Klaus Keil

Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI 96822, USA

Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO₂ (~36–39 wt.%), high in MgO (~27–30 wt.%) and notably high in FeO (~26–37 wt.%), and low in Al₂O₃(~0.2–2.5 wt.%) and combined alkalis Na₂O and K₂O (~0–0.7 wt.%). They consist mostly of olivine (~71–96 vol.%; ~Fo_64–73), major clinopyroxene (minor to ~15 vol.%; ~En_40–63Wo_36–48), with variable small amounts of plagioclase (0 to ~10 vol.%; ~An_15–33), and minor to trace amounts of orthopyroxene (none to ~20 vol.%; En_69–73Wo_2–4), Fe-sulfides (trace to ~7 vol.%), chromite (none to ~5 vol.%), phosphates (none to ~3 vol.%) and metallic Fe,Ni (trace to ~2 vol.%). Minerals tend to be homogeneous, and textures are medium to coarse-grained (~0.1–1.5 mm), with olivine commonly displaying triple junctions. Brachina has near-chondritic lithophile element abundances, whereas other brachinites show variable depletions in Al, Ca, Rb, K, Na, and LREE. Siderophile element abundance patterns vary and range from ~0.01 to ~0.9 CI. Oxygen isotope composition (Δ¹⁷O) ranges from ~−0.09 to −0.39‰, with the mean = −0.23 ± 0.14‰. Brachinites are ancient rocks, as was recognized early by the detection, in some brachinites, of excess ¹²⁹Xe from the decay of short-lived ¹²⁹I (half-life 17 Ma) and of fission tracks from the decay of ²⁴⁴Pu (half-life 82 Ma) in phosphate, high-Ca clinopyroxene and olivine. The first precise crystallization age was determined for Brachina using ⁵³Mn–⁵³Cr systematics, relative to the Pb–Pb age of the angrite LEW 86010, and yielded an age of 4563.7 ± 0.9 Ma. Thus, Brachina is at most ~4 Ma younger that the CAIs whose age is 4567.2 ± 0.6 Ma. There is no consensus on the origin of brachinites, but they most likely are primitive achondrites, i.e., ultra-mafic residues from various low degrees of partial melting. Partial melting experiments suggest that they possibly formed from a parent lithology chemically similar but not identical to the Rumuruti (R) chondrites, although the different oxygen isotopic compositions of the R chondrites and the brachinites put a serious constraint on this hypothesis. The apparent lack of abundant rocks representing the partial melts suggests that brachinites may have formed on a parent body <~100 km in radius, where early partial melts were removed from the parent body by explosive pyroclastic volcanism. Graves Nunataks 06128 and 06129 are felsic, andesitic basalts which have properties that suggest a relationship to brachinites and thus, may be samples of the elusive partial melts.

Reference
Keil K (in press) Brachinite meteorites: Partial melt residues from an FeO-rich asteroid. Chemie der Erde
[doi:10.1016/j.chemer.2014.02.001]
Copyright Elsevier

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Sebastiano Padovan¹, Jean-Luc Margot^1,2, Steven A. Hauck II³, William B. Moore^4,5 and Sean C. Solomon^6,7

¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Department of Physics and Astronomy, University of California, Los Angeles, California, USA
³Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, Ohio, USA
⁴Department of Atmospheric and Planetary Sciences, Hampton University, Hampton, Virginia, USA
⁵National Institute of Aerospace, Hampton, Virginia, USA
⁶Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA
⁷Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, District of Columbia, USA

The combination of the radio tracking of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft and Earth-based radar measurements of the planet’s spin state gives three fundamental quantities for the determination of the interior structure of Mercury: mean density ρ, moment of inertia C, and moment of inertia of the outer solid shell C_m. This work focuses on the additional information that can be gained by a determination of the change in gravitational potential due to planetary tides, as parameterized by the tidal potential Love number k₂. We investigate the tidal response for sets of interior models that are compatible with the available constraints (ρ, C, and C_m). We show that the tidal response correlates with the size of the liquid core and the mean density of material below the outer solid shell and that it is affected by the rheology of the outer solid shell of the planet, which depends on its temperature and mineralogy. For a mantle grain size of 1 cm, we calculate that the tidal k₂ of Mercury is in the range 0.45 to 0.52. Some of the current models for the interior structure of Mercury are compatible with the existence of a solid FeS layer at the top of the core. Such a layer, if present, would increase the tidal response of the planet.

