here. That includes new global mineral distributions, quantitative mineral abundance distributions and maps of Martian surface chemistry (wt% oxide) detailed in a companion paper (Riu et al, 2018). Here we present the method to derive the global hyperspectral cube from several hundred millions of spectra. Global maps of some mafic minerals are then shown, and compared to previous works.

A melt inclusion study on volatile abundances in the lunar mantle

1Peng Ni(倪鹏), 1Youxue Zhang(张有学), 1Sha Chen(陈沙),2Joel Gagnon
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.12.034]
1Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA
2Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
Copyright Elsevier

Earth’s Moon was thought to be highly depleted in volatiles due to its formation by a giant impact. Over the last decade, however, evidence has been found in apatites, lunar volcanic glass beads, nominally anhydrous minerals and olivine-hosted melt inclusions, to support a relatively “wet” Moon. In particular, based on H2O/Ce, F/Nd, and S/Dy ratios, recent melt inclusion (MI) work estimated volatile (H2O, F, and S) abundances in lunar rocks to be similar to or slightly lower than the terrestrial depleted mantle. Uncertainties still occur, however, in whether the limited numbers of lunar samples studied are representative of the primitive lunar mantle, and whether the high H2O/Ce ratio for 74220 is due to local heterogeneity. In this paper, we report major element, trace element, volatile, and transition metal data in MIs for 5 mare basalt samples (10020, 12040, 15016, 15647 and 74235) and a pyroclastic deposit (74220).

With our new lunar MI data, H2O/Ce ratios are still found to vary significantly among different lunar samples, from ∼ 50 for 74220, to ∼ 9 for 10020, ∼ 3 for 74235, 1.7 to 0.9 for 12008, 15016, and 15647, and 0.5 for 12040. H2O/Ce ratios for these samples show positive correlation with their cooling rates, indicating a possible effect of post-eruptive loss of H on their H2O/Ce variations. It is evident that most other lab and lunar processes, including loss of H2O during homogenization, mantle partial melting, magma evolution, and ingassing during or post eruption are unlikely the causes of high H2O/Ce variations among different lunar samples. By comparing ratios of F/Nd, S/Dy, Zn/Fe, Pb/Ce, Cs/Rb, Rb/Ba, Cl/K, Na/Sr, Ga/Lu, K/Ba, and Li/Yb between 74220 and other lunar samples, the possibility of 74220 originating from a volatile-enriched heterogeneity in the lunar mantle can also be excluded. With all the above considerations, we think that the H2O/Ce ratio for 74220 best represents the pre-degassing lunar basaltic melt and primitive lunar mantle, either because it was formed by a rapid eruption process, or it was sourced from a deeper part of the lunar mantle that experienced less degassing H2O loss during lunar magma ocean crystallization. With an H2O/Ce ratio of ∼50, the primitive lunar mantle is estimated to contain ∼84 ppm H2O.

Comparing volatile abundances in melt inclusions, glassy embayments, and glass beads in 74220 yields the following volatility trend for volcanic eruptions on the lunar surface:

H2O >> Cl >> Zn ≈ Cu ≈ F > S ≈ Ga ≈ Cs > Rb ≈ Pb > Na > K ≈ Li.

Using the melt inclusion data obtained thus far, the volatile depletion trend for the Moon from a MI perspective is estimated. Our results show that most of the volatile elements in the lunar mantle are depleted relative to the bulk silicate Earth by a factor of 2 to 20, however, a good correlation between half condensation temperature and the volatile depletion trend is not observed. The relatively flat pattern for the lunar volatile depletion trend requires a lunar formation model that can reconcile the abundances of these volatiles in the lunar mantle.

