Richard A. Kerr

The simple organic compounds discovered by the Curiosity Mars rover either came with the tons of never-alive cosmic debris that sifts onto every planetary body or are something far more exciting: remains of martian life from eons ago, when a habitable lake graced the rover’s landing site. Researchers could learn more about the complex organic molecules that yielded these first finds if Curiosity’s drilling strikes a richer vein of organics in the coming months or if a more sophisticated analytical technique is employed.

Reference
Kerr RA (2014) Search for Martian Life Clears Another Hurdle. Science 343:1419.
[doi:10.1126/science.343.6178.1419]
Reprinted with permission from AAAS

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Eleanor R. Mare¹, Andrew G. Tomkins¹ and Belinda M. Godel²

¹School of Geosciences, Monash University, Melbourne, Australia
²CSIRO Earth Science and Resource Engineering, Australian Resources Research Centre, Kensington, Western Australia, Australia

Ordinary chondrite meteorites contain silicates, Fe,Ni-metal grains, and troilite (FeS). Conjoined metal-troilite grains would be the first phase to melt during radiogenic heating in the parent body, if temperatures reached over approximately 910–960 °C (the Fe,Ni-FeS eutectic). On the basis of two-pyroxene thermometry of 13 ordinary chondrites, we argue that peak temperatures in some type 6 chondrites exceeded the Fe,Ni-FeS eutectic and thus conjoined metal-troilite grains would have begun to melt. Melting reactions consume energy, so thermal models were constructed to investigate the effect of melting on the thermal history of the H, L, and LL parent asteroids. We constrained the models by finding the proportions of conjoined metal-troilite grains in ordinary chondrites using high-resolution X-ray computed tomography. The models show that metal-troilite melting causes thermal buffering and inhibits the onset of silicate melting. Compared with models that ignore the effect of melting, our models predict longer cooling histories for the asteroids and accretion times that are earlier by 61, 124, or 113 kyr for the H, L, and LL asteroids, respectively. Because the Ni/Fe ratio of the metal and the bulk troilite/metal ratio is higher in L and LL chondrites than H chondrites, thermal buffering has the greatest effect in models for the L and LL chondrite parent bodies, and least effect for the H chondrite parent. Metal-troilite melting is also relevant to models of primitive achondrite parent bodies, particularly those that underwent only low degrees of silicate partial melting. Thermal models can predict proportions of petrologic types formed within an asteroid, but are systematically different from the statistics of meteorite collections. A sampling bias is interpreted to explain these differences.

Reference
Mare ER, Tomkins AG and Godel BM (in press) Restriction of parent body heating by metal-troilite melting: Thermal models for the ordinary chondrites. Meteoritics & Planetary Science
[doi:10.1111/maps.12280]
Published by arrangement with John Wiley & Sons

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Alan W. Harris and Line Drube

German Aerospace Center (DLR) Institute of Planetary Research, Rutherfordstrasse 2, D-12489 Berlin, Germany

The metal content of asteroids is of great interest, not only for theories of their origins and the evolution of the solar system but, in the case of near-Earth objects (NEOs), also for impact mitigation planning and endeavors in the field of planetary resources. However, since the reflection spectra of metallic asteroids are largely featureless, it is difficult to identify them and relatively few are known. We show how data from the Wide-field Infrared Survey Explorer (WISE)/NEOWISE thermal-infrared survey and similar surveys, fitted with a simple thermal model, can reveal objects likely to be metal rich. We provide a list of candidate metal-rich NEOs. Our results imply that future infrared surveys with the appropriate instrumentation could discover many more metal-rich asteroids, providing valuable data for assessment of the impact hazard and the potential of NEOs as reservoirs of vital materials for future interplanetary space activities and, eventually perhaps, for use on Earth.

Reference
Harris AW and Drube L (2014) How to find metal-rich asteroids. The Astrophysical Journal – Letters 785:L4.
[doi:10.1088/2041-8205/785/1/L4]

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W. D. Pesnell¹ and P. Bryans²

¹NASA Goddard Space Flight Center, Code 671, Greenbelt, MD 20771, USA
²ADNET Systems Inc., NASA Goddard Space Flight Center, Code 671, Greenbelt, MD 20771, USA

