Des grains cométaires en laboratoire: premiers résultats de la mission Stardust.

My thesis is dedicated to SXRM (Synchrotron X-Ray Microscopy) and µ-FTIR (micro Fourier Transformed InfraRed) analyses of comet Wild 2 samples brought back to Earth by the Stardust mission. SXRM experiments allowed the study of both the elemental composition of Wild 2 grains and their slowing down into the aerogel. A XANES study at the edge of iron shows that the information concerning the incident particle is preserved in the terminal particle found at the end of the track. Fluorescence mappings were used to determine the elemental composition of comet Wild 2, showing that, normalised to iron, Ca, Ti, Cr, Mn, Ni, Ge and Se are present in chondritic abundances. Enrichments in Cu, Zn and Ga are observed, indicating that at least a part of the refractory phase of Wild 2 formed during a late stage of nebular condensation from a gas phase depleted in most refractory elements. Sulphur seems depleted, which is surely a clue for an incomplete condensation process, stopped before all the sulphur is condensed. The µ-FTIR study coupled to Raman studies of grains extracted from the aerogel shows that organic matter in Wild 2 is dominated by aromatic rings linked together by aliphatic chains. The study of the 3.4 µm band shows that the aliphatic chains are longer (or less branched) than in IDPs and grains from the interstellar medium (ISM). Comparison with observations of the diffuse ISM reveals that organic matter in Wild 2 is clearly not interstellar and appears simpler than that contained in interstellar grains.Ma thèse porte sur l'analyse par SXRM (Synchrotron X-Ray Microscopy) et spectroscopie µ-FTIR (micro Fourier Transformed InfraRed) des échantillons de la comète Wild 2 collectés par la mission Stardust. Les analyses SXRM ont permis de déterminer la composition élémentaire des grains de Wild 2, tout en étudiant leur ralentissement dans l'aérogel. Une étude XANES (X-Ray Absorption Near Edge Structure) au seuil du fer d'analogues des échantillons Stardust a permis de valider la méthode de collecte des grains en se basant sur l'état d'oxydation du fer. Malgré l'élévation de température subie par la particule lors de son entrée dans l'aérogel, les informations concernant la particule incidente, en particulier concernant sa minéralogie, sont préservées dans la particule finale. Des cartographies de µ-fluorescence X ont permis de déterminer la composition élémentaire des grains de Wild 2. Les abondances élémentaires, normalisées au fer, sont chondritiques pour Ca, Ti, Cr, Mn, Ni, Ge, et Se. Un enrichissement en Cu, Zn et Ga est en revanche observé et indiquerait qu'au moins une partie de la matière réfractaire de Wild 2 s'est formée tardivement, à partir d'un gaz appauvri en les éléments les plus réfractaires. Le soufre semble quant à lui sous-abondant, ce qui signe sans doute un processus de condensation incomplet, stoppé avant que la totalité du soufre ait pu se condenser. L'analyse par µ-FTIR, complétée par de la spectroscopie Raman, de grains extraits de l'aérogel montre que la matière organique de Wild 2 est dominée par la présence d'îlots de carbone aromatique reliés entre eux par des ponts aliphatiques. L'étude de la bande à 3.4 µm montre que les chaînes aliphatiques sont plus longues (ou moins ramifiées) que dans les IDPs et les grains du milieu interstellaire (MIS). La comparaison avec les observations du MIS diffus montre que la matière organique de Wild 2 n'est clairement pas interstellaire et semble plus simple que celle contenue dans les grains du MIS

Des grains cométaires en laboratoire (premiers résultats de la mission Stardust)