Extreme 13C,15N and 17O isotopic enrichment in the young planetary nebula K4-47

1D. R. Schmidt,1 N. J. Woolf, 2T. J. Zega, 2,3,4L. M. Ziurys
Nature 564, 378-381 Link to Article [https://doi.org/10.1038/s41586-018-0763-1]
1Department of Astronomy, Steward Observatory, University of Arizona, Tucson, AZ, USA
2Department of Planetary Science, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
3Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ, USA
4Arizona Radio Observatory, Steward Observatory, University of Arizona, Tucson, AZ, USA

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Tunguska eyewitness accounts, injuries, and casualties

1Peter Jenniskens, 2Olga P.Popova, 2Dmitry O.Glazachev, 2Elena D.Podobnaya, 3Anna P.Kartashova
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.01.001]
1SETI Institute, 189 Bernardo Ave, Mountain View, CA 94043, United States
2Institute for Dynamics of Geospheres R. A. S., Moscow, Russia
3Institute of Astronomy R. A. S., Moscow, Russia
Copyright Elsevier

The airburst events at Chelyabinsk and Tunguska in Russia are the best-documented asteroid impacts of recent times. Models that assess the potential danger from such events rely on an accurate picture of their aftermath. Here, we re-examine the most critical eyewitness accounts of the Tunguska airburst, namely those that describe injuries and casualties, and those that paint a picture of what events were responsible. Not all relevant information has survived in the written record and there are contradictions that create some ambiguity. We find that inside and near the tree-fall area were at least 30 people. Many lost consciousness and at least 3 passed away (immediately or later) as a direct consequence of the Tunguska event. The airburst created a butterfly-shaped pattern of glass damage extending 4–5 times wider than that seen at Chelyabinsk. At these larger distances, there were no reported injuries from falls, shattering glass cuts, or from UV radiation exposure.

Spectrophotometric modeling and mapping of Ceres

1Jian-YangLi(李荐扬) et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.12.038]
1Planetary Science Institute, Tucson, AZ 85719, USA
Copyright Elsevier

We report a comprehensive analysis of the global spectrophotometric properties of Ceres using the images collected by the Dawn Framing Camera through seven color filters from April to June 2015 during the RC3 (rotational characterization 3) and Survey mission phases. We derived the Hapke model parameters for all color filters. The single-scattering albedo of Ceres at 555 nm wavelength is 0.14 ± 0.04, the geometric albedo is 0.096 ± 0.006, and the bolometric Bond albedo is 0.037 ± 0.002. The asymmetry factors calculated from the best-fit two-term Henyey-Greenstein (HG) single-particle phase functions (SPPFs) show a weak wavelength dependence from −0.04 at 438 nm increasing to 0.002 at >900 nm, suggesting that the phase reddening of Ceres is dominated by single-particle scattering rather than multiple scattering or small-scale surface roughness. The Hapke roughness parameter of Ceres is derived to be 20° ± 6°, with no wavelength dependence. The phase function of Ceres presents appreciably strong scattering around 90° phase angle that cannot be fitted with a single-term HG SPPF, suggesting possible stronger forward scattering component than other asteroids previously analyzed with spacecraft data. We speculate that such a scattering characteristic of Ceres might be related to its ubiquitous distribution of phyllosilicates and high abundance of carbonates on the surface. We further grouped the reflectance data into a 1° latitude-longitude grid over the surface of Ceres, and fitted each grid independently with both empirical models and the Hapke model to study the spatial variations of photometric properties. Our derived albedo maps and color maps are consistent with previous studies [Nathues, A., et al., 2016, Planet. Space Sci. 134, 122–127; Schröder, S.E., et al., 2017, Icarus 288, 201–225]. The SPPF over the surface of Ceres shows an overall correlation with albedo distribution, where lower albedo is mostly associated with stronger backscattering and vice versa, consistent with the general trend among asteroids. On the other hand, the Hapke roughness parameter does not vary much across the surface of Ceres, except for the ancient Vendimia Planitia region that is associated with a slightly higher roughness. Furthermore, the spatial distributions of the SPPF and the Hapke roughness do not depend on wavelength. Based on the wavelength dependence of the SPPF of Ceres, we hypothesize that the regolith grains on Ceres either contain a considerable fraction of μm-sized or smaller particles, or are strongly affected by internal scatterers of this size.