Comet 81P/Wild 2 Under a Microscope

NEW PUBLICATION IN SCIENCE

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

Back from San Francisco

Me voilà de retour de San Francisco. Mon oral s’est bien passé. Il y avait peu de monde dans la salle…. normal pour une toute fin de session (le dernier jour en avant avant dernier), ça n’attire plus grand monde. Le lendemain, petite visite de San Francisco. Les photos sont un petit peu plus bas.

J’ai modifié aussi la page Travaux car je ne sais pas trop si je peux mettre les articles en ligne, pour des questions de copyright.
J’ai aussi fait une mise à jour avec de nouveaux articles.

4th Street Union Square Cable Car Alcatraz
Musée Golden gate San Francisco Grand Magasin

Pallasite

J’ai eu un joli cadeau de la part de certains de mes amis pour mon mariage: un morceau de pallasite (Merci Brigite, Cédric, Hélène, Tim, Clem, Guillaume et Xav). Je mets une miniature de la photo ci dessous. Une pallasite c’est un genre de météorite qui pourrait être un analogue de ce qui se passe à 2900km sous nos pieds, la limite entre le manteau et le noyau. Ces météorites sont constituées d’olivines flottant dans une matrice de fer.

Pallasite

Section de la Pallasite Brahim

Compiz-Beryl

Comme je le disais sur mon ancienne page, j’ai maintenant un Toshiba Satellite A100-165 sur lequel Kubuntu est installé.
Globalement tout marche bien et j’ai même pu installer le nouvel environnement graphique Xgl. C’est presque un gadget mais cela fait entrer Linux dans la véritable 3D. J’ai fait une vidéo des possibilités de l’environnement que vous pouvez regarder ici.
Le gestionnaire de fenêtres associé à l’environnement graphique Xgl était jusqu’à présent Compiz. Maintenant, le projet s’est divisé en deux et seul le projet Beryl me semble actif. J’ai donc choisi d’installer celui-ci. Je dois dire que le gain de rapidité est impressionant mais je n’arrive pas à avoir un menu K transparent….

Equation of state and phase transition in KAlSi3O8 hollandite at high pressure

NEW PUBLICATION IN AMERICAN MINERALOGIST

The tetragonal hollandite structure (KAlSi3O8 hollandite) has been studied up to 32 GPa at room temperature using high-pressure in-situ X-ray diffraction techniques. A phase transformation from tetragonal I4/m phase to a new phase was found to occur at about 20 GPa. This transition is reversible on release of pressure without noticeable hysteresis and hence this new high-pressure phase is unquenchable to ambient conditions. The volume change associated with the transition is found to be small (not measurable), suggesting a second order transition. The diffraction pattern of the high-pressure phase can be indexed in a monoclinic unit cell (space group I2/m), which is isostructual with BaMn8O16 hollandite. The  » angle of the monoclinic unit cell increases continuously above the transition. A Birch-Murnaghan equation of state Þ t to pressure-volume data obtained for KAlSi3O8 hollandite yields a bulk modulus K0 = 201.4 (7) GPa with K’0 = 4.0.

A new high-pressure form of KAlSi3O8 under lower mantle conditions

NEW PUBLICATION

In situ X-ray diffraction measurements have been made on KAlSi3O8 hollandite using diamond anvil cell and multianvil apparatus combined with synchrotron radiation.
Both of the measurements with different techniques demonstrated that K-hollandite transforms to a new highpressure phase (hollandite II) at 22 GPa upon increasing pressure at room temperature. The X-ray diffraction peaks of the new phase were reasonably indexed on the basis of a monoclinic cell with I2/m space group. Hollandite II was also confirmed to be formed at high temperatures to 1200°C and pressures to 35 GPa, which was quenched to room temperature under pressure but converted back to hollandite at about 20 GPa on release of pressure. The present result is contradictory to earlier studies based mainly on quench method, which concluded that hollandite is stable up to 95 GPa at both room temperature and high temperatures up to 2300°C.

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Motion of a solid object through a pasty (thixotropic)… fluid

NEW PUBLICATION IN PHYSCIS OF FLUIDS

For materials assumed to be simple yield stress fluids the velocity of an object should continuously increase from zero as the applied force increases from the critical value for incipient motion. We carried out experiments of fall of a sphere in a typical, thixotropic, pasty material ~a laponite suspension!. We either left a sphere falling in the fluid in different initial states of structure or vibrated the fluid in a given state of structure at different frequencies. In each case three analogous regimes appear either for increasing restructuring states of the fluid or decreasing frequencies: A rapid fall at an almost constant rate; a slower fall at a progressively decreasing velocity; a slow fall at a rapidly decreasing rate finally leading to apparent stoppage. These results show that the motion of an object, due to gravity in a pasty material, is a more complex dynamical process than generally assumed for simple yield stress fluids. A simple model using the basic features of the ~thixotropic! rheological behavior of these pasty materials makes it possible to explain these experimental trends. The fall of an object in such a fluid thus appears to basically follow a bifurcation process: For a sufficiently large force applied onto the object its rapid motion tends to sufficiently liquify the fluid around it so that its subsequent motion is more rapid and so on until reaching a constant velocity; on the contrary if the force applied onto the object is not sufficiently large the fluid around has enough time to restructure, which slows down the motion and so on until the complete stoppage of the object.