364 research outputs found
Preliminary studies of electromagnetic sounding of cometary nuclei
The internal structure of a comet could be determined with a spacecraft borne electromagnetic sounder. A dielectric profile of the comet could be produced in direct analogy with terrestrial glacier and ice sheet sounding experiments. This profile would allow the detection of a rocky core or ice layers if they exist, just as layers in the ice and the bedrock interface have been clearly observed through the Greenland ice sheet. It would also provide a gross estimate of the amount of dust in the icy region. Models for the response of the nucleus and cometary plasma to electromagnetic sounding are developed and used to derive experimental parameters. A point system design was completed. Preliminary engineering study results indicate that the sounder is well within the bounds of current space technology
New Design Techniques for Dynamic Reconfigurable Architectures
L'abstract Ăš presente nell'allegato / the abstract is in the attachmen
Logan Medallist 4. Large-Scale Impact and Earth History
The current record of large-scale impact on Earth consists of close to 200 impact structures and some 30 impact events recorded in the stratigraphic record, only some of which are related to known structures. It is a preservation sample of a much larger production population, with the impact rate on Earth being higher than that of the moon. This is due to the Earthâs larger physical and gravitational cross-sections, with respect to asteroidal and cometary bodies entering the inner solar system. While terrestrial impact structures have been studied as the only source of ground-truth data on impact as a planetary process, it is becoming increasingly acknowledged that large-scale impact has had its effects on the geologic history of the Earth, itself. As extremely high energy events, impacts redistribute, disrupt and reprocess target lithologies, resulting in topographic, structural and thermal anomalies in the upper crust. This has resulted in many impact structures being the source of natural resources, including some world-class examples, such as gold and uranium at Vredefort, South Africa, NiâCuâPGE sulphides at Sudbury, Canada and hydrocarbons from the Campeche Bank, Mexico. Large-scale impact also has the potential to disrupt the terrestrial biosphere. The most devastating known example is the evidence for the role of impact in the CretaceousâPaleocene (KâPg) mass extinction event and the formation of the Chicxulub structure, Mexico. It also likely had a role in other, less dramatic, climatic excursions, such as the PaleoceneâEoceneâThermal Maximum (PETM) event. The impact rate was much higher in early Earth history and, while based on reasoned speculation, it is argued that the early surface of the Hadean Earth was replete with massive impact melt pools, in place of the large multiring basins that formed on the lower gravity moon in the same time-period. These melt pools would differentiate to form more felsic upper lithologies and, thus, are a potential source for Hadean-aged zircons, without invoking more modern geodynamic scenarios. The Earth-moon system is unique in the inner solar system and currently the best working hypothesis for its origin is a planetary-scale impact with the proto-Earth, after core formation at ca. 4.43 Ga. Future large-scale impact is a low probability event but with high consequences and has the potential to create a natural disaster of proportions unequalled by other geologic processes and threaten the extended future of human civilization, itself.RĂSUMĂLe bilan actuel de traces de grands impacts sur la Terre se compose de prĂšs de 200 astroblĂšmes et d'une trentaine dâimpacts enregistrĂ©s dans la stratigraphie, dont seulement certains sont liĂ©s Ă des astroblĂšmes connus. Il s'agit d'Ă©chantillons prĂ©servĂ©s sur une population dâĂ©vĂ©nements beaucoup plus