Reference
Padovan S, Jean-Luc Margot J-L, Hauck II SA, Moore WB and Sean C. Solomon SC (in press) The tides of Mercury and possible implications for its interior structure. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004459]
Published by arrangement with John Wiley & Sons

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L.J. Hicks^a, J.C. Bridges^a and S.J. Gurman^b

^aSpace Research Centre, Dept. of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK.
^bDept. of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK.

Transmission Electron Microscopy and Fe-K X-ray Absorption Spectroscopy have been used to determine structure and ferric content of the secondary phase mineral assemblages in the nakhlite martian meteorites, NWA 998, Lafayette, Nakhla, GV, Y 000593, Y 000749, MIL 03346, NWA 817, and NWA 5790. The secondary phases are a rapidly cooled, metastable assemblage that has preserved Mg# and Ca fractionation related to distance from the fluid source, for most of the nakhlites, though one, NWA 5790, appears not to have experienced a fluid pathway. All nine nakhlite samples have also been analysed with scanning electron microscopy, electron probe micro analysis, Bright Field high-resolution transmission electron microscopy (HRTEM) and Selected Area Electron Diffraction. By measuring the energy position of the Fe-K XANES 1s→3d pre-edge transition centroid we calculate the ferric content of the minerals within the nakhlite meteorites. The crystalline phyllosilicates and amorphous silicate of the hydrothermal deposits filling the olivine fractures are found to have variable Fe³⁺/ΣFe values ranging from 0.4 to 0.9. In Lafayette, the central silicate gel parts of the veins are more ferric than the phyllosilicates around it, showing that the fluid became increasingly oxidised. The mesostasis of Lafayette and NWA 817 also have phyllosilicate, which have a higher ferric content than the olivine fracture deposits, with Fe³⁺/ΣFe values of up to 1.0. Further study, via TEM analyses, reveal the Lafayette and NWA 817 olivine phyllosilicates to have 2:1 T-O-T lattice structure with a the d₀₀₁-spacing of 0.96 nm, whereas the Lafayette mesostasis phyllosilicates have 1:1 T-O structure with d₀₀₁-spacings of 0.7 nm. Based on our analyses, the phyllosilicate found within the Lafayette olivine fractures is trioctahedral ferric saponite (Ca_0.2K_0.1)_∑0.3(Mg_2.6Fe²⁺_1.3Fe³⁺_1.7Mn_0.1)_∑5.7[(Si_6.7Al^IV_0.9Fe³⁺_0.4)_∑8.0O₂₀](OH)₄.nH₂O, and that found in the mesostasis fractures is an Fe-serpentine (Ca_0.1Mg_0.7Fe³⁺_1.0Al^VI_0.4)_∑2.2[Si₂O₅]OH₄, with a ferric gel of similar composition in Lafayette and found as fracture fills throughout the other nakhlites.

