80 research outputs found
Préparation à la caractérisation in-situ de la matiÚre organique cométaire par spectrométrie de masse : application à l'instrument COSIMA
This thesis has been prepared in the frame of the time of flight secondary ions mass spectrometry experiment called COSIMA on board the ESA's Rosetta cometary mission. This instrument will collect and measure the chemical nature of the grains of the comet 67P/Churyumov-Gerasimenko. The aim of my thesis is to prepare and facilitate the interpretation of mass spectra of COSIMA, especially their refractory organic component. Therefore, the mass spectra of two families of pure compounds (nitrogen heterocycles and carboxylic acids) have been measured with a laboratory instrument tailor-made to have instrumental characteristics similar to those of COSIMA.. We have established that those data can be considered as reference mass spectra to be used by the COSIMA team. This database has been a starting point for the search for markers to distinguish molecules composed of specific atoms. Two markers have been found. Nevertheless they should be handled with caution because of bias that can be induced by the presence of minerals in grains or contaminants on the targets. Cometary organic matter analogs have also been analyzed. These measurements show that the spectra collected with COSIMA can have significant biases. Identification of HCN polymers is compromised. However, the analysis of a grain from the Murchison meteorite shows that the distinction between inorganic and organic compounds is feasible. In addition, the signatures of key compounds such as hexamethylenetetramine or polyoxymethylene (POM) are clearly identifiable in the positive mode of the instrument. Since the POM is thermally unstable, a kinetic study of its thermal degradation has been carried out to constrain the operational strategy to be adopted for its detection. To maximize the chances to detect it in the cometary grains, analyses by COSIMA must be made within fifteen days after the collection of grains.Cette thĂšse s'inscrit dans le cadre de l'expĂ©rience de spectromĂ©trie de masse d'ions secondaires Ă temps de vol, COSIMA, se trouvant Ă bord de la mission europĂ©enne comĂ©taire Rosetta. Cet instrument a pour vocation de collecter puis de mesurer la nature chimique des grains de la comĂšte 67P/Churyumov-Gerasimenko. L'objectif de ma thĂšse est de prĂ©parer et de faciliter l'interprĂ©tation des spectres de masse de COSIMA, et plus particuliĂšrement de leur composante organique rĂ©fractaire. Pour cela, Ă partir d'un instrument de laboratoire conçu spĂ©cialement pour prĂ©senter des caractĂ©ristiques instrumentales similaires Ă COSIMA, les spectres de masse de deux familles de composĂ©s purs (hĂ©tĂ©rocycles azotĂ©s et acides carboxyliques) ont Ă©tĂ© mesurĂ©s. Ces donnĂ©es ont Ă©tĂ© qualifiĂ©es en tant que spectres de masse de rĂ©fĂ©rence utilisables par l'Ă©quipe COSIMA. Cette base de donnĂ©es est le point de dĂ©part de la recherche de marqueurs permettant la distinction des molĂ©cules constituĂ©es d'atomes particuliers. Deux marqueurs ont Ă©tĂ© trouvĂ©s. NĂ©anmoins ils sont Ă manipuler avec prudence compte tenu des biais pouvant ĂȘtre induits par la prĂ©sence de minĂ©raux au sein des grains ou de contaminants sur les cibles d'analyse. L'analyse d'analogues de la matiĂšre organique comĂ©taire a Ă©galement Ă©tĂ© effectuĂ©e. Il en ressort que les mesures rĂ©alisĂ©es avec COSIMA peuvent prĂ©senter des biais importants. L'identification des polymĂšres de HCN en est d'ailleurs compromise. Toutefois, l'analyse d'un grain de la mĂ©tĂ©orite de Murchison montre que la distinction entre les composantes minĂ©rale et organique est facilement rĂ©alisable. De plus, les signatures de composĂ©s clĂ©s comme l'hexamĂ©thylĂšnetĂ©tramine ou le polyoxymĂ©thylĂšne (POM) sont clairement identifiables dans le mode positif de l'instrument. Le POM Ă©tant thermiquement instable, une Ă©tude cinĂ©tique de sa dĂ©gradation thermique a Ă©tĂ© rĂ©alisĂ©e en vue de contraindre la stratĂ©gie opĂ©rationnelle Ă adopter pour sa dĂ©tection. Pour maximiser les chances de le dĂ©tecter dans les grains comĂ©taires, les analyses par COSIMA doivent ĂȘtre effectuĂ©es dans les quinze jours aprĂšs la collecte des grains
Solar wind sputtering of dust on the surface of 67P/Churyumov-Gerasimenko