Recent improvements in solar observations have greatly progressed the study of sungrazing comets. They can now be imaged along the entirety of their perihelion passage through the solar atmosphere, revealing details of their composition and structure not measurable through previous observations in the less volatile region of the orbit further from the solar surface. Such comets are also unique probes of the solar atmosphere. The debris deposited by sungrazers is rapidly ionized and subsequently influenced by the ambient magnetic field. Measuring the spectral signature of the deposited material highlights the topology of the magnetic field and can reveal plasma parameters such as the electron temperature and density. Recovering these variables from the observable data requires a model of the interaction of the cometary species with the atmosphere through which they pass. The present paper offers such a model by considering the time-dependent chemistry of sublimated cometary species as they interact with the solar radiation field and coronal plasma. We expand on a previous simplified model by considering the fully time-dependent solutions of the emitting species’ densities. To compare with observations, we consider a spherically symmetric expansion of the sublimated material into the corona and convert the time-dependent ion densities to radial profiles. Using emissivities from the CHIANTI database and plasma parameters derived from a magnetohydrodynamic simulation leads to a spatially dependent emission spectrum that can be directly compared with observations. We find our simulated spectra to be consistent with observation.

Reference
Pesnell WD and Bryans P (2014) The Time-dependent Chemistry of Cometary Debris in the Solar Corona. The Astrophysical Journal 785:50.
[doi:10.1088/0004-637X/785/1/50]

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Joshua F. Snape^1,2,3, Katherine H. Joy^2,4, Ian A. Crawford^2,5 and Louise Alexander^2,5

¹Department of Physical Sciences, Open University, Milton Keynes, UK
²The Centre for Planetary Sciences at UCL-Birkbeck, London, UK
³Department of Earth Sciences, University College London, London, UK
⁴School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
⁵Department of Earth and Planetary Sciences, Birkbeck College, University of London, London, UK

A detailed petrologic survey has been made of 17 basaltic chips (sized between 1 and 10 mm) from the 12003 soil sample as part of an ongoing study of basaltic diversity at the Apollo 12 landing site. An attempt has been made to classify these samples according to the well-established grouping of olivine, pigeonite, ilmenite, and feldspathic basalts. Particular attention has been paid to variations in major, minor, and trace element mineral chemistry (determined by electron microprobe analysis and laser ablation ICP-MS), which may be indicative of particular basaltic suites and less susceptible to sampling bias than bulk sample characteristics. Examples of all three main (olivine, pigeonite, and ilmenite) basaltic suites have been identified within the 12003 soil. One sample is identified as a possible new addition to the feldspathic suite, which currently consists of only one other confirmed sample. Identification of additional feldspathic basalts strengthens the argument that they represent a poorly sampled basaltic flow local to the Apollo 12 site, rather than exotic material introduced to the site by impact mixing processes. Three samples are identified as representing members of one or two previously unrecognized basaltic suites.

Reference
Snape JF, Joy KH, Crawford IA and Alexander L (in press) Basaltic diversity at the Apollo 12 landing site: Inferences from petrologic examinations of the soil sample 12003. Meteoritics & Planetary Science
[doi:10.1111/maps.12285]
Published by arrangement with John Wiley & Sons

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Stanley G. Love¹, Donald R. Pettit¹ and Scott R. Messenger²

¹NASA Lyndon B. Johnson Space Center, Houston, Texas, USA
²Robert M. Walker Laboratory for Space Science, Astromaterials Research and Exploration Science Directorate, NASA Lyndon B. Johnson Space Center, Houston, Texas, USA

We conducted experiments in space to investigate the aggregation of millimeter- and submillimeter-sized particles in microgravity, an important early step in planet formation. Particulate materials included salt (NaCl), sugar (sucrose), coffee, mica, ice, Bjurböle chondrules, ordinary and carbonaceous chondrite meteorite fragments, and acrylic and glass beads, all triply confined in clear plastic containers. Angular submillimeter particles rapidly and spontaneously formed clusters strong enough to survive turbulence in a protoplanetary nebula. Smaller particles generally aggregated more strongly and quickly than larger ones. We observed only a weak dependence of aggregation time on particle number density. We observed no strong dependence on composition. Round, smooth particles aggregated weakly or not at all. In a mixture of particle types, some phases aggregated more readily than others, creating selection effects that controlled the composition of the growing clumps. The physical process of aggregation appears to be electrostatic in nature.