Ma thèse porte sur l analyse par SXRM (Synchrotron X-Ray Microscopy) et spectroscopie -FTIR (micro Fourier Transformed InfraRed) de grains de la comète Wild 2 collectés par la mission Stardust. Les analyses SXRM ont permis de déterminer leur composition élémentaire, tout en étudiant leur ralentissement dans l aérogel. Une étude XANES (X-Ray Absorption Near Edge Structure) au seuil du fer montre que les informations concernant la particule incidente sont préservées dans la particule finale. Des cartographies de -fluorescence X ont permis de déterminer la composition élémentaire des grains de Wild 2, montrant que, normalisés au fer, Ca, Ti, Cr, Mn, Ni, Ge, et Se sont présents en abondances chondritiques. Un enrichissement en Cu, Zn et Ga est observé et indiquerait qu au moins une partie de la matière réfractaire de Wild 2 s est formée tardivement, à partir d un gaz appauvri en les éléments les plus réfractaires. Le soufre semble sous-abondant, ce qui signe sans doute un processus de condensation incomplet, stoppé avant que la totalité du soufre ait pu se condenser. L analyse par -FTIR, complétée par de la spectroscopie Raman, de grains extraits de l aérogel montre que la matière organique de Wild 2 est dominée par la présence d îlots de carbone aromatique reliés entre eux par des ponts aliphatiques. L étude de la bande à 3.4 m montre que les chaînes aliphatiques sont plus longues (ou moins ramifiées) que dans les IDPs et les grains du milieu interstellaire (MIS). La comparaison avec les observations du MIS diffus montre que la matière organique de Wild 2 n est clairement pas interstellaire et semble plus simple, moins mature, que celle contenue dans les grains du MIS.My thesis is dedicated to SXRM (Synchrotron X-Ray Microscopy) and -FTIR (micro Fourier Transformed InfraRed) analyses of comet Wild 2 samples brought back to Earth by the Stardust mission. SXRM experiments allowed the study of both the elemental composition of Wild 2 grains and their slowing down into the aerogel. A XANES study at the edge of iron shows that the information concerning the incident particle is preserved in the terminal particle found at the end of the track. Fluorescence mappings were used to determine the elemental composition of comet Wild 2, showing that, normalised to iron, Ca, Ti, Cr, Mn, Ni, Ge and Se are present in chondritic abundances. Enrichments in Cu, Zn and Ga are observed, indicating that at least a part of the refractory phase of Wild 2 formed during a late stage of nebular condensation from a gas phase depleted in most refractory elements. Sulphur seems depleted, which is surely a clue for an incomplete condensation process, stopped before all the sulphur is condensed. The -FTIR study coupled to Raman studies of grains extracted from the aerogel shows that organic matter in Wild 2 is dominated by aromatic rings linked together by aliphatic chains. The study of the 3.4 m band shows that the aliphatic chains are longer (or less branched) than in IDPs and grains from the interstellar medium (ISM). Comparison with observations of the diffuse ISM reveals that organic matter in Wild 2 is clearly not interstellar and appears less mature than in interstellar grains.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

In-situ Fe XANES of extraterrestrial grains trapped in aerogel collectors: An analytical test for the interpretation of Stardust samples analyses

On 15 January 2006, the NASA Stardust Capsule Sample Return came back to Earth with its load of cometary and contemporary interstellar grains trapped in aerogel collectors. These cometary grains are the first samples of known parent body and their study in the laboratory will give new clues on the nature of the cometary materials. Using Synchrotron X-ray Microscopy (SXRM), some analogues of Stardust's samples were analysed. The aim was to develop an analytical protocol and to study the effects of the slowing down of hypervelocity particles into aerogel on the physical and chemical properties of the collected grains. Our samples originate either from the NASA Orbital Debris Collection Experiment (ODCE) deployed outside the MIR station, or from light gas gun shots of Allende meteorite grains into aerogel at velocities of 6 km/s. They consist of grains trapped in pieces of aerogel, a few hundreds of microns large. Using synchrotron X-ray microbeam, micro-fluorescence mappings and X-ray absorption near-edge structure (XANES) spectra were performed, bringing information on elemental analysis, repartition and speciation of Fe in our samples. In particular, the XANES studies obtained at the iron K-edge show that iron is present in different oxidation states in the samples, rather in a ferric form at the track entrance while rather in a ferrous form at the end of the track as well as in the final grain. The tests performed on the Allende meteorite dust grains for which the Fe2+/Fe3+ ratio is a priori known, tend to show that the final particle presents the same oxidation state as the initial incident one, a very encouraging clue for the validity of the future interpretation of Stardust samples analyses

Infrared spectroscopy of comet 81P/Wild 2 samples returned by Stardust

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/ Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/ protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements

Research article - Comet 81P/Wild 2 under a microscope

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