¹Siddharth Hegde,²Ivan G. Paulino-Lima,³Ryan Kent,^1,4Lisa Kaltenegger,⁵Lynn Rothschild
¹Max Planck Institute for Astronomy, Heidelberg 69117, Germany;
²National Aeronautics and Space Administration Postdoctoral Program Fellow, National Aeronautics and Space Administration Ames Research Center, Moffett Field, CA 94035;
³University of California, Santa Cruz University Affiliated Research Center, National Aeronautics and Space Administration Ames Research Center, Moffett Field, CA 94035;
⁴Institute for Pale Blue Dots, Department of Astronomy, Cornell University, Ithaca, NY 14853; and
⁵National Aeronautics and Space Administration Ames Research Center, Moffett Field, CA 94035

Exoplanet discovery has made remarkable progress, with the first rocky planets having been detected in the central star’s liquid water habitable zone. The remote sensing techniques used to characterize such planets for potential habitability and life rely solely on our understanding of life on Earth. The vegetation red edge from terrestrial land plants is often used as a direct signature of life, but it occupies only a small niche in the environmental parameter space that binds life on present-day Earth and has been widespread for only about 460 My. To more fully exploit the diversity of the one example of life known, we measured the spectral characteristics of 137 microorganisms containing a range of pigments, including ones isolated from Earth’s most extreme environments. Our database covers the visible and near-infrared to the short-wavelength infrared (0.35–2.5 µm) portions of the electromagnetic spectrum and is made freely available from biosignatures.astro.cornell.edu. Our results show how the reflectance properties are dominated by the absorption of light by pigments in the visible portion and by strong absorptions by the cellular water of hydration in the infrared (up to 2.5 µm) portion of the spectrum. Our spectral library provides a broader and more realistic guide based on Earth life for the search for surface features of extraterrestrial life. The library, when used as inputs for modeling disk-integrated spectra of exoplanets, in preparation for the next generation of space- and ground-based instruments, will increase the chances of detecting life.

Reference
Hegde S, Paulino-Lima IG, Kent R, Kaltenegger L, Rothschild L (2015) Surface biosignatures of exo-Earths: Remote detection of extraterrestrial life. Proceedings of the National Academy of Sciences 112, 3886–3891
Link to Article [doi: 10.1073/pnas.1421237112]

¹M. Campbell-Brown
¹University of Western Ontario, Department of Physics and Astronomy, London, ON, Canada N6A 3K7 +15196612111

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

Reference
Campbell-Brown M (2015) A population of small refractory meteoroids in asteroidal Orbits. Planetary and Space Science (in Press)
Link to Article [doi:10.1016/j.pss.2015.03.022]

¹Deepak Dhingra,¹Carle M. Pieters,¹James W. Head
¹Earth, Environmental and Planetary Sciences, Brown University, Brook Street, Box 1846, Providence, RI 02912, USA

Multiple origins for olivine-bearing lithologies at Copernicus crater are recognized based on integrated analysis of data from Chandrayaan-1 Moon Mineralogy Mapper (M3), Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC) and Kaguya Terrain Camera (TC). We report the diverse morphological and spectral character of previously known olivine-bearing exposures as well as the new olivine occurrences identified in this study. Prominent albedo differences exist between olivine-bearing exposures in the central peaks and a northern wall unit (the latter being ∼40% darker). The low-albedo wall unit occurs as a linear mantling deposit and is interpreted to be of impact melt origin, in contrast with the largely unmodified nature of olivine-bearing peaks. Small and localized occurrences of olivine-bearing lithology have also been identified on the impact melt-rich floor, representing a third geologic setting (apart from crater wall and peaks). Recent remote sensing missions have identified olivine-bearing exposures around lunar basins (e.g. Yamamoto et al., 2010, Pieters et al., 2011 and Kramer et al., 2013) and at other craters (e.g. Sun and Li, 2014), renewing strong interest in its origin and provenance. A direct mantle exposure has commonly been suggested in this regard. Our detailed observations of the morphological and spectral diversity in the olivine-bearing exposures at Copernicus have provided critical constraints on their origin and source regions, emphasizing multiple formation mechanisms. These findings directly impact the interpretation of olivine exposures elsewhere on the Moon. Olivine can occur in diverse environments including an impact melt origin, and therefore it is unlikely for all olivine exposures to be direct mantle occurrences as has generally been suggested.