International audienceFar away from the Sun, at around 3 AU, the activity of comet 67P/Churyumov-Gerasimenko is low and changes with local time (solar insolation), with location (chemical heterogeneity of the surface), and with season. When the activity is very low because the total cross section of the comet against the Sun is small, the solar wind has access to the surface of the comet and causes ion-induced sputtering of surface material, which we wish to observe.Methods. We used the Double Focussing Mass Spectrometer (DFMS) of the ROSINA experiment on ESAâs Rosetta mission to search for mass spectrometric evidence of sputtered refractory species. In high-resolution mode, DFMS can separate some of the mass peaks of refractory elements from the many volatile species present in the coma.Results. At present, the locations of solar wind surface access are in the southern hemisphere of the comet (the local winter). Of particular interest is sputtering of dust grains on the surface. We observe global averages over the winter hemisphere of the refractory elements Na, K, Si, and Ca, presumably sputtered from grains residing on the surface. Compared to carbonaceous chondrites, the comet has the same Na abundance, is depleted in Ca, and has an excess of K. In addition, for Si the signal strength is strong enough to compile a coarse compositional map of the southern hemisphere. Most, perhaps all, of the observed variation can be explained by the solar wind being affected by the atmosphere of the comet
Sulphur-bearing species in the coma of comet 67P/ChuryumovâGerasimenko
Several sulphur-bearing species have already been observed in different families of comets. However, the knowledge on the minor sulphur species is still limited. The cometâs sulphur inventory is closely linked to the pre-solar cloud and holds important clues to the degree of reprocessing of the material in the solar nebula and during comet accretion. Sulphur in pre-solar clouds is highly depleted, which is quite puzzling as the S/O ratio in the diffuse interstellar medium is cosmic. This work focuses on the abundance of the previously known species H2S, OCS, SO, S2, SO2 and CS2 in the coma of comet 67P/ChuryumovâGerasimenko measured by Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double Focusing Mass Spectrometer between equinox and perihelion 2015. Furthermore, we present the first detection of S3, S4, CH3SH and C2H6S in a comet, and we determine the elemental abundance of S/O in the bulk ice of (1.47 ± 0.05) Ă 10â2. We show that SO is present in the coma originating from the nucleus, but not CS in the case of 67P, and for the first time establish that S2 is present in a volatile and a refractory phase. The derived total elemental sulphur abundance of 67P is in agreement with solar photospheric elemental abundances and shows no sulphur depletion as reported for dense interstellar clouds. Also the presence of S2 at heliocentric distances larger than 3 au indicates that sulphur-bearing species have been processed by radiolysis in the
pre-solar cloud and that at least some of the ice from this cloud has survived in comets up the present
Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta
Cometary ices are rich in CO2, CO and organic volatile
compounds, but the carbon content of cometary dust was only measured for
the Oort Cloud comet 1P/Halley, during its flyby in 1986. The COmetary
Secondary Ion Mass Analyzer (COSIMA)/Rosetta mass spectrometer
analysed dust particles with sizes ranging from 50 to 1000Â ÎŒm, collected
over 2 yr, from 67P/Churyumov-Gerasimenko (67P), a Jupiter family
comet. Here, we report 67P dust composition focusing on the elements C
and O. It has a high carbon content (atomic | |â )
close to the solar value and comparable to the 1P/Halley data. From
COSIMA measurements, we conclude that 67P particles are made of nearly
50 per cent organic matter in mass, mixed with mineral phases that are
mostly anhydrous. The whole composition, rich in carbon and non-hydrated
minerals, points to a primitive matter that likely preserved its
initial characteristics since the comet accretion in the outer regions
of the protoplanetary disc.</p
Nitrogen-to-carbon atomic ratio measured by COSIMA in the particles of comet 67P/ChuryumovâGerasimenko
The COmetary Secondary Ion Mass Analyzer (COSIMA) on board the Rosetta mission has analysed numerous cometary dust particles collected at very low velocities (a few m sâ1)
in the environment of comet 67P/ChuryumovâGerasimenko (hereafter 67P).
