F. Braga-Ribas et al. (>10)*
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Reference
Braga-Ribas et al. (2014) A ring system detected around the Centaur (10199) Chariklo. Nature 508:72
[doi:10.1038/nature13155]

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John Chambers

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Reference
Chambers J (2014) Planetary science: A chronometer for Earth’s age. Nature 508:51.
[doi:10.1038/508051a]

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Seth A. Jacobson, Alessandro Morbidelli, Sean N. Raymond, David P. O’Brien, Kevin J. Walsh and David C. Rubie

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

Reference
Jacobson SA, Morbidelli A, Raymond SN, O’Brien DP, Walsh KJ and Rubie DC (2014) Highly siderophile elements in Earth’s mantle as a clock for the Moon-forming impact. Nature 508:84.
[doi:10.1038/nature13172]

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Alastair W. Tait^a, Andrew G. Tomkins^a, Bélinda M. Godel^b, Siobhan A. Wilson^a and Pavlina Hasalova^a,c

^aSchool of Geosciences, Monash University, Melbourne, Victoria 3800, Australia
^bCSIRO Earth Science and Resource Engineering, Australian Resources Research Centre, 26 Dick Perry Ave., Kensington, Western Australia 6151, Australia
^cCurrent Address: Česká Geologická Služba, 118 21 Praha 1, Czech Republic

Despite the fact that the number of officially classified meteorites is now over 45,000, we lack a clearly defined sequence of samples from a single parent body that records the entire range in metamorphic temperatures from pristine primitive meteorites up to the temperatures required for extensive silicate partial melting. Here, we conduct a detailed analysis of Watson 012, an H7 ordinary chondrite, to generate some clarity on the textural and chemical changes associated with equilibrium-based silicate partial melting in chondritic meteorites. To do this we compare the textures in the meteorite with those preserved in metamorphic contact aureoles on Earth. The most distinctive texture generated by the partial melting that affected Watson 012 is an extensively interconnected plagioclase network, which is clearly observable with a petrographic microscope. Enlarged metal-troilite grains are encapsulated at widenings in this plagioclase network, and this is clearly visible in reflected light. Together with these features, we define a series of other characteristics that can be used to more clearly classify chondritic meteorites as being of petrologic Type 7. To provide comprehensive evidence of silicate partial melting and strengthen the case for using simple petrographic observations to classify similar meteorites, we use high-resolution X-ray computed tomography to demonstrate that the plagioclase network has a high degree of interconnectedness and crystallised as large (cm-scale) skeletal crystals within an olivine-orthopyroxene-clinopyroxene framework, essentially pseudomorphing a melt network. Back-scattered electron imaging and element mapping are used to show that some of the clino- and orthopyroxene in Watson 012 also crystallised from silicate melt, and the order of crystallisation was orthopyroxene clinopyroxene plagioclase. X-ray diffraction data, supported by bulk geochemistry, are used to show that plagioclase and ortho- and clinopyroxene were added to the Watson 012 sample by through-flowing basaltic melt. Along with the absence of glass and granophyre, this interconnected network of coarse-grained skeletal plagioclase indicates that the sample cooled slowly at depth within the parent body. The evidence of melt flux indicates that Watson 012 formed in the presence of a gravitational gradient, and thus at significant distance from the centre of the H chondrite parent body (the gravitational gradient at the centre would be zero). Our interpretation is that incipient silicate partial melting in Watson 012 occurred when a region of radiogenically heated H6 material located at considerable depth (possibly at ~15-20 km from surface) was heated by an additional ca. 200-300°C in association with a large shock event. Due to insulation at depth within an already hot parent body, the post-shock temperature equilibrated and remained above the solidus long enough for widespread equilibrium-based silicate partial melting, and for melt to migrate. Although the observed melting may have been facilitated by additional heating from an impact event, this is not an example of instantaneous shock melting, which produces thermal disequilibrium at short length scales and distinctly different textures. A small number of H, L and LL chondrites have been previously classified as being of petrologic Type 7; with our new criteria to support that classification, these represent our best opportunity to explore the transition from high temperature sub-solidus metamorphism through the onset of silicate partial melting in three different parent bodies.

Reference
Tait AW, Tomkins AG, Godel BM, Wilson SA and Hasalova P (in press) Investigation of the H7 ordinary chondrite, Watson 012: Implications for recognition and classification of Type 7 meteorites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.039]
Copyright Elsevier

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Gwendolyn D. Bart

Univ. of Idaho, Dept. of Physics, 875 Perimeter Drive MS 0903, Moscow, ID, 83844, USA

Small impact craters (~10-300 m) that encounter a strength transition in the target (like a regolith over bedrock) have unique morphologies. Previous studies have used these morphologies as indicators of regolith depth. This paper reports on several new analyses that expand our understanding of the quantitative relationship between small crater morphology and target layering. I describe three practical situations where the application of the updated method is ambiguous because the specific relationship between the target layering and the crater morphology has never been analyzed. In order to resolve the ambiguity, I report on new analyses of computer models and lunar data that demonstrate how the dimensions of the crater shape relate to layer depth. I also analyze the boundary conditions under which the crater-layering relationship will enable determination of layering depth. Finally, in light of the greater understanding of the crater-layering relationship, I discuss the possible application of this method to Mars.

