¹Alexey A. Berezhnoy,
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.08.034]
¹Sternberg Astronomical Institute, Moscow State University, Universitetskij pr., 13, 119234 Moscow, Russia
Copyright Elsevier

Based on the equilibrium thermochemical approach and quenching theory, formation of molecules and dust grains in impact-produced clouds formed after collisions between meteoroids and Mercury is considered. Based on observations of Al, Fe, and Mn atoms in the exosphere of Mercury and new results of studies of the elemental composition of the surface of Mercury, quenching temperatures and pressures of main chemical reactions and condensation of dust particles were estimated. The behavior of the main Na-, K-, Ca-, Fe-, Al-, Mn-, Mg-, Si-, Ti, Ni-, Cr-, Co, Zn-, O-, H-, S-, C-, Cl-, N-, and P-containing species delivered to the Hermean exosphere during meteoroid impacts was studied. The importance of meteoroid bombardment as a source of Na, K, Ca, Fe, Al, Mn, Mg, and O atoms in the exosphere of Mercury is discussed.

¹Paul G. Lucey, ¹David Trang, ²Jeffrey R. Johnson, ³Timothy D. Glotch
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.09.010]
¹University of Hawai‘i at Mānoa, Hawai‘i Institute of Geophysics and Planetology, Honolulu, HI, 96822
²Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723
³Department of Geological Sciences, Stony Brook University, Stony Brook, NY 11794
Copyright Elsevier

Several studies have detected the presence of nanophase ferric oxide, such as nanophase hematite, across the martian surface through spacecraft and rover data. In this study, we used the radiative transfer method to detect and quantify the abundance of these nanophase particles. Because the visible/near-infrared spectral characteristics of hematite >10 nm in size are different from nanophase hematite <10 nm, there are not any sufficient optical constants of nanophase hematite to study visible to near-infrared rover/spacecraft data of the martian surface. Consequently, we found that radiative transfer models based upon the optical constants of crystalline hematite are unable to reproduce laboratory spectra of nanophase hematite. In order to match the model spectra to the laboratory spectra, we developed a new set of optical constants of nanophase hematite in the visible and near-infrared and found that radiative transfer models based upon these optical constants consistently model the laboratory spectra. We applied our model to the passive bidirectional reflectance spectra data from the Chemistry and Camera (ChemCam) instrument onboard the Mars Science Laboratory rover, Curiosity. After modeling six spectra representing different major units identified during the first year of rover operations, we found that the nanophase hematite abundance was no more than 4 wt%.

¹Hamed Pourkhorsandi, ¹Jérôme Gattacceca, ¹Bertrand Devouard, ^bMassimo D’Orazio, ¹Pierre Rochette, ³Pierre Beck, ¹Corinne Sonzogni, ^4,5Millarca Valenzuela
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.013]
¹CNRS, Aix-Marseille Univ., IRD, Coll. France, CEREGE, Aix-en-Provence, France
²Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
³Institut de Planétologie et d’Astrophysique de Grenoble, Grenoble, France
⁴Millennium Institute of Astrophysics (MAS), Pontificia Universidad Católica de Chile, Santiago, Chile
⁵Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Santiago, Chile
Copyright Elsevier

El Médano 301 (EM 301) is an ungrouped chondrite with overall texture and trace-element distribution similar to those of ordinary chondrites (OCs), but with silicate (olivine and low-Ca pyroxene) compositions that are more reduced than those in OCs, with average olivine and low-Ca pyroxene of Fa_3.9±0.3 and Fs_12.8±4.9, respectively. These values are far lower than the values for OCs and even for chondrites designed as “reduced” chondrites. Low-Ca pyroxene is the dominant mineral phase and shows zoning with higher MgO contents along the crystal rims and cracks (reverse zoning). The Co content of kamacite is also much lower than the concentrations observed in OCs (below detection limit of 0.18 wt% versus 0.44-37 wt%). Oxygen isotopic composition is Δ¹⁷O = +0.79, +0.78‰ and slightly different from that of OCs. The lower modal olivine/pyroxene ratio, different Infrared (IR) spectra, lower Co content of kamacite, lower mean FeO contents of olivine and pyroxene, different kamacite texture, and different oxygen-isotopic composition show that EM 301 does not belong to a known OC group. EM 301 shows similarities with chondritic clasts in Cumberland Falls aubrite, and with Northwest Africa 7135 (NWA 7135) and Acfer 370 ungrouped chondrites. However, dissimilar to NWA 7135 and the clasts, it does not contain highly reduced mineral phases like daubréelite.

