Noam R. Izenberg^a, Rachel L. Klima^a, Scott L. Murchie^a, David T. Blewett^a, Gregory M. Holsclaw^b, William E. McClintock^b, Erick Malaret^c, Calogero Mauceri^c, Faith Vilas^d, Ann L. Sprague^e, Jörn Helbert^f, Deborah L. Domingue^d, James W. Head III^g, Timothy A. Goudge^g, Sean C. Solomon^h,i, Charles A. Hibbitts^a, M. Darby Dyar^j

^aPlanetary Exploration Group, The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
^bLaboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
^cApplied Coherent Technology Corporation, Herndon, VA 20170, USA
^dPlanetary Science Institute, Tucson, AZ 85719, USA
^eLunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
^fInstitute of Planetary Research, Deutsches Zentrum für Luft und Raumfahrt, D-12489 Berlin, Germany
^gDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA
^hDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
ⁱLamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
^jDepartment of Astronomy, Mount Holyoke College, South Hadley, MA 01075, USA

The MESSENGER spacecraft’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) obtained more than 1.6 million reflectance spectra of Mercury’s surface from near-ultraviolet to near-infrared wavelengths during the first year of orbital operations. A global analysis of spectra in the wavelength range 300–1450 nm shows little regional variation in absolute reflectance or spectral slopes and a lack of mineralogically diagnostic absorptions. In particular, reflectance spectra show no clear evidence for an absorption band centered near 1 μm that would be associated with the presence of ferrous iron in silicates. There is, however, evidence for an ultraviolet absorption possibly consistent with a very low iron content (2–3 wt% FeO or less) in surface silicates and for the presence of small amounts of metallic iron or other opaque minerals in the form of nano- or micrometer-sized particles. These findings are consistent with MESSENGER X-ray and gamma-ray measurements of Mercury’s surface iron abundance. Although X-ray and gamma-ray observations indicate higher than expected quantities of sulfur on the surface, reflectance spectra show no absorption bands diagnostic of sulfide minerals. Whereas there is strong evidence of water ice in permanently shadowed craters near Mercury’s poles, MASCS spectra provide no evidence for hydroxylated materials near permanently shadowed craters.

Reference
Izenberg NR, Klima RL, Murchie SL, Blewett DT, Holsclaw GM, McClintock WE, Malaret E, Mauceri C, Vilas F, Sprague AL, Helbert J, Domingue DL, Head III JW, Goudge TA, Solomon SC, Hibbitts CA and Dyar MD (in press) The low-iron, reduced surface of Mercury as seen in spectral reflectance by MESSENGER. Icarus
[doi:10.1016/j.icarus.2013.10.023]
Copyright Elsevier

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J. M. Paque^1,*, S. R. Sutton², S. B. Simon², J. R. Beckett¹, D. S. Burnett¹, L. Grossman^2,3, H. Yurimoto⁴, S. Itoh⁴, H. C. Connolly Jr.^5,6,7,8

¹Division of Geological and Planetary Sciences, Caltech, Pasadena, California, USA
²Department of Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
³Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, USA
⁴Department of Natural History Science, Hokkaido University, Sapporo, Hokkaido, Japan
⁵Department of Physical Sciences, Kingsborough Community College of the City University of New York, Brooklyn, New York, USA
⁶Department of Earth and Environmental Sciences, The Graduate Center of the City University of New York, New York, New York, USA
⁷Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
8Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

Ti valence measurements in MgAl₂O₄ spinel from calcium-aluminum-rich inclusions (CAIs) by X-ray absorption near-edge structure (XANES) spectroscopy show that many spinels have predominantly tetravalent Ti, regardless of host phases. The average spinel in Allende type B1 inclusion TS34 has 87% Ti⁺⁴. Most spinels in fluffy type A (FTA) inclusions also have high Ti valence. In contrast, the rims of some spinels in TS34 and spinel grain cores in two Vigarano type B inclusions have larger amounts of trivalent titanium. Spinels from TS34 have approximately equal amounts of divalent and trivalent vanadium. Based on experiments conducted on CAI-like compositions over a range of redox conditions, both clinopyroxene and spinel should be Ti⁺³-rich if they equilibrated with CAI liquids under near-solar oxygen fugacities. In igneous inclusions, the seeming paradox of high-valence spinels coexisting with low-valence clinopyroxene can be explained either by transient oxidizing conditions accompanying low-pressure evaporation or by equilibration of spinel with relict Ti⁺⁴-rich phases (e.g., perovskite) prior to or during melting. Ion probe analyses of large spinel grains in TS34 show that they are enriched in heavy Mg, with an average Δ²⁵Mg of 4.25 ± 0.028‰, consistent with formation of the spinel from an evaporating liquid. Δ²⁵Mg shows small, but significant, variation, both within individual spinels and between spinel and adjacent melilite hosts. The Δ²⁵Mg data are most simply explained by the low-pressure evaporation model, but this model has difficulty explaining the high Ti⁺⁴ concentrations in spinel.