Reference
Dhingra D, Pieters CM, Head JW (2015) Multiple origins for olivine at Copernicus crater. Earth and Planetary Science Letters 420, 95–101
Link to Article [doi:10.1016/j.epsl.2015.03.039]

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¹Martin Schiller,¹James N. Connelly,¹Aslaug C. Glad,²Takashi Mikouchi,¹Martin Bizzarro
¹Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5-7, DK-1350, Denmark
²Department of Earth & Planetary Science, University of Tokyo, Tokyo, Japan

The mechanisms and timescales of accretion of 10–1000 km sized planetesimals, the building blocks of planets, are not yet well understood. With planetesimal melting predominantly driven by the decay of the short-lived radionuclide 26Al (26Al→26Mg; t1/2=0.73 Mat1/2=0.73 Ma), its initial abundance determines the permissible timeframe of planetesimal-scale melting and its subsequent cooling history. Currently, precise knowledge about the initial 26Al abundance [(26Al/27Al)0] exists only for the oldest known solids, calcium aluminum-rich inclusions (CAIs) – the so-called canonical value. We have determined the 26Al/27Al of three angrite meteorites, D’Orbigny, Sahara 99555 and NWA 1670, at their time of crystallization, which corresponds to (3.98±0.15)×10−7(3.98±0.15)×10−7, (3.64±0.18)×10−7(3.64±0.18)×10−7, and (5.92±0.59)×10−7(5.92±0.59)×10−7, respectively. Combined with a newly determined absolute U-corrected Pb–Pb age for NWA 1670 of 4564.39±0.24 Ma4564.39±0.24 Ma and published U-corrected Pb–Pb ages for the other two angrites, this allows us to calculate an initial (26Al/27Al)0 of View the MathML source(1.33−0.18+0.21)×10−5 for the angrite parent body (APB) precursor material at the time of CAI formation, a value four times lower than the accepted canonical value of 5.25×10−55.25×10−5. Based on their similar 54Cr/52Cr ratios, most inner solar system materials likely accreted from material containing a similar 26Al/27Al ratio as the APB precursor at the time of CAI formation. To satisfy the abundant evidence for widespread planetesimal differentiation, the subcanonical 26Al budget requires that differentiated planetesimals, and hence protoplanets, accreted rapidly within 0.25±0.15 Ma0.25±0.15 Ma of the formation of canonical CAIs.

Reference
Schiller M, Connelly JN, Glad AC, Mikouchi T, Bizzarro M (2015) Early accretion of protoplanets inferred from a reduced inner solar system 26Al inventory. Earth and Planetary Science Letters 420, 45–54.
Link to Article [doi:10.1016/j.epsl.2015.03.028]

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¹Corentin Le Guillou,²Hitesh G. Changela,²Adrian J. Brearley
¹Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, Germany
²Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA

The CR chondrites carry one of the most pristine records of the solar nebula materials that accreted to form planetesimals. They have experienced very variable degrees of aqueous alteration, ranging from incipient alteration in their matrices to the complete hydration of all of their components. In order to constrain their chemical alteration pathways and the conditions of alteration, we have investigated the mineralogy and Fe oxidation state of silicates in the matrices of 8 CR chondrites, from type 3 to type 1. Fe-L edge X-ray Absorption Near Edge Structure (XANES) was performed on matrix FIB sections using synchrotron-based scanning transmission X-ray microscopy (STXM). The Fe3+/∑FeFe3+/∑Fe ratio of submicron silicate particles was obtained and coordinated with TEM observations.
In all the least altered CR chondrites (QUE 99177, EET 87770, EET 92042, LAP 02342, GRA 95229 and Renazzo), we find that the matrices consist of abundant submicron Fe-rich hydrated amorphous silicate grains, mixed with nanometer-sized phyllosilicates. The Fe3+/∑FeFe3+/∑Fe ratios of both amorphous and nanocrystalline regions are very high with values ranging from 68 to 78%. In the most altered samples (Al Rais and GRO 95577), fine-grained phyllosilicates also have a high Fe3+/∑FeFe3+/∑Fe ratio (around 70%), whereas the coarse, micrometer-sized phyllosilicates are less oxidized (down to 55%) and have a lower iron content.
These observations suggest the following sequence: submicron Fe2+-amorphous silicate particles were the building blocks of CR matrices; after accretion they were quickly hydrated and oxidized, leading to a metastable, amorphous gel-like phase. Nucleation and growth of crystalline phyllosilicates was kinetically-limited in most type 3 and 2 CRs, but increased as alteration became more extensive in Al Rais and GRO 95577. The decreasing Fe3+/∑FeFe3+/∑Fe ratio is interpreted as a result of the transfer of Fe3+ from silicates to oxides during growth, while aqueous alteration progressed (higher temperature, longer duration, change of fluid composition). In a fully closed system, equilibrium thermodynamics suggest that the water to rock ratios, typically assumed to be low (<1) for chondrites, should primarily control the iron valency of the silicates and predict a lower Fe3+/∑FeFe3+/∑Fe ratio. Such a high Fe3+/∑FeFe3+/∑Fe value could be accounted for, however, if the system was partially open, at least with respect to H2 (and other gases as well). Rapid degassing of the fluid would have favored more oxidizing fluid conditions. Recently proposed scenarios involving some degree of water D/H increase through Rayleigh isotopic fractionation are supported by these results.

Reflectance
Le Guillou C, Changela HG, Brearley AJ (2015) Widespread oxidized and hydrated amorphous silicates in CR chondrites matrices: Implications for alteration conditions and H2 degassing of asteroids. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.02.031]

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^1,2,3,4Johan Villeneuve, ^1,5,6Guy Libourel,¹Camille Soulié
¹CRPG-Université de Lorraine, CNRS, UMR 7358, 15 Rue Notre-Dame des Pauvres, BP20, 545401 Vandoeuvre-lès-Nancy, France
²Université d’Orléans, ISTO, UMR 7327, 45071 Orléans, France
³CNRS/INSU, ISTO, UMR 7327, 45071 Orléans, France
⁴BRGM, BP 36009, 45060 Orléans, France
⁵Ecole Nationale Supérieure de Géologie – Université de Lorraine, Rue du Doyen Marcel Roubault, BP40, 54501, Vandoeuvre-lès-Nancy, France
⁶Geoazur, OCA, Université de Nice – Sophia Antipolis, CNRS/IRD, 250 rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France

In unequilibrated chondrites, the ferromagnesian silicates in chondrules exhibit wide ranges of mg# = Mg/(Mg + Fe), allowing to sub-divide porphyritic chondrules into either type I (mg# > 0.9) or type II (mg# < 0.9). Although both chondrule types formed under oxidizing conditions relative to the canonical solar nebula, it is generally inferred that type II chondrules formed in more oxidizing conditions than type I. In order to check whether this redox difference was established during chondrule formation, or reflects differences in their precursors, we have undertaken a set of experiments aimed at heating type I olivine-rich (A) chondrule proxy, i.e. forsterite + Fe metal + Ca-Mg-Si-Al glass mixtures, under oxidizing conditions. We show that high temperature (isothermal) oxidation of type IA-like assemblages is a very efficient and rapid process (e.g., few tens of minutes) to form textures similar to type IIA chondrules. Due to the rapid dissolution of Fe metal blebs, a FeO increase in the melt and in combination with the dissolution of magnesian olivine allows the melt to reach ferroan olivine saturation. Crystallization of ferroan olivine occurs either as new crystal in the mesostasis or as overgrowths on the remaining unresorbed forsterite grains (relicts). Interruption of this process at any time before its completion by rapid cooling allows to reproduce the whole range of textures and chemical diversity observed in type A chondrules, i.e., from type I to type II.