Our observations suggest the formation of EM 301 in a nebular region compositionally similar to OCs but with a different redox state, with oxygen fugacity (ƒO₂) in this region lower than that of OCs and higher than that of enstatite chondrites condensation region. A second, possibly nebular, phase of reduction by the production of reducing gas phases (e.g., C-rich) could be responsible for the subsequent reduction of the primary material and the occurrence of reverse zoning in the low-Ca pyroxene and lower average Fa/Fs ratio. Based on the IR spectra of EM 301 we suggest the possibility that the parent body of this chondrite was a V-type asteroid.

^1,2Matthew J. Genge, ¹Bridie Davies, ^1,2Martin D. Suttle, ³Matthias van Ginneken, ⁴Andrew G. Tomkins
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.004]
¹Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 11BW, UK
²Earth Sciences Department, The Natural History Museum, Cromwell Road, London SW7 2BW, UK
³Earth System Science, Vrije Universiteit Brussel, Pleinlaan, 2 B-1050 Brussel, Belgium
⁴School of Earth, Atmosphere & Environment, Monash University, Melbourne, Victoria 3800, Australia
Copyright Elsevier

I-type cosmic spherules are micrometeorites that formed by melting during atmospheric entry and consist mainly of iron oxides and FeNi metal. I-types are important because they can readily be recovered from sedimentary rocks allowing study of solar system events over geological time. We report the results of a study of the mineralogy and petrology of 88 I-type cosmic spherules recovered from Antarctica in order to evaluate how they formed and evolved during atmospheric entry, to constrain the nature of their precursors and to establish rigorous criteria by which they may be conclusively identified within sediments and sedimentary rocks. Two textural types of I-type cosmic spherule are recognised: (1) metal bead-bearing (MET) spherules dominated by Ni-poor (<1.5 w%) wüstite and FeNi metal (10-95 wt% Ni) with minor magnetite, and (2) metal bead-free (OX) spherules dominated by Ni-rich wüstite (0.5-22.5 wt%) and magnetite. Two varieties of OX spherule are distinguished, magnetite-poor dendritic spherules and magnetite-rich coarse spherules. Six OXMET particles having features of both MET and OX spherules were also observed. The wüstite to magnetite ratios and metal contents of the studied particles testify to their formation by melting of extraterrestrial FeNi grains during progressive oxidation in the atmosphere. Precursors are suggested to be mainly kamacite and rare taenite grains. Vesicle formation within metal beads and extrusion of metallic liquid into surrounding wüstite grain boundaries suggests an evaporated iron sulphide or carbide component within at least 23% of particles. The Ni/Co ratios of metal vary from 14 to >100 and suggest that metal from H-group ordinary, CM, CR and iron meteorites may form the majority of particles. Oxidation during entry heating increases in the series MET<magnetite-poor OX<magnetite-rich OX spherules owing to differences in particle size, entry angle and velocity. Magnetite-poor OX spherules are shown to form by crystallisation of non-stoichiometric wüstite at the liquidus followed by sub-solidus decomposition to magnetite, whilst in magnetite-rich OX spherules magnetite crystallises directly at the liquidus. Magnetite rims found on most particles are suggested to form by oxidation during sub-solidus flight. The separation of metal beads due to deceleration is proposed to have been minor with most OX spherules shown to have been in equilibrium with metal beads containing >80 wt% Ni comprising a particle mass fraction of <0.2. Non-equilibrium effects in the exchange of Ni between wüstite and metal, and magnetite and wüstite are suggested as proxies for the rate of oxidation and cooling rate respectively. Variations in magnetite and wüstite crystal sizes are also suggested to relate to cooling rate allowing relative entry angle of particles to be evaluated. The formation of secondary metal in the form of sub-micron Ni-rich or Pt-group nuggets and as symplectite with magnetite was also identified and suggested to occur largely due to the exsolution of metallic alloys during decomposition of non-stoichiometric wüstite. Weathering is restricted to replacement of metal by iron hydroxides. The following criteria are recommended for the conclusive identification of I-type spherules within sediments and sedimentary rocks: (i) spherical particle morphologies, (ii) dendritic crystal morphologies, (iii) the presence of wüstite and magnetite, (iv) Ni-bearing wüstite and magnetite, and (v) the presence of relict FeNi metal.