importante, le taux d'impact sur Terre Ă©tant supĂ©rieur Ă celui de la lune. Cela tient aux plus grandes sections transversales physiques et gravitationnelles de la Terre sur la trajectoire des astĂ©roĂŻdes et comĂštes qui pĂ©nĂštrent le systĂšme solaire interne. Alors que les astroblĂšmes terrestres ont Ă©tĂ© Ă©tudiĂ©s comme Ă©tant la seule source de donnĂ©es avĂ©rĂ©e dâimpacts en tant que processus planĂ©taire, de plus en plus on reconnaĂźt que les grands impacts ont eu des effets sur l'histoire gĂ©ologique de la Terre. Ă lâinstar des Ă©vĂ©nements d'Ă©nergie extrĂȘme, les impacts redistribuent, perturbent et remanient les lithologies impliquĂ©es, provoquant dans la croĂ»te terrestre supĂ©rieure des anomalies topographiques, structurelles et thermiques. Il en a rĂ©sultĂ© de nombreux astroblĂšmes Ă lâorigine de ressources naturelles, dont certains exemples de classe mondiale tels que l'or et l'uranium Ă Vredefort en Afrique du Sud, les sulfures de NiâCuâPGE Ă Sudbury au Canada, et les hydrocarbures du Banc de Campeche au Mexique. Les grands impacts peuvent Ă©galement perturber la biosphĂšre terrestre. L'exemple le plus dĂ©vastateur connu nous est donnĂ© des indices du rĂŽle de l'impact dans l'extinction de masse au CrĂ©tacĂ©âPalĂ©ogĂšne (KâPg) et la formation de la structure de Chicxulub, au Mexique. Il a Ă©galement probablement jouĂ© un rĂŽle dans d'autres Ă©vĂ©nements climatiques extraordinaires moins dramatiques, comme le Maximum thermal du PaleocĂšneâEocĂšne (PETM). Le taux d'impact Ă©tait beaucoup plus Ă©levĂ© au dĂ©but de l'histoire de la Terre et, tout en Ă©tant basĂ© sur une spĂ©culation raisonnĂ©e, on fait valoir que la surface prĂ©coce de la Terre Ă lâHadĂ©en Ă©tait tapissĂ©e de grands bassins en fusion, au lieu de grands bassins Ă couronnes multiples tels ceux qui se sont formĂ©s Ă la mĂȘme pĂ©riode sur la lune ayant une gravitĂ© infĂ©rieure. Ces bassins en fusion se seraient diffĂ©renciĂ©es pour constituer des lithologies plus felsiques sur le dessus, devenant ainsi une source potentielle de zircons dâĂąge HadĂ©en, sans quâil soit nĂ©cessaire dâinvoquer des scĂ©narios gĂ©odynamiques plus rĂ©cents. Le systĂšme Terre-lune est unique dans le systĂšme solaire interne. Actuellement la meilleure hypothĂšse de travail pour son origine est un impact planĂ©taire avec la proto-Terre, aprĂšs la formation du noyau Ă env. 4,43 Ga. La probabilitĂ© dâun futur grand impact est faible mais comporte des consĂ©quences capables dâengendrer un dĂ©sastre naturel aux proportions inĂ©galĂ©es comparĂ© Ă d'autres processus gĂ©ologiques, menaçant l'avenir de la civilisation humaine elle-mĂȘme
Workshop on the Analysis of Interplanetary Dust Particles
Great progress has been made in the analysis of interplanetary dust particles (IDP's) over the past few years. This workshop provided a forum for the discussion of the following topics: observation and modeling of dust in the solar system, mineralogy and petrography of IDP's, processing of IDP's in the solar system and terrestrial atmosphere, comparison of IDP's to meteorites and micrometeorites, composition of IDP's, classification, and collection of IDP's
The Twenty-Fifth Lunar and Planetary Science Conference. Part 2: H-O
Various papers on lunar and planetary science are presented, covering such topics as: planetary geology, lunar geology, meteorites, shock loads, cometary collisions, planetary mapping, planetary atmospheres, chondrites, chondrules, planetary surfaces, impact craters, lava flow, achondrites, geochemistry, stratigraphy, micrometeorites, tectonics, mineralogy, petrology, geomorphology, and volcanology
Etude de la structure nucléaire de noyaux exotiques à ALTO : développements et résultats de deux nouvelles installations