Several implications on chondrule formation processes can be inferred from the presented experiments. Type I chondrules or fragments of type I chondrules are very likely the main precursor material involved in the formation of most type II chondrules. Formation of porphyritic olivine type II chondrules is very likely the result of processes generating crystal growth by chemical disequilibrium at high temperature rather than processes generating crystallization only by cooling rates. This questions the reliability of chondrule thermal history (e.g., cooling rate values) hitherto inferred for producing porphyritic textures from dynamical cooling rate experiments only. Type A chondrule formation can be a very fast process. After periods of sub-isothermal heating or slow cooling (< 50 K/h) as short as several tens of minutes and no longer than few hundreds of minutes at 1500 -1800°C, type A chondrules terminates their formation by a fast cooling (> 103-104 K/h) in order to preserve their glassy mesostasis. Such inferred thermal history being at odds with nebular shock models, we thus advocate that impacts on planetesimals causing rapid melting and vaporization may provide the high density and highly volatile-enriched gaseous environments required to form chondrules. In this scenario, chondrules and their diversity should result from various degrees of interaction of the ejected fragments with the impact vapor plume; the most oxidizing conditions recorded in type IIA chondrules being very likely the closest to those imposed by the impact vapor plume.

Reference
Villeneuve J, Libourel G, Souliéa C (2015) Relationships between type I and type II chondrules: Implications on chondrule formation processes. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.03.033]

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¹G.J. Consolmagno,^2,3G.J. Golabek,⁴D. Turrini,⁵M. Jutzi,⁶S. Sirono,⁷V. Svetsov, ⁸K. Tsiganis
¹Specola Vaticana, V-00120, Vatican City State
²Institute of Geophysics, ETH Zurich, Sonneggstrasse 5, CH-8092 Zürich, Switzerland
³Bayerisches Geoinstitut, University of Bayreuth, D-95440 Bayreuth, Germany
⁴Istituto di Astrofisica e Planetologia Spaziali INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Rome, Italy
⁵Physics Institute, Space Research and Planetary Sciences, Center for Space and Habitability, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
⁶Earth and Environmental Sciences, Nagoya University, Tikusa-ku, Furo-cho, Nagoya 464-8601 Japan
⁷Institute for Dynamics of Geospheres, Leninskiy Prospekt 38-1, Moscow, Russia
⁸Unit of Mechanics, Section of Astrophysics, Astronomy & Mechanics, Department of Physics, Aristotle University of Thessaloniki, GR 54 124 Thessaloniki, Greece

It is difficult to find a Vesta model of iron core, pyroxene and olivine-rich mantle, and HED crust that can match the joint constraints of (a) Vesta’s density and core size as reported by the Dawn spacecraft team; (b) the chemical trends of the HED meteorites, including the depletion of sodium, the FeO abundance, and the trace element enrichments; and (c) the absence of exposed mantle material on Vesta’s surface, among Vestoid asteroids, or in our collection of basaltic meteorites. These conclusions are based entirely on mass-balance and density arguments, independent of any particular formation scenario for the HED meteorites themselves. We suggest that Vesta either formed from source material with non-chondritic composition or underwent after its formation a radical physical alteration, possibly caused by collisional processes, that affected its global composition and interior structure.

Reference
Consolmagno GJ, Golabek GJ, Turrini D, Jutzi M, Sirono S, Svetsov V, Tsiganis K (2015) Is Vesta an Intact and Pristine Protoplanet? Icarus (in Press)
Link im Article [doi:10.1016/j.icarus.2015.03.029]

Copyright Elsevier

¹Alexander Hubbard
¹Department of Astrophysics, American Museum of Natural History, New York, NY 10024-5192, USA

