A. Belloche¹, H. S. P. Müller^1,2, K. M. Menten¹, P. Schilke^1,2 and C. Comito¹

¹Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Context. The discovery of amino acids in meteorites fallen to Earth and the detection of glycine, the simplest of them, in samples returned from a comet to Earth strongly suggest that the chemistry of the interstellar medium is capable of producing such complex organic molecules and that they may be widespread in our Galaxy.
Aims. Our goal is to investigate the degree of chemical complexity that can be reached in the interstellar medium, in particular in dense star-forming regions.
Methods. We performed an unbiased, spectral line survey toward Sgr B2(N) and (M), two regions where high-mass stars are formed, with the IRAM 30 m telescope in the 3 mm atmospheric transmission window. Partial surveys at 2 and 1.3 mm were performed in parallel. The spectra were analyzed with a simple radiative transfer model that assumes local thermodynamic equilibrium but takes optical depth effects into account.
Results. About 3675 and 945 spectral lines with a peak signal-to-noise ratio higher than 4 are detected at 3 mm toward Sgr B2(N) and (M), i.e. about 102 and 26 lines per GHz, respectively. This represents an increase by about a factor of two over previous surveys of Sgr B2. About 70% and 47% of the lines detected toward Sgr B2(N) and (M) are identified and assigned to 56 and 46 distinct molecules as well as to 66 and 54 less abundant isotopologues of these molecules, respectively. In addition, we report the detection of transitions from 59 and 24 catalog entries corresponding to vibrationally or torsionally excited states of some of these molecules, respectively, up to a vibration energy of 1400 cm^-1 (2000 K). Excitation temperatures and column densities were derived for each species but should be used with caution. The rotation temperatures of the detected complex molecules typically range from ~50 to 200 K. Among the detected molecules, aminoacetonitrile, n-propyl cyanide, and ethyl formate were reported for the first time in space based on this survey, as were five rare isotopologues of vinyl cyanide, cyanoacetylene, and hydrogen cyanide. We also report the detection of transitions from within twelve new vibrationally or torsionally excited states of known molecules. Absorption features produced by diffuse clouds along the line of sight are detected in transitions with low rotation quantum numbers of many simple molecules and are modeled with ~30–40 velocity components with typical linewidths of ~3–5 km s^-1.
Conclusions. Although the large number of unidentified lines may still allow future identification of new molecules, we expect most of these lines to belong to vibrationally or torsionally excited states or to rare isotopologues of known molecules for which spectroscopic predictions are currently missing. Significant progress in extending the inventory of complex organic molecules in Sgr B2(N) and deriving tighter constraints on their location, origin, and abundance is expected in the near future thanks to an ongoing spectral line survey at 3 mm with ALMA in its cycles 0 and 1. The present single-dish survey will serve as a solid basis for the line identification and analysis of such an interferometric survey.

Reference
Belloche A, Müller HSP, Menten KM, Schilke P and Comito C (in press) Complex organic molecules in the interstellar medium: IRAM 30 m line survey of Sagittarius B2(N) and (M). Astronomy & Astrophysics 559:A47.
[doi:10.1051/0004-6361/201321096]
Reproduced with permission © ESO

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James A. Van Orman^a,*, Daniele J. Cherniak^b and Noriko T. Kita^c

^aDepartment of Earth, Environmental and Planetary Sciences, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
^bDepartment of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
^cDepartment of Geoscience, University of Wisconsin–Madison, Madison, WI 53706, USA

Experimental data are reported on Mg diffusion in plagioclase crystals with a range of anorthite content (x_An), at temperatures between 800 and 1150 °C. Oriented and polished single crystals of anorthite (x_An=0.93), labradorite (x_An=0.67), andesine (x_An=0.43) and oligoclase (x_An=0.23) were each embedded in powered source material enriched in natural MgO or ²⁵MgO and suspended in a furnace at constant temperature. Diffusion profiles in quenched samples were measured from the polished surface using SIMS depth profiling. The diffusion coefficient does not depend significantly on the Mg concentration gradient, and little anisotropy is observed between the b and c directions in labradorite. Diffusion coefficients increase systematically with decreasing x_An, and the entire data set is described by , where R is the gas constant, T is absolute temperature, and the diffusion coefficient D is in m²/s. ²⁶Al–²⁶Mg ages in albite-rich plagioclase are much more easily reset than in anorthite, with closure temperatures up to 120–150 K lower.