Reference
Bart GD (in press) The Quantitative Relationship Between Small Impact Crater Morphology and Regolith Depth. Icarus
[doi:10.1016/j.icarus.2014.03.020]
Copyright Elsevier

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J.-H. Wang and R. Brasser

Institute for Astronomy and Astrophysics, Academia Sinica; 11F AS/NTU building, 1 Roosevelt Rd., Sec. 4, 10617 Taipei, Taiwan

The origin of the Halley-type comets (HTCs) is one of the last mysteries of the dynamical evolution of the solar system. Prior investigation into their origin has focused on two source regions: the Oort Cloud and the scattered disc. From the former it has been difficult to reproduce the non-isotropic, prograde skew in the inclination distribution of the observed HTCs without invoking a multi-component Oort Cloud model and specific fading of the comets. The scattered disc origin fares better, but needs an order of magnitude more mass than is consistent with theory and observations. Here we revisit the Oort Cloud origin and include cometary fading. Our observational sample stems from the JPL catalogue. We only keep comets discovered and observed after 1950, but place no a priori restriction on the maximum perihelion distance of observational completeness. We then numerically evolve half a million comets from the Oort Cloud through the realm of the giant planets and keep track of their number of perihelion passages with perihelion distance q < 2.5 AU, below which the activity is supposed to increase considerably. We can simultaneously fit the HTC inclination and semi-major axis distribution very well with a power-law fading function of the form m^-k, where m is the number of perihelion passages with q < 2.5 AU and k is the fading index. We match both the inclination and semi-major axis distributions when k ~ 1 and the maximum imposed perihelion distance of the observed sample is q_m ~ 1.8 AU. The value of k is higher than the one obtained for the long-period comets (LPCs), for which typically k ~ 0.7. This increase in k is most likely the result of cometary surface processes. We argue the HTC sample is now most likely complete for q_m < 1.8 AU. We calculate that the steady-state number of active HTCs with diameter D > 2.3 km and q < 1.8 AU is on the order of 100.

Reference
Wang J-H and Brasser R (2014) An Oort Cloud origin of the Halley-type comets. Astronomy & Astrophysics 563:A122.
[doi:10.1051/0004-6361/201322508]
Reproduced with permission © ESO

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Stephen M. Seddio^a,b, Bradley L. Jolliff^a, Randy L. Korotev^a and Paul K. Carpenter^a

^aDepartment of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, 1 Brookings Dr., St. Louis, Missouri 63130, United States of America
^bThermo Fisher Scientific, Inc., 5225 Verona Rd., Fitchburg, WI 53711, United States of America

We present the first quantitative compositional analysis of thorite in a lunar sample. The sample, a granitic assemblage, also contains monazite and yttrobetafite grains, all with concentrations of U, Th, and Pb sufficiently high to determine reliably with the electron microprobe. The assemblage represents the first documented occurrence of these three minerals together and only the second reported occurrence of thorite in a lunar rock. Sample 12023,147-10 is a small, monomict rock fragment recovered from an Apollo 12 regolith sample. It comprises graphic intergrowths of K-feldspar and quartz, and plagioclase and quartz, along with minor or accessory hedenbergite, fayalite, ilmenite, zircon, yttrobetafite, thorite, monazite, and Fe metal. Thorite, ideally ThSiO₄, occurs in the assemblage adjacent to quartz and plagioclase, and includes a 12% xenotime ([Y,HREE]PO₄) component. From quantitative electron-probe microanalysis (EPMA) of Th, U, and Pb in thorite, assuming that all of the measured Pb is radiogenic, we calculate an age of 3.87 ± 0.03 Ga. Yttrobetafite and monazite, which contain lesser concentrations of U, Th, and Pb than the thorite, yield ages of 3.78 ± 0.06 Ga and 3.9 ± 0.3 Ga, respectively. These dates are consistent with formation of the granitic material around 3.9 Ga, possibly associated with, or after, the formation of the Imbrium basin. This age falls within a group of younger ages for granitic samples, measured mainly by ion microprobe analysis of zircon, compared to a suite of older ages, ca. 4.2-4.32 Ga, also from zircons (Meyer et al., 1996). A 3.9 Ga age may reflect an origin following the Imbrium event whereby granitic melt formed as a result of heating and melting, and was mobilized and emplaced along an Imbrium-related ring-fracture system. Silicic volcanic or exposed intrusive materials occur in several circum-Imbrium locations such as the Mairan and Gruithuisen Domes and in ejecta excavated by Aristarchus crater. Perhaps sample 12023,147-10 and some of the other granitic materials sampled at the Apollo 12 site represent rocks similar to the rocks that make up these large silicic rock occurrences.