Reference
Hicks LJ, Bridges JC and Gurman SJ (in press) Ferric saponite and serpentine in the nakhlite martian meteorites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.010]
Copyright Elsevier

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M. de Val-Borro^1,2 et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

Context. Cometary activity at large heliocentric distances is thought to be driven by outgassing of molecular species more volatile than water that are present in the nucleus. The long-period comet C/2006 W3 (Christensen) was an exceptional target for a detailed study of its distant gaseous and dust activity.
Aims. We aimed to measure the H₂O and dust production rates in C/2006 W3 (Christensen) with the Herschel Space Observatory at a heliocentric distance of ~5 AU and compared these data with previous post-perihelion Herschel and ground-based observations at ~3.3 AU from the Sun.
Methods. We have searched for emission in the H_O and NH₃ ground-state rotational transitions, J_KaKc (1₁₀–1₀₁) at 557 GHz and J_K(1₀–0₀) at 572 GHz, simultaneously, toward comet C/2006 W3 (Christensen) with the Heterodyne Instrument for the Far Infrared (HIFI) onboard Herschel on UT 1.5 September 2010. Photometric observations of the dust coma in the 70 μm and 160 μm channels were acquired with the Photodetector Array Camera and Spectrometer (PACS) instrument on UT 26.5 August 2010.
Results. A tentative 4σ H₂O line emission feature was found in the spectra obtained with the HIFI wide-band and high-resolution spectrometers, from which we derive a water production rate of (2.0 ± 0.5) × 10²⁷ molec s^-1. A 3σ upper limit for the ammonia production rate of <1.5 × 10²⁷ molec s^-1 is obtained taking into account the contribution from all hyperfine components. The dust thermal emission was detected in the 70 μm and 160 μm filters, with a more extended emission in the blue channel. We fit the radial dependence of the surface brightness with radially symmetric profiles for the blue and red bands. The dust production rates, obtained for a dust size distribution index that explains the fluxes at the photocenters of the 70 μm and 160 μm PACS images, lie in the range 70 kg s^-1 to 110 kg s^-1. Scaling the CO production rate measured post-perihelion at 3.20 AU and 3.32 AU, these values correspond to a dust-to-gas production rate ratio in the range 0.3–0.4.
Conclusions. The blueshift of the water line detected by HIFI suggests preferential emission from the subsolar point. However, it is also possible that water sublimation occurs in small ice-bearing grains that are emitted from an active region on the nucleus surface at a speed of ~0.2 km s^-1. The dust production rates derived in August 2010 are roughly one order of magnitude lower than in September 2009, suggesting that the dust-to-gas production rate ratio remained approximately constant during the period when the activity became increasingly dominated by CO outgassing.

Reference
Val-Borro et al. (2014) Herschel observations of gas and dust in comet C/2006 W3 (Christensen) at 5 AU from the Sun.  Astronomy & Astrophysics 564:A124.
[doi:10.1051/0004-6361/201423427]
Reproduced with permission © ESO

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Kazushige Tomeoka and Ichiro Ohnishi¹

Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan
¹Present address: EM Business Unit, JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan.

Fine-grained rims surrounding chondrules and inclusions in the Mokoia CV3 carbonaceous chondrite can be divided into phyllosilicate-rich and olivine-rich types. We present a petrographic and electron microscopic study of the olivine-rich rims and their host objects (referred to as chondrules/olivine-rich rims). The olivine-rich rims consist mainly of Fe-rich olivine and very minor phyllosilicate (saponite). Their host chondrules contain minor saponite and phlogopite, which resulted from aqueous alteration of anhydrous silicates. Mineralogical and compositional characteristics of the chondrules/olivine-rich rims suggest that they experienced mild thermal metamorphic effects. The rims commonly contain veins of coarse-grained Fe-rich olivine, magnetite, and Fe−(Ni) sulfides.
The chondrules show abundant evidence of alteration along their peripheries, and the alteration textures suggest a mechanism for rim formation by replacement of the chondrules. Initially, enstatite and opaque nodules preferentially reacted to form coarse, platy, Fe-rich olivine crystals, which were subsequently divided into finer grains. Forsterite was also replaced by Fe-rich olivine. As the alteration advanced, these Fe-rich olivines were disaggregated, mixed with simultaneously produced saponite, and formed rims. In contrast, the surrounding matrix shows no evidence of such alteration and metamorphism. These observations indicate that the chondrules/olivine-rich rims did not experience these secondary processes in their present setting.
The results suggest that the chondrules/olivine-rich rims experienced extensive replacement reactions in an environment in which aqueous fluids existed but only in minor amounts. They have probably also undergone simultaneous and/or subsequent mild thermal metamorphism. We suggest that the chondrules/olivine-rich rims are actually clasts transported from a relatively dry region in the parent body that was different from the region where Mokoia was finally lithified.