Reference
Van Orman JA, Cherniak DJ and Kita NT (2013) Magnesium diffusion in plagioclase: Dependence on composition, and implications for thermal resetting of the ²⁶Al–²⁶Mg early solar system chronometer. Earth and Planetary Science Letters 385:79–88.
[doi:10.1016/j.epsl.2013.10.026]
Copyright Elsevier

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C. Koike¹, R. Noguchi², H. Chihara^3,5, H. Suto⁴, O. Ohtaka², Y. Imai², T. Matsumoto² and A. Tsuchiyama³

¹Department of Physics, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
²Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
³Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-8052, Japan
⁴National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
⁵Currently at College of General Education, Osaka Sangyo University, Daito, Osaka 574-8530, Japan.

The existence of silica within several debris disks has been suggested. Data on both the spectroscopy and annealing conditions of the various polymorphs of silica need to be investigated, as these data are lacking and incomplete in the literature. We investigate the annealing conditions of silica and prepare various types of silica, including α-cristobalite, α-quartz, coesite, stishovite, and fused quartz, which are natural, synthetic, or commercial samples. This paper presents a new study of both the spectroscopy of relevant silica polymorphs and the conditions under which they form. We compare the results to previous studies and find that there are discrepancies. The interesting result of features similar to those of forsterite should be highlighted, where α-cristobalite and coesite showed similar peaks at 16, 33, and 69 μm as forsterite. The 69 μm band for α-cristobalite is especially very broad and strong and shifts largely to a shorter wavelengths under cooling to low temperatures. The band for coesite, however, is very sharp and shifts only a small amount to longer wavelengths under cooling to low temperatures. We discuss the possibility of silica detection around debris disks.

Reference
Koike C, Noguchi R, Chihara H, Suto H, Ohtaka O, Imai Y, Matsumoto T and Tsuchiyama A (2013) Infrared Spectra of Silica Polymorphs and the Conditions of Their Formation. The Astrophysical Journal 778:60.
[doi:10.1088/0004-637X/778/1/60]

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Yonaton Goldsmith^a,b,c, Mordechai Stein^a,b, Yehouda Enzel^a

^aThe Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel
^bGeological Survey of Israel, 30 Malkhei Israel St., Jerusalem 95501, Israel
^cLamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA

Chemical compositions of rock varnish from the Negev Desert of Israel and local settled dust were used to constrain the mechanisms of varnish formation and patterns of Mn enrichment and accumulation in the varnish. Rock varnish was sampled from coeval, undisturbed prehistoric flint artifacts along a south–north climatic transect (∼30 to 120 mm/yr of rain). Our analyses indicate that Mn, Ba and Pb in the varnish are significantly enriched (∼100×) in respect to the local settling dust and that Mn content systematically fluctuates with depth in the varnish. The varnish and settled dust data combined with basic thermodynamic and kinetic reasoning are used to constrain the following geochemical model of rock varnish formation: dust accumulates in micro-basins on exposed rock surfaces, under pH∼8 (common Negev value) and during wetting by dew and rain, Mn in the dust is mobilized and leached to a depth of ∼5 μm under the varnish surface where Hollandite Mn-oxides precipitate and are adsorbed onto and between the porous clay minerals that comprise most of the varnish. During its mobile phase Mn-oxide is negatively charged and adsorbs rare earth elements. Once the solution dries abrasion removes the upper, weakly cemented dust sediment, which contains mainly Si, Al and Fe (which are not mobile at pH ∼ 8). Ca is also removed in large quantities. Mn, Ba, Pb and the REE are deposited at a depth and thus, protected from erosion. Reoccurrences of these processes result in a noticeable accumulation of these elements, but not of Si, Al or Fe. The alternating Mn-rich and Mn-poor laminas form as a result of a competition between the leaching rate of Mn and the adhesion rate of the clay minerals. When moisture is high (low), lamina with high (low) Mn/clay mineral ratio forms.

Reference
Goldsmith Y, Stein M and Enzel Y (in press) From dust to varnish: Geochemical constraints on rock varnish formation in the Negev Desert, Israel. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.040]
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Patrick Höhener^a,* and Olivier Atteia^b

^aAix-Marseille Université – Laboratoire Chimie Environnement, FRE 3416 –CNRS, Marseille, France
^bENSEGID, 1 allée Daguin 33607 Pessac, France

In isotope geochemistry, the Rayleigh equation describes the evolution of isotope ratios in a parent compound as a function of reaction progress, and associated equations describe isotope ratios in an instantaneous product and an accumulated product. The Rayleigh equation is commonly used for fitting fractionation factors of processes undergoing kinetic isotope fractionation such as biochemical reactions. This work extends the equations associated with the Rayleigh equation for describing the isotope ratios in intermediate products in a chain of reacting species degrading with first-order kinetics. A general solution is presented for decay chains of any length, and explicit examples are presented for the biodegradation of a substrate or a mixture of substrates through 3 intermediate products to a final product. Applications of these analytical solutions for the fitting of enrichment factors for intermediate compounds in laboratory experiments are demonstrated with a spreadsheet. This avoids separate experiments to measure each intermediate product. The utility of the equations for the assessment of slopes in dual isotope plots is furthermore illustrated, and limitations of its use are critically discussed.