Reference
Paque JM, Sutton SR, Simon SB, Beckett JR, Burnett DS, Grossman L, Yurimoto H, Itoh S and Connolly HC (in press) XANES and Mg isotopic analyses of spinels in Ca-Al-rich inclusions: Evidence for formation under oxidizing conditions. Meteoritics & Planetary Science
[doi:10.1111/maps.12216]
Published by arrangement with John Wiley & Sons

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Mariana V. Maziviero^1,*, Marcos A. R. Vasconcelos¹, Alvaro P. Crosta¹, Ana M. Góes², Wolf U. Reimold^3,4, Cleyton de C. Carneiro¹

¹Department of Geology and Natural Resources, Institute of Geosciences, University of Campinas, Campinas, São Paulo, Brazil
²Department of Sedimentary and Environmental Geology, Institute of Geosciences, University of São Paulo, São Paulo, Brazil
³Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity, Berlin, Germany
⁴Humboldt-Universität zu Berlin, Berlin, Germany

Riachão, located at S7°42′/W46°38′ in Maranhão State, northeastern Brazil, is a complex impact structure of about 4.1 km diameter, formed in Pennsylvanian to Permian sedimentary rocks of the Parnaíba Basin sequence. Although its impact origin was already proposed in the 1970s, information on its geology and shock features is still scarce in the literature. We present here the main geomorphological and geological characteristics of the Riachão impact structure obtained by integrated geophysical and remote sensing analysis, as well as geological field work and petrographic analysis. The identified lithostratigraphic units consist of different levels of the Pedra de Fogo Formation and, possibly, the Piauí Formation. Our petrographic analysis confirms the presence of shock-diagnostic planar microdeformation structures in quartz grains of sandstone from the central uplift as evidence for an impact origin of the Riachão structure. The absence of crater-filling impact breccias and melt rocks, shatter cones, as well as the restricted occurrence of microscopic shock effects, suggests that intense and relatively deep erosion has occurred since crater formation.

Reference
Maziviero MV, Vasconcelos MAR, Crosta A., Góes AM, Reimold WU and de C Carneiro C (in press) Geology and impact features of Riachão structure, northern Brazil. Meteoritics & Planetary Science
[doi:10.1111/maps.12213]
Published by arrangement with John Wiley & Sons

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B.C. Johnson^a and H.J. Melosh^a,b

^aDepartment of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907
^bDepartment of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907

We present a model that describes the formation of melt droplets, melt fragments, and accretionary impact lapilli during a hypervelocity impact. Using the iSALE hydrocode, coupled to the ANEOS equation of state for silica, we create high-resolution two-dimensional impact models to track the motion of impact ejecta. We then estimate the size of the ejecta products using simple analytical expressions and information derived from our hydrocode models. Ultimately, our model makes predictions of how the size of the ejecta products depends on impactor size, impact velocity, and ejection velocity. In general, we find that larger impactor sizes result in larger ejecta products and higher ejection velocities result in smaller ejecta product sizes. We find that a 10 km diameter impactor striking at a velocity of 20 km/s creates millimeter scale melt droplets comparable to the melt droplets found in the Chicxulub ejecta curtain layer. Our model also predicts that melt droplets, melt fragments, and accretionary impact lapilli should be found together in well preserved ejecta curtain layers and that all three ejecta products can form even on airless bodies that lack significant volatile content. This prediction agrees with observations of ejecta from the Sudbury and Chicxulub impacts as well as the presence of accretionary impact lapilli in lunar breccia.