Reference
Seddio SM, Jolliff BL, Korotev RL and Carpenter PK (in press) Thorite in an Apollo 12 granite fragment and age determination using the electron microprobe. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.03.020]
Copyright Elsevier

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J. Blum^a, B. Gundlach^a, S. Mühle^a and J.M. Trigo-Rodriguez^b

^aInstitut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany
^aInstitute of Space Sciences (CSIC), Campus UAB, Facultat de Ciéncies, Torre C-5 pares, 2a pl., 08193 Bellaterra (Barcelona), Spain

When comet nuclei approach the Sun, the increasing energy flux through the surface layers leads to sublimation of the underlying ices and subsequent outgassing that promotes the observed emission of gas and dust. While the release of gas can be straightforwardly understood by solving the heat-transport equation and taking into account the finite permeability of the ice-free dust layer close to the surface of the comet nucleus, the ejection of dust additionally requires that the forces binding the dust particles to the comet nucleus must be overcome by the forces caused by the sublimation process. This relates to the question of how large the tensile strength of the overlying dust layer is. Homogeneous layers of micrometer-sized dust particles reach tensile strengths of typically 10³ to 10⁴ Pa. This exceeds by far the maximum sublimation pressure of water ice in comets. It is therefore unclear how cometary dust activity is driven.
To solve this paradox, we used the model by Skorov and Blum (Icarus 221, 1-11, 2012), who assumed that cometesimals formed by gravitational instability of a cloud of dust and ice aggregates and calculated for the corresponding structure of comet nuclei tensile strength of the dust-aggregate layers on the order of 1 Pa. Here we present evidence that the emitted cometary dust particles are indeed aggregates with the right properties to fit the model by Skorov and Blum. Then we experimentally measure the tensile strengths of layers of laboratory dust aggregates and confirm the values derived by the model. To explain the comet activity driven by the evaporation of water ice, we derive a minimum size for the dust aggregates of ~1 mm, in agreement with meteoroid observations and dust-agglomeration models in the solar nebula. Finally we conclude that cometesimals must have formed by gravitational instability, because all alternative formation models lead to higher tensile strengths of the surface layers.

Reference
Blum J, Gundlach B, Mühle S and Trigo-Rodriguez JM (in press) Comets formed in solar-nebula instabilities! – An experimental and modeling attempt to relate the activity of comets to their formation process. Icarus
[doi:10.1016/j.icarus.2014.03.016]
Copyright Elsevier

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R. Martín-Doménech¹, G. M. Muñoz Caro¹, J. Bueno² and F. Goesmann³

¹Centro de Astrobiología (INTA-CSIC), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
²Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, Netherlands
³Max Planck Institute for Solar System Research, Justus von Liebig Weg 3, 370077 Göttingen, Germany

Context. Thermal annealing of interstellar ices takes place in several stages of star formation. Knowledge of this process comes from a combination of astronomical observations and laboratory simulations under astrophysically relevant conditions.
Aims. For the first time we present the results of temperature programmed desorption (TPD) experiments with pre-cometary ice analogs composed of up to five molecular components: H₂O, CO, CO₂, CH₃OH, and NH₃.
Methods. The experiments were performed with an ultra-high vacuum chamber. A gas line with a novel design allows the controlled preparation of mixtures with up to five molecular components. Volatiles desorbing to the gas phase were monitored using a quadrupole mass spectrometer, while changes in the ice structure and composition were studied by means of infrared spectroscopy.
Results. The TPD curves of water ice containing CO, CO₂, CH₃OH, and NH₃ present desorption peaks at temperatures near those observed in pure ice experiments, volcano desorption peaks after water ice crystallization, and co-desorption peaks with water. Desorption peaks of CH₃OH and NH₃ at temperatures similar to the pure ices takes place when their abundance relative to water is above ~3% in the ice matrix. We found that CO, CO₂, and NH₃ also present co-desorption peaks with CH₃OH, which cannot be reproduced in experiments with binary water-rich ice mixtures. These are extensively used in the study of thermal desorption of interstellar ices.
Conclusions. These results reproduce the heating of circumstellar ices in hot cores and can be also applied to the late thermal evolution of comets. In particular, TPD curves represent a benchmark for the analysis of the measurements that mass spectrometers on board the ESA-Rosetta cometary mission will perform on the coma of comet 67P/Churyumov-Gerasimenko, which will be active before the arrival of Rosetta according to our predictions.

Reference
Martín-Doménech R, Muñoz Caro GM, Bueno J and F. Goesmann F (2014) Thermal desorption of circumstellar and cometary ice analogs. Astronomy & Astrophysics 564:A8.
[doi:10.1051/0004-6361/201322824]
Reproduced with permission © ESO

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Tamara Goldin

Review of the Republic of the Moon touring exhibition

Reference
Goldin T (2014) Exhibition: Lunar reflections and astronaut geese. Nature Geoscience 7:162.
[doi:10.1038/ngeo2103]

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