Reference
Höhener P and Atteia O (in press) Rayleigh equation for evolution of stable isotope ratios in contaminant decay chains. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.036]
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Katarina Miljković^1,*, Mark A. Wieczorek¹, Gareth S. Collins², Matthieu Laneuville¹, Gregory A. Neumann³, H. Jay Melosh⁴, Sean C. Solomon^5,6, Roger J. Phillips⁷, David E. Smith⁸, Maria T. Zuber⁸

¹Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, Case 7011, Lamarck A, 5, 35 rue Hélène Brion, 75205 Paris cedex 13, France.
²Department of Earth Sciences and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.
³Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
⁴Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA.
⁵Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
⁶Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.
⁷Planetary Science Directorate, Southwest Research Institute, Boulder, CO 80302, USA.
⁸Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

Maps of crustal thickness derived from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission revealed more large impact basins on the nearside hemisphere of the Moon than on its farside. The enrichment in heat-producing elements and prolonged volcanic activity on the lunar nearside hemisphere indicate that the temperature of the nearside crust and upper mantle was hotter than that of the farside at the time of basin formation. Using the iSALE-2D hydrocode to model impact basin formation, we found that impacts on the hotter nearside would have formed basins with up to twice the diameter of similar impacts on the cooler farside hemisphere. The size distribution of lunar impact basins is thus not representative of the earliest inner solar system impact bombardment.

Reference
Miljković K, Wieczorek MA, Collins GS, Laneuville M, Neumann GA, Melosh HJ, Solomon SC, Phillips RJ, Smith DE and Zuber MT (2013) Asymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties. Science 342:724-726.
[doi:10.1126/science.1243224]
Reprinted with permission from AAAS

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Yudhijit Bhattacharjee

The Mars Atmosphere and Volatile EvolutioN (MAVEN), a NASA spacecraft to be launched to Mars later this month, will try to decipher billions of years of planetary history from careful study of the martian atmosphere. Eons ago, planetary scientists believe, Mars had a thick atmosphere that sheltered a surface awash with water—conditions in which life could have emerged and thrived. Today, that atmosphere is thin and depleted, and Mars is a cold, barren desert. What caused this remarkable transformation? Until now, planetary scientists have attempted to answer such questions mainly from the planet’s surface. MAVEN will take a new course: flying through the outer fringes of Mars’s atmosphere, measuring gases and monitoring conditions with eight instruments. The measurements should help researchers figure out how the solar wind, asteroid impacts, and chemical reactions gradually depleted the Red Planet’s atmosphere.

Reference
Bhattacharjee Y (2013) Orbiting MAVEN Mission Set to Trace a Planet’s History in Thin Martian Air. Science 342:681.
[doi:10.1126/science.342.6159.681]
Reprinted with permission from AAAS

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Mariek E. Schmidt^a, Christian M. Schrader^b,* and Timothy J. McCoy^c

^aDepartment of Earth Sciences, Brock University, Saint Catharines, ON L2S 3A1, Canada
^bNASA, Marshall Space Flight Center, Huntsville, AL 35813, Unites States
^cDepartment of Mineral Sciences, Smithsonian Institution, MRC-0119, PO Box 37012, Washington, DC 20013-7012, United States

The primary oxygen fugacity (fO₂) of basaltic melts reflects the mantle source oxidation state, dictates the crystallizing assemblage, and determines how the magma will evolve. Basalts examined by the Spirit Mars Exploration Rover in Gusev Crater range from the K-poor Adirondack class (0.02 wt% K₂O) to K-rich Backstay class (up to 1.2 wt% K₂O) and exhibit substantially more variation than observed in martian basaltic meteorites. The ratios of ferric to total iron (Fe³⁺/Fe_T) measured by the Mössbauer spectrometer are high (equivalent to −0.76 to +2.98 ΔQFM; quartz-fayalite-magnetite buffer as defined by Wones and Gilbert, 1969), reflecting secondary Fe³⁺ phases. By combining the Fe³⁺/Fe_T of the igneous minerals (olivine, pyroxene, and magnetite) determined by Mössbauer spectrometer, we estimate primary fO₂ for the Gusev basalts to be −3.6 to 0.5 ΔQFM. Estimating the fO₂ as a function of the dependence of the CIPW normative fayalite/magnetite ratios on Fe³⁺/Fe_T yields a slightly smaller range of −2.58 to +0.57 ΔQFM. General similarity between the fO₂ estimated for the Gusev basalts and ranges in fO₂ for the shergottitic meteorites (−3.8 to 0.2 ΔQFM; Herd, 2003 and Goodrich et al., 2003) suggests that the overall range of fO₂ for the martian igneous rocks and mantle is relatively restricted. Like the shergottites (Herd, 2003), estimated fO₂ of three Gusev classes (Adirondack, Barnhill and Irvine) correlates with a proxy for LREE enrichment (K₂O/TiO₂). This suggests mixing between melts or fluids derived from reservoirs with contrasting fO₂ and REE characteristics. Oxygen fugacity estimates for the martian interior suggest that tectonic processes have not led to sufficient recycling of oxidized surface material into the martian interior to entirely affect the overall oxidation state of the mantle.