Reference
Johnson BC and Melosh HJ (in press) Formation of melt droplets, melt fragments, and accretionary impact lapilli during a hypervelocity impact. Icarus
[doi:10.1016/j.icarus.2013.10.022]
Copyright Elsevier

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Masahiko Tanaka^1,*, Tomoki Nakamura², Takaaki Noguchi³, Aiko Nakato², Hatsumi Ishida², Toru Yada⁴, Kei Shirai⁴, Akio Fujimura⁴, Yukihiro Ishibashi⁴, Masanao Abe⁴, Tatsuaki Okada⁴, Munetaka Ueno⁴, Toshifumi Mukai⁴

¹Synchrotron X-ray Station at SPring-8, National Institute for Materials Science (NIMS), Sayo, Hyogo, Japan
²Department of the Earth and Planetary Material Science, Tohoku University, Sendai, Miyagi, Japan
³Department of Materials and Biological Sciences, Ibaraki University, Mito, Ibaraki, Japan
⁴JAXA-ISAS, Sagamihara, Kanagawa, Japan

The crystallization temperatures of Itokawa surface particles recovered by the space probe Hayabusa were estimated by a plagioclase geothermometer using sodic plagioclase triclinicity. The Δ131-index required for the thermometer, which is the difference in X-ray diffraction peak positions between the 131 and 131 reflections of plagioclase, was obtained by a high-resolution synchrotron Gandolfi camera developed for the third generation synchrotron radiation beamline, BL15XU at SPring-8. Crystallization temperatures were successfully determined from the Δ131-indices for four particles. The observed plagioclase crystallization temperatures were in a range from 655 to 660 °C. The temperatures indicate crystallization temperatures of plagioclases in the process of prograde metamorphism before the peak metamorphic stage.

Reference
Tanaka M, Nakamura T, Noguchi T, Nakato A, Ishida H, Yada T, Shirai K, Fujimura A, Ishibashi Y, Abe M, Okada T, Ueno M and Mukai T (in press) Crystallization temperature determination of Itokawa particles by plagioclase thermometry with X-ray diffraction data obtained by a high-resolution synchrotron Gandolfi camera. Meteoritics & Planetary Science
[doi:10.1111/maps.12215]
Published by arrangement with John Wiley & Sons

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W. Akram^1,2, M. Schönbächler^1,2, P. Sprung³, and N. Vogel^2,4

¹School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
²Institute for Geochemistry and Petrology, ETH, Clausiusstrasse 25, 8092 Zürich, Switzerland
³Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany
⁴Eawag, Swiss Federal Institute of Aquatic Science and Technology, Water Resources and Drinking Water, Ueberlandstrasse 133, 8600 Dübendorf, Switzerland

Recent work based on analyses of meteorite and terrestrial whole-rock samples showed that the r- and s- process isotopes of Hf were homogeneously distributed throughout the inner solar system. We report new Hf isotope data for Calcium-Aluminum-rich inclusions (CAIs) of the CV3 carbonaceous chondrite Allende, and novel high-precision Zr isotope data for these CAIs and three carbonaceous chondrites (CM, CO, CK). Our Zr data reveal enrichments in the neutron-rich isotope ⁹⁶Zr (≤1ε in ⁹⁶Zr/⁹⁰Zr) for bulk chondrites and CAIs (~2ε). Potential isotope effects due to incomplete sample dissolution, galactic and cosmic ray spallation, and the nuclear field shift are assessed and excluded, leading to the conclusion that the ⁹⁶Zr isotope variations are of nucleosynthetic origin. The ⁹⁶Zr enrichments are coupled with ⁵⁰Ti excesses suggesting that both nuclides were produced in the same astrophysical environment. The same CAIs also exhibit deficits in r-process Hf isotopes, which provides strong evidence for a decoupling between the nucleosynthetic processes that produce the light (A ≤ 130) and heavy (A > 130) neutron-rich isotopes. We propose that the light neutron-capture isotopes largely formed in Type II supernovae (SNeII) with higher mass progenitors than the supernovae that produced the heavy r-process isotopes. In the context of our model, the light isotopes (e.g. ⁹⁶Zr) are predominantly synthesized via charged-particle reactions in a high entropy wind environment, in which Hf isotopes are not produced. Collectively, our data indicates that CAIs sampled an excess of materials produced in a normal mass (12-25 M_☉) SNII.

