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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