Reference
Schmidt ME, Schrader CM and McCoy TJ (2013) The primary fO2 of basalts examined by the Spirit rover in Gusev Crater, Mars: Evidence for multiple redox states in the martian interior. Earth and Planetary Science Letters 384:198–208.
[doi:10.1016/j.epsl.2013.10.005]
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Yan-Fei Jiang (姜燕飞)¹, James M. Stone¹ and Shane W. Davis²

¹Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
²Canadian Institute for Theoretical Astrophysics, Toronto, ON M5S3H4, Canada

We study the long-term thermal stability of radiation-dominated disks in which the vertical structure is determined self-consistently by the balance of heating due to the dissipation of MHD turbulence driven by magneto-rotational instability (MRI) and cooling due to radiation emitted at the photosphere. The calculations adopt the local shearing box approximation and utilize the recently developed radiation transfer module in the Athena MHD code based on a variable Eddington tensor rather than an assumed local closure. After saturation of the MRI, in many cases the disk maintains a steady vertical structure for many thermal times. However, in every case in which the box size in the horizontal directions are at least one pressure scale height, fluctuations associated with MRI turbulence and dynamo action in the disk eventually trigger a thermal runaway that causes the disk to either expand or contract until the calculation must be terminated. During runaway, the dependence of the heating and cooling rates on total pressure satisfy the simplest criterion for classical thermal instability. We identify several physical reasons why the thermal runaway observed in our simulations differ from the standard α disk model; for example, the advection of radiation contributes a non-negligible fraction to the vertical energy flux at the largest radiation pressure, most of the dissipation does not happen in the disk mid-plane, and the change of dissipation scale height with mid-plane pressure is slower than the change of density scale height. We discuss how and why our results differ from those published previously. Such thermal runaway behavior might have important implications for interpreting temporal variability in observed systems, but fully global simulations are required to study the saturated state before detailed predictions can be made.

Reference
Jiang Y-F, Stone JM and Davis SW (2013) On the Thermal Stability of Radiation-dominated Accretion Disks. The Astrophysical Journal 778:65.
[doi:10.1088/0004-637X/778/1/65]

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Philip J. Armitage^1,2, Jacob B. Simon¹ and Rebecca G. Martin^1,3

¹JILA, University of Colorado and NIST, 440 UCB, Boulder, CO 80309-0440, USA
²Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0391, USA
³Sagan Fellow.

Protoplanetary disks are likely to be threaded by a weak net flux of vertical magnetic field that is a remnant of the much larger fluxes present in molecular cloud cores. If this flux is approximately conserved its dynamical importance will increase as mass is accreted, initially by stimulating magnetorotational disk turbulence and subsequently by enabling wind angular momentum loss. We use fits to numerical simulations of ambipolar dominated disk turbulence to construct simplified one-dimensional evolution models for weakly magnetized protoplanetary disks. We show that the late onset of significant angular momentum loss in a wind can give rise to “two timescale” disk evolution in which a long phase of viscous evolution precedes rapid dispersal as the wind becomes dominant. The wide dispersion in disk lifetimes could therefore be due to varying initial levels of net flux. Magnetohydrodynamic (MHD) wind triggered dispersal differs from photoevaporative dispersal in predicting mass loss from small (<1 AU) scales, where thermal winds are suppressed. Our specific models are based on a limited set of simulations that remain uncertain, but qualitatively similar evolution appears likely if mass is lost from disks more quickly than flux, and if MHD winds become important as the plasma β decreases.

Reference
Armitage PJ, Simon JB and Martin RG (2013) Two Timescale Dispersal of Magnetized Protoplanetary Disks. The Astrophysical Journal – Letters 778:L14.
[doi:10.1088/2041-8205/778/1/L14]

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