Reference
Akram W, Schönbächler M, Sprung P and Vogel N (2013) Zirconium—Hafnium Isotope Evidence from Meteorites for the Decoupled Synthesis of Light and Heavy Neutron-rich Nuclei. The Astrophysical Journal 777:169.
[doi:10.1088/0004-637X/777/2/169]

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Subrata Chakraborty^*, Petia Yanchulova, Mark H. Thiemens

Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093–0356, USA.

We currently seek a copyright agreement with Science to display abstracts of their cosmochemistry related publications.

Reference
Chakraborty S, Yanchulova P and Thiemens MH (in press) Mass-Independent Oxygen Isotopic Partitioning During Gas-Phase SiO2 Formation. Science 342:463-466.
[doi:10.1126/science.1242237]

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Florian J. Zurfluh^1,*, Beda A. Hofmann², Edwin Gnos³, Urs Eggenberger¹

¹Institut für Geologie, Universität Bern, Bern, Switzerland
²Naturhistorisches Museum der Burgergemeinde Bern, Bern, Switzerland
³Muséum d’histoire naturelle de la Ville de Genève, Genève 6, Switzerland

The common appearance of hygroscopic brine (“sweating”) on ordinary chondrites (OCs) from Oman during storage under room conditions initiated a study on the role of water-soluble salts on the weathering of OCs. Analyses of leachates from OCs and soils, combined with petrography of alteration features and a 11-month record of in situ meteorite and soil temperatures, are used to evaluate the role of salts in OC weathering. Main soluble ions in soils are Ca²⁺, SO₄²⁻, HCO₃⁻, Na⁺, and Cl⁻, while OC leachates are dominated by Mg²⁺ (from meteoritic olivine), Ca²⁺ (from soil), Cl⁻ (from soil), SO₄²⁻ (from meteoritic troilite and soil), and iron (meteoritic). “Sweating meteorites” mainly contain Mg²⁺ and Cl⁻. The median Na/Cl mass ratio of leachates changes from 0.65 in soils to 0.07 in meteorites, indicating the precipitation of a Na-rich phase or loss of an efflorescent Na-salt. The total concentrations of water-soluble ions in bulk OCs ranges from 600 to 9000 μg g⁻¹ (median 2500 μg g⁻¹) as compared to 187–14140 μg g⁻¹ in soils (median 1148 μg g⁻¹). Soil salts dissolved by rain water are soaked up by meteorites by capillary forces. Daily heating (up to 66.3 °C) and cooling of the meteorites cause a pumping effect, resulting in a strong concentration of soluble ions in meteorites over time. The concentrations of water-soluble ions in meteorites, which are complex mixtures of ions from the soil and from oxidation and hydrolysis of meteoritic material, depend on the degree of weathering and are highest at W3. Input of soil contaminants generally dominates over the ions mobilized from meteorites. Silicate hydrolysis preferentially affects olivine and is enhanced by sulfide oxidation, producing local acidic conditions as evidenced by jarosite. Plagioclase weathering is negligible. After completion of troilite oxidation, the rate of chemical weathering slows down with continuing Ca-sulfate contamination.

Reference
Zurfluh FJ, Hofmann BA, Gnos E and Eggenberger U (in press) “Sweating meteorites”—Water-soluble salts and temperature variation in ordinary chondrites and soil from the hot desert of Oman. Meteoritics & Planetary Science
[doi:10.1111/maps.12211]
Published by arrangement with John Wiley & Sons

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M. Jadhav¹, M. Pignatari^2,6, F. Herwig^3,6,7, E. Zinner⁴, R. Gallino⁵ and G. R. Huss¹

¹Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA
²Department of Physics, University of Basel, CH-4056 Basel, Switzerland
³Department of Physics & Astronomy, University of Victoria, Victoria, BC V8P5C2, Canada
⁴Laboratory for Space Sciences & the Physics Department, Washington University, St. Louis, MO 63130, USA
⁵Dipartimento di Fisica Generale, Università di Torino & INAF, Via Pietro Giuria 1, 10125 Torino, Italy
⁶NuGrid Collaboration: http://www.nugridstars.org.
⁷Also at The Joint Institute for Nuclear Astrophysics, Notre Dame, IN 46556, USA.