Reference
Tomeoka K and Ohnishi I (in press) Olivine-rich rims surrounding chondrules in the Mokoia CV3 carbonaceous chondrite: Further evidence for parent-body processes. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.004]
Copyright Elsevier

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Richard G. Arendt^1,2, Eli Dwek², Gladys Kober^2,3, Jeonghee Rho^4,5 and Una Hwang^6,7

¹CRESST, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
²NASA Goddard Space Flight Center, Code 665, Greenbelt, MD 20771, USA
³Department of Physics, IACS, Catholic University of America, Washington, DC 20064, USA
⁴SETI Institute, 189 Bernardo Avenue, Mountain View, CA 94043, USA
⁵SOFIA Science Center, NASA Ames Research Center, MS 211-3, Moffett Field, CA 94035, USA
⁶NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
⁷The Henry A. Rowland Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA

Infrared continuum observations provide a means of investigating the physical composition of the dust in the ejecta and swept up medium of the Cas A supernova remnant (SNR). Using low-resolution Spitzer IRS spectra (5-35 μm), and broad-band Herschel PACS imaging (70, 100, and 160 μm), we identify characteristic dust spectra, associated with ejecta layers that underwent distinct nuclear burning histories. The most luminous spectrum exhibits strong emission features at ~9 and 21 μm and is closely associated with ejecta knots with strong Ar emission lines. The dust features can be reproduced by magnesium silicate grains with relatively low Mg to Si ratios. Another dust spectrum is associated with ejecta having strong Ne emission lines. It has no indication of any silicate features and is best fit by Al₂O₃ dust. A third characteristic dust spectrum shows features that are best matched by magnesium silicates with a relatively high Mg to Si ratio. This dust is primarily associated with the X-ray-emitting shocked ejecta, but it is also evident in regions where shocked interstellar or circumstellar material is expected. However, the identification of dust composition is not unique, and each spectrum includes an additional featureless dust component of unknown composition. Colder dust of indeterminate composition is associated with emission from the interior of the SNR, where the reverse shock has not yet swept up and heated the ejecta. Most of the dust mass in Cas A is associated with this unidentified cold component, which is  0.1 M_☉. The mass of warmer dust is only ~0.04 M_☉.

Reference
Arendt RG, Dwek E, Kober G, Rho J and Hwang U (2014) Interstellar and Ejecta Dust in the Cas A Supernova Remnant. The Astrophysical Journal 786:55.
[doi:10.1088/0004-637X/786/1/55]

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Nan Liu¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA

We present barium, carbon, and silicon isotopic compositions of 38 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing barium contamination. Strong depletions in δ(¹³⁸Ba/¹³⁶Ba) values are found, down to –400‰, which can only be modeled with a flatter ¹³C profile within the ¹³C pocket than is normally used. The dependence of δ(¹³⁸Ba/¹³⁶Ba) predictions on the distribution of ¹³C within the pocket in asymptotic giant branch (AGB) models allows us to probe the ¹³C profile within the ¹³C pocket and the pocket mass in AGB stars. In addition, we provide constraints on the ²²Ne (α, n) ²⁵Mg rate in the stellar temperature regime relevant to AGB stars, based on δ(¹³⁴Ba/¹³⁶Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ(¹³⁴Ba/¹³⁶Ba) values that cannot be explained by any of the current AGB model calculations. Instead, such negative values are consistent with the intermediate neutron capture process (i process), which is activated by the very late thermal pulse during the post-AGB phase and characterized by a neutron density much higher than the s process. These two grains may have condensed around post-AGB stars. Finally, we report abundances of two p-process isotopes, ¹³⁰Ba and ¹³²Ba, in single SiC grains. These isotopes are destroyed in the s process in AGB stars. By comparing their abundances with respect to that of ¹³⁵Ba, we conclude that there is no measurable decay of ¹³⁵Cs (t 1/2 = 2.3 Ma) to ¹³⁵Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before ¹³⁵Cs decayed.