About 4-5% of chondrules are compound: two separate chondrules stuck together. This is commonly believed to be the result of the two component chondrules having collided shortly after forming, while still molten. This allows high velocity impacts to result in sticking. However, at T ∼1100 K, the temperature below which chondrules collide as solids (and hence usually bounce), coalescence times for droplets of appropriate composition are measured in tens of seconds. Even at 1025 K, at which temperature theory predicts that the chondrules must have collided extremely slowly to have stuck together, the coalescence time scale is still less than an hour. These coalescence time scales are too short for the collision of molten chondrules to explain the observed frequency of compound chondrules. We suggest instead a scenario where chondrules stuck together in slow collisions while fully solid; and the resulting chondrule pair was subsequently briefly heated to a temperature in the range of 900-1025 K. In that temperature window the coalescence time is finite but long, covering a span of hours to a decade. This is particularly interesting because those temperatures are precisely the critical window for thermally ionized MRI activity, so compound chondrules provide a possible probe into that vital regime.

Reference
Hubbard A (2015) Compound Chondrules fused Cold. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.02.030]

Copyright Elsevier

¹Scott L. Murchie et al (>10)*
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
*Find the extensive, full author and affiliation list on the publishers website

A principal data product from MESSENGER’s primary orbital mission at Mercury is a global multispectral map in eight visible to near-infrared colors, at an average pixel scale of 1 km, acquired by the Mercury Dual Imaging System. The constituent images have been calibrated, photometrically corrected to a standard geometry, and map projected. Global analysis reveals no spectral units not seen during MESSENGER’s Mercury flybys and supports previous conclusions that most spectral variation is related to changes in spectral slope and reflectance between spectral end-member high-reflectance red plains (HRP) and low-reflectance material (LRM). Comparison of color properties of plains units mapped on the basis of morphology shows that the two largest unambiguously volcanic smooth plains deposits (the interior plains of Caloris and the northern plains) are close to HRP end members and have average color properties distinct from those of most other smooth plains and intercrater plains. In contrast, smaller deposits of smooth plains are nearly indistinguishable from intercrater plains on the basis of their range of color properties, consistent with the interpretation that intercrater plains are older equivalents of smooth plains. LRM having nearly the same reflectance is exposed in crater and basin ejecta of all ages, suggesting impact excavation from depth of material that is intrinsically dark or darkens very rapidly, rather than the product of gradual darkening of exposed material purely by space weathering. A global search reveals no definitive absorptions attributable to Fe2+-containing silicates or to sulfides over regions 20 km or more in horizontal extent, consistent with results from MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer. The only absorption-like feature identified is broad upward curvature of the spectrum centered near 600 nm wavelength. The feature is strongest in freshly exposed LRM and weak or absent in older exposures of LRM. We modeled spectra of LRM as intimate mixtures of HRP with candidate low-reflectance phases having a similar 600-nm spectral feature, under the assumption that the grain size is 1 μm or larger. Sulfides measured to date in the laboratory and coarse-grained iron are both too bright to produce LRM from HRP. Ilmenite is sufficiently dark but would require Ti abundances too high to be consistent with MESSENGER X-Ray Spectrometer measurements. Three phases or mixtures of phases that could be responsible for the low reflectance of LRM are consistent with our analyses. Graphite, in amounts consistent with upper limits from the Gamma-Ray Spectrometer, may be consistent with geochemical models of Mercury’s differentiation calling for a graphite-enriched primary flotation crust from an early magma ocean and impact mixing of that early crust before or during the late heavy bombardment (LHB) into material underlying the volcanic plains. The grain size of preexisting iron or iron sulfide could have been altered to a mix of nanophase and microphase grains by shock during those impacts, lowering reflectance. Alternatively, iron-bearing phases and carbon in a late-accreting carbonaceous veneer may have been stirred into the lower crust or upper mantle. Decoupling of variations in color from abundances of major elements probably results from the very low content and variation of Fe2+ in crustal silicates, such that reflectance is controlled instead by minor opaque phases and the extent of space weathering.

Reference
Murchie SL et al. (2015) Orbital Multispectral Mapping of Mercury with the MESSENGER Mercury Dual Imaging System: Evidence for the Origins of Plains Units and Low-Reflectance Material. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.03.027]

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