Graphite is one of the many presolar circumstellar condensate species found in primitive meteorites. While the isotopic compositions of low-density graphite grains indicate an origin in core-collapse supernovae, some high-density grains have extreme isotopic anomalies in C, Ca, and Ti, which cannot be explained by envelope predictions of asymptotic giant branch (AGB) stars or theoretical supernova models. The Ca and Ti isotopic anomalies, however, match the predictions of He-shell abundances in AGB stars. In this study, we show that the C, Ca, and Ti isotopic anomalies are consistent with nucleosynthesis predictions of the H-ingestion phase during a very late thermal pulse (VLTP) event in post-AGB stars. The low ¹²C/¹³C isotopic ratios in these grains are a result of abundant ¹²C efficiently capturing the protons that are being ingested during the VLTP. Very high neutron densities of ~10¹⁵ cm⁻³, typical of the i-process, are achieved during this phase in post-AGB stars. The large ^42,43,44Ca excesses in some graphite grains are indicative of neutron capture nucleosynthesis during VLTP. The comparison of VLTP nucleosynthesis calculations to the graphite data also indicate that apparent anomalies in the Ti isotopic ratios are due to large contributions from ^46,48Ca, which cannot be resolved from the isobars ^46,48Ti during the measurements. We conclude that presolar graphite grains with moderate to extreme Ca and Ti isotopic anomalies originate in post-AGB stars that suffer a VLTP.

Reference
Jadhav M, Pignatari M, Herwig F, Zinner E, Gallino R, and Huss GR (in press) Relics of Ancient Post-AGB Stars in a Primitive Meteorite. The Astrophysical Journal – Letters
[doi:10.1088/2041-8205/777/2/L27]

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Emma C. Marcucci^1,2, Brian M. Hynek^1,2,*, Kathryn S. Kierein-Young², K. L. Rogers^3,4

¹Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado, USA
²Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA
³Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA
⁴Now at Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York, USA

Acid-sulfate weathering at Nicaraguan hydrothermal sites Cerro Negro, Momotombo, and Telica volcanoes and Hervidores de San Jacinto mudpots was characterized as an analog for similar processes that likely operated on early Mars. In situ mineralogical analyses were conducted with a field portable visible near-infrared spectrometer for comparison to similar Martian data sets. Three classes of alteration minerals were identified: sulfates (gypsum and natroalunite), oxides/hydroxides (hematite and goethite), and phyllosilicates (kaolinite/halloysite, montmorillonite, and saponite), as well as elemental sulfur and hydrated silica phases. Our sites had similar suites of minerals, but frequencies varied with location. The results of this field campaign allow inferences regarding the paleo-environmental conditions that were likely present at similar relic hydrothermal sites identified on Mars. In particular, sulfates and phyllosilicates could have coevolved under hydrothermal conditions at Noctis Labyrinthus as is seen in Nicaragua. Fe/Mg smectites were detected in areas with pH of 3–4. Alunite spectra at Terra Sirenum demonstrated mineral mixing effects on spectroscopy. Mineral mixing can cause uncertainties in spectral identification due to a dominant spectrum, such as iron minerals, masking another or the suppression of weaker bands. When viewed from orbit, our field sites would likely be dominated by hydrated silica and Mars sites, such as one in Syrtis Major, could have a more diverse mineralogy than the data reveal. Concentrated amorphous silica, such as at Gusev crater, can result from acidic fumarolic activity, while Mg sulfates may indicate a lack of reworking by water. This field spectroscopy study helps confirm and provide insight into hydrothermal processes on ancient Mars.

Reference
Marcucci EC, Hynek BM, Kierein-Young KS and Rogers KL (in press) Visible-near-infrared reflectance spectroscopy of volcanic acid-sulfate alteration in Nicaragua: Analogs for early Mars. Journal of Geophysical Research – Planets
[doi:10.1002/jgre.20159]
Published by arrangement with John Wiley & Sons

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