ALTO (AccĂ©lĂ©rateur LinĂ©aire et Tandem dâOrsay) is a facility composed of two accelerators dedicated to research and industrial applications. There is a 15 MV tandem and a linear accelerator. My PhD work was to develop the instrumentation of the linear accelerator part of ALTO which provides radioactive beams for fundamental research. These radioactive beams are produced using the Isotope Separation On-Line method (ISOL). This technique allows three kinds of experiments: mass measurement, nuclear orientation and radioactivity experiments. Among those three types of experiments, I worked on the development of two new experimental platforms for the ALTO instrumentation. The first one, BEDO (BEta Decay studies in Orsay) is an ensemble of detectors dedicated to ÎČ-Îł spectroscopy of ÎČ-decaying nuclei produced by ALTO. I present in this thesis, the commissioning of this new experimental set-up, its technical characteristics and the tools development leading to the first results. For this commissioning experiment a mass 82 radioactive beam was produced, taking this opportunity the âžÂČGe vers âžÂČAs decay was re-investigated allowing to establish a new level scheme for âžÂČAs and giving the first evidences for the presence of intruder states in the N=49 odd-odd isotones. The second project, which is developed, is POLAREX (POLARization of EXotic nuclei), a new facility for nuclear orientation experiments. My thesis deals with the entire reconditioning of a ÂłHe-âŽHe dilution refrigerator (major and most complex element of the facility) and R&D and technical developments of the platform. These contributions allowed the successful commissioning of the new experimental platform with the first physical measurements on â”âŽMn, â”â¶Co, â”â·Co created by activation of an iron foil with deuterons produced by the Tandem.ALTO (AccĂ©lĂ©rateur LinĂ©aire et Tandem dâOrsay) est une installation Ă©quipĂ©e de deux accĂ©lĂ©rateurs pour la recherche et les applications industrielles (un tandem de 15 MV et une accĂ©lĂ©rateur linĂ©aire). Mon travail de thĂšse consiste Ă lâinstrumentation pour la recherche fondamentale de la partie accĂ©lĂ©rateur linĂ©aire dâALTO qui fournir des faisceaux de noyaux radioactifs. Les faisceaux de noyaux radioactifs riches en neutrons sont produits par la technique de sĂ©paration isotopique en ligne (ISOL). Cette mĂ©thode de production permet trois types dâexpĂ©rience : la mesure de masse, lâorientation nuclĂ©aire et les expĂ©riences de dĂ©croissances radioactives. Parmi ces trois types dâexpĂ©riences, jâai participĂ© aux dĂ©veloppements de deux nouvelles plateformes expĂ©rimentales dans le cadre du projet de lâinstrumentation de lâinstallation ISOL dâALTO. Le premier, BEDO (BEta Decay studies in Orsay) est un ensemble de dĂ©tecteurs dĂ©diĂ© Ă la spectroscopie ÎČ-Îł des noyaux dĂ©croissants par dĂ©sintĂ©gration ÎČ produits par ALTO. Je prĂ©sente ici, la mise en fonctionnement de cette plateforme expĂ©rimentale, ses caractĂ©ristiques techniques et les dĂ©veloppements dâoutils permettant dâaboutir aux premiers rĂ©sultats. Pour cette expĂ©rience un faisceau de la masse 82 a Ă©tĂ© produit, saisissant cette opportunitĂ©, une rĂ©-investigation de la dĂ©croissance de âžÂČGe vers âžÂČAs a permis dâĂ©tablir un nouveau schĂ©ma de niveaux pour âžÂČAs et de donner les premiĂšres indications de la prĂ©sence dâĂ©tats issus de configurations intruses dans les isotones impair-impair N=49. Le second projet dĂ©veloppĂ© est POLAREX (POLARization of EXotic nuclei), il sâagit dâune plateforme expĂ©rimentale dĂ©diĂ©e aux expĂ©riences dâorientation nuclĂ©aire. Mon travail traite ici de lâentiĂšre rĂ©habilitation du cryostat Ă dilution ÂłHe-âŽHe (Ă©lĂ©ment principal et le plus complexe de lâinstallation) et des dĂ©veloppements techniques et R&D apportĂ©s Ă lâensemble de la plateforme. Lâensemble de ces contributions a permis la validation du fonctionnement de lâinstallation avec les premiĂšres mesures physiques sur les noyaux de â”âŽMn, â”â¶Co, â”â·Co crĂ©Ă©s par activation dâune feuille de Fer avec des deutons produits par le Tandem
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