Reference
Love SG, Pettit DR and Messenger SR (in press) Particle aggregation in microgravity: Informal experiments on the International Space Station. Meteoritics & Planetary Science
[doi:10.1111/maps.12286]
Published by arrangement with John Wiley & Sons

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Monique C. Aller¹, Varsha P. Kulkarni¹, Donald G. York², Daniel E. Welty², Giovanni Vladilo³ and Nicholas Liger¹

¹Department of Physics and Astronomy, University of South Carolina, 712 Main Street, Columbia, SC 29208, USA
²Department of Astronomy & Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
³Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34143 Trieste, Italy

We report the detection of interstellar silicate dust in the z_abs = 0.685 absorber along the sightline toward the gravitationally lensed blazar TXS 0218+357. Using Spitzer Space Telescope Infrared Spectrograph data, we detect the 10 μm silicate absorption feature with a detection significance of 10.7σ. We fit laboratory-derived silicate dust profile templates obtained from the literature to the observed 10 μm absorption feature and find that the best single-mineral fit is obtained using an amorphous olivine template with a measured peak optical depth of τ₁₀ = 0.49 ± 0.02, which rises to τ₁₀ ~ 0.67 ± 0.04 if the covering factor is taken into account. We also detected the 18 μm silicate absorption feature in our data with a >3σ significance. Due to the proximity of the 18 μm absorption feature to the edge of our covered spectral range, and associated uncertainty about the shape of the quasar continuum normalization near 18 μm, we do not independently fit this feature. We find, however, that the shape and depth of the 18 μm silicate absorption are well matched to the amorphous olivine template prediction, given the optical depth inferred for the 10 μm feature. The measured 10 μm peak optical depth in this absorber is significantly higher than those found in previously studied quasar absorption systems. However, the reddening, 21 cm absorption, and velocity spread of Mg II are not outliers relative to other studied absorption systems. This high optical depth may be evidence for variations in dust grain properties in the interstellar medium between this and the previously studied high redshift galaxies.

Reference
Aller MC, Kulkarni VP, York DG, Welty DE, Vladilo G and Liger N (2014) Interstellar Silicate Dust in the z = 0.685 Absorber Toward TXS 0218+357. The Astrophysical Journal 785:36.
[doi:10.1088/0004-637X/785/1/36]

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Megan E. Schwamb

Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan.

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

Reference
Schwamb ME (2014) Solar System: Stranded in no-man’s-land. Nature 507:435–436.
[doi:10.1038/507435a]

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Chadwick A. Trujillo¹ and Scott S. Sheppard²

¹Gemini Observatory, 670 North A‘ohoku Place, Hilo, Hawaii 96720, USA
²Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington DC 20015, USA

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

Reference
Trujillo CA and Sheppard SS (2014) A Sedna-like body with a perihelion of 80 astronomical units. Nature 507:471–474.
[doi:10.1038/nature13156]

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Aaron S. Burton¹, Daniel P. Glavin², Jamie E. Elsila², Jason P. Dworkin², Peter Jenniskens^3,4 and Qing-Zhu Yin⁵

¹NASA Johnson Space Center, Houston, Texas, USA
²NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
³SETI Institute, Mountain View, California, USA
⁴NASA Ames Research Center, Moffett Field, California, USA
⁵Department of Earth and Planetary Sciences, University of California at Davis, Davis, California, USA

We determined the abundances and enantiomeric compositions of amino acids in Sutter’s Mill fragment #2 (designated SM2) recovered prior to heavy rains that fell April 25–26, 2012, and two other meteorite fragments, SM12 and SM51, that were recovered postrain. We also determined the abundance, enantiomeric, and isotopic compositions of amino acids in soil from the recovery site of fragment SM51. The three meteorite stones experienced terrestrial amino acid contamination, as evidenced by the low D/L ratios of several proteinogenic amino acids. The D/L ratios were higher in SM2 than in SM12 and SM51, consistent with rain introducing additional l-amino acid contaminants to SM12 and SM51. Higher percentages of glycine, β-alanine, and γ-amino-n-butyric acid were observed in free form in SM2 and SM51 compared with the soil, suggesting that these free amino acids may be indigenous. Trace levels of D+L-β-aminoisobutyric acid (β-AIB) observed in all three meteorites are not easily explained as terrestrial contamination, as β-AIB is rare on Earth and was not detected in the soil. Bulk carbon and nitrogen and isotopic ratios of the SM samples and the soil also indicate terrestrial contamination, as does compound-specific isotopic analysis of the amino acids in the soil. The amino acid abundances in SM2, the most pristine SM meteorite analyzed here, are approximately 20-fold lower than in the Murchison CM2 carbonaceous chondrite. This may be due to thermal metamorphism in the Sutter’s Mill parent body at temperatures greater than observed for other aqueously altered CM2 meteorites.

Reference
Burton AS, Glavin DP, Elsila JE, Dworkin JP, Jenniskens P and Yin Q-Z (in press) The amino acid composition of the Sutter’s Mill CM2 carbonaceous chondrite. Meteoritics & Planetary Science
[doi:10.1111/maps.12281]
Published by arrangement with John Wiley & Sons

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