Reference
Liu et al. (2014) Barium Isotopic Composition of Mainstream Silicon Carbides from Murchison: Constraints for s-process Nucleosynthesis in Asymptotic Giant Branch Stars. The Astrophysical Journal 786:66.
[doi:10.1088/0004-637X/786/1/48]

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D. Bodewits¹, T. L. Farnham¹, M. F. A’Hearn¹, L. M. Feaga¹, A. McKay^2,3, D. G. Schleicher⁴ and J. M. Sunshine¹

¹Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
²Astronomy Department, New Mexico State University, Las Cruces, NM 88003, USA
³University of Texas Austin/McDonald Observatory, 1 University Station, Austin, TX 78712, USA
⁴Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA

We used the Ultraviolet-Optical Telescope on board Swift to observe the dynamically young comet C/2009 P1 (Garradd) from a heliocentric distance of 3.5 AU pre-perihelion until 4.0 AU outbound. At 3.5 AU pre-perihelion, comet Garradd had one of the highest dust-to-gas ratios ever observed, matched only by comet Hale-Bopp. The evolving morphology of the dust in its coma suggests an outburst that ended around 2.2 AU pre-perihelion. Comparing slit-based measurements and observations acquired with larger fields of view indicated that between 3 AU and 2 AU pre-perihelion a significant extended source started producing water in the coma. We demonstrate that this source, which could be due to icy grains, disappeared quickly around perihelion. Water production by the nucleus may be attributed to a constantly active source of at least 75 km², estimated to be >20% of the surface. Based on our measurements, the comet lost 4 × 10¹¹ kg of ice and dust during this apparition, corresponding to at most a few meters of its surface. Even though this was likely not the comet’s first passage through the inner solar system, the activity of Garradd was complex and changed significantly during the time it was observed.

Reference
Bodewits D, Farnham TL, A’Hearn MF, Feaga LM, McKay A, Schleicher DG and Sunshine JM (2014) The Evolving Activity of the Dynamically Young Comet C/2009 P1 (Garradd). The Astrophysical Journal 786:48.
[doi:10.1088/0004-637X/786/1/48]

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Yudhijit Bhattacharjee

Astronomers have discovered an Earth-sized planet in the habitable zone of a red dwarf—a star cooler than the sun—500 light-years away. Because such stars make up three-quarters of all stars in the Milky Way, the finding could open a wide new hunting ground for extraterrestrial life.

Reference
Bhattacharjee Y (2014) Almost-Earth Tantalizes Astronomers With Promise of Worlds to Come. Science 344:249.
[doi:10.1126/science.344.6181.249]
Reprinted with permission from AAAS

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Elisa V. Quintana¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043, USA.
²NASA Ames Research Center, Moffett Field, CA 94035, USA.

The quest for Earth-like planets is a major focus of current exoplanet research. Although planets that are Earth-sized and smaller have been detected, these planets reside in orbits that are too close to their host star to allow liquid water on their surfaces. We present the detection of Kepler-186f, a 1.11 ± 0.14 Earth-radius planet that is the outermost of five planets, all roughly Earth-sized, that transit a 0.47 ± 0.05 solar-radius star. The intensity and spectrum of the star’s radiation place Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form.

Reference
Quintana et al. (2014) An Earth-Sized Planet in the Habitable Zone of a Cool Star. Science 344:277.
[doi:10.1126/science.1249403]
Reprinted with permission from AAAS

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