In these particles, carbon and nitrogen are expected mainly to be part
of the organic matter. We have measured the nitrogen-to-carbon (N/C)
atomic ratio of 27 cometary particles. It ranges from 0.018 to 0.06 with
an averaged value of 0.035 ± 0.011. This is compatible with the
measurements of the particles of comet 1P/Halley and is in the lower
range of the values measured in comet 81P/Wild 2 particles brought back
to Earth by the Stardust mission. Moreover, the averaged value
found in 67P particles is also similar to the one found in the insoluble
organic matter extracted from CM, CI and CR carbonaceous chondrites and
to the bulk values measured in most interplanetary dust particles and
micrometeorites. The close agreement of the N/C atomic ratio in all
these objects indicates that their organic matters share some
similarities and could have a similar chemical origin. Furthermore,
compared to the abundances of all the detected elements in the particles
of 67P and to the elemental solar abundances, the nitrogen is depleted
in the particles and the nucleus of 67P as was previously inferred also
for comet 1P/Halley. This nitrogen depletion could constrain the
formation scenarios of cometary nuclei.</p
Halogens as tracers of protosolar nebula material in comet 67P/ChuryumovâGerasimenko
We report the first in situ detection of halogens in a cometary coma, that of 67P/ChuryumovGerasimenko. Neutral gas mass spectra collected by the European Space Agencyâs Rosetta spacecraft during four periods of interest from the first comet encounter up to perihelion indicate that the main halogen-bearing compounds are HF, HCl and HBr. The bulk elemental abundances relative to oxygen are ~8.9 Ă 10â»â” for F/O, ~1.2 Ă 10â»âŽ for Cl/O and ~2.5 Ă 10â»â¶ for Br/O, for the volatile fraction of the comet. The cometary isotopic ratios for Âłâ·Cl/Âłâ”Cl and âžÂčBr/â·âčBr match the Solar system values within the error margins. The observations point to an origin of the hydrogen halides in molecular cloud chemistry, with frozen hydrogen halides on dust grains, and a subsequent incorporation into comets as the cloud condensed and the Solar system formed
Préparation à la caractérisation in-situ de la matiÚre organique cométaire par spectrométrie de masse : application à l'instrument COSIMA
This thesis has been prepared in the frame of the time of flight secondary ions mass spectrometry experiment called COSIMA on board the ESA's Rosetta cometary mission. This instrument will collect and measure the chemical nature of the grains of the comet 67P/Churyumov-Gerasimenko. The aim of my thesis is to prepare and facilitate the interpretation of mass spectra of COSIMA, especially their refractory organic component. Therefore, the mass spectra of two families of pure compounds (nitrogen heterocycles and carboxylic acids) have been measured with a laboratory instrument tailor-made to have instrumental characteristics similar to those of COSIMA.. We have established that those data can be considered as reference mass spectra to be used by the COSIMA team. This database has been a starting point for the search for markers to distinguish molecules composed of specific atoms. Two markers have been found. Nevertheless they should be handled with caution because of bias that can be induced by the presence of minerals in grains or contaminants on the targets. Cometary organic matter analogs have also been analyzed. These measurements show that the spectra collected with COSIMA can have significant biases. Identification of HCN polymers is compromised. However, the analysis of a grain from the Murchison meteorite shows that the distinction between inorganic and organic compounds is feasible. In addition, the signatures of key compounds such as hexamethylenetetramine or polyoxymethylene (POM) are clearly identifiable in the positive mode of the instrument. Since the POM is thermally unstable, a kinetic study of its thermal degradation has been carried out to constrain the operational strategy to be adopted for its detection. To maximize the chances to detect it in the cometary grains, analyses by COSIMA must be made within fifteen days after the collection of grains.Cette thĂšse s'inscrit dans le cadre de l'expĂ©rience de spectromĂ©trie de masse d'ions secondaires Ă temps de vol, COSIMA, se trouvant Ă bord de la mission europĂ©enne comĂ©taire Rosetta. Cet instrument a pour vocation de collecter puis de mesurer la nature chimique des grains de la comĂšte 67P/Churyumov-Gerasimenko. L'objectif de ma thĂšse est de prĂ©parer et de faciliter l'interprĂ©tation des spectres de masse de COSIMA, et plus particuliĂšrement de leur composante organique rĂ©fractaire. Pour cela, Ă partir d'un instrument de laboratoire conçu spĂ©cialement pour prĂ©senter des caractĂ©ristiques instrumentales similaires Ă COSIMA, les spectres de masse de deux familles de composĂ©s purs (hĂ©tĂ©rocycles azotĂ©s et acides carboxyliques) ont Ă©tĂ© mesurĂ©s. Ces donnĂ©es ont Ă©tĂ© qualifiĂ©es en tant que spectres de masse de rĂ©fĂ©rence utilisables par l'Ă©quipe COSIMA. Cette base de donnĂ©es est le point de dĂ©part de la recherche de marqueurs permettant la distinction des molĂ©cules constituĂ©es d'atomes particuliers. Deux marqueurs ont Ă©tĂ© trouvĂ©s. NĂ©anmoins ils sont Ă manipuler avec prudence compte tenu des biais pouvant ĂȘtre induits par la prĂ©sence de minĂ©raux au sein des grains ou de contaminants sur les cibles d'analyse. L'analyse d'analogues de la matiĂšre organique comĂ©taire a Ă©galement Ă©tĂ© effectuĂ©e. Il en ressort que les mesures rĂ©alisĂ©es avec COSIMA peuvent prĂ©senter des biais importants. L'identification des polymĂšres de HCN en est d'ailleurs compromise. Toutefois, l'analyse d'un grain de la mĂ©tĂ©orite de Murchison montre que la distinction entre les composantes minĂ©rale et organique est facilement rĂ©alisable. De plus, les signatures de composĂ©s clĂ©s comme l'hexamĂ©thylĂšnetĂ©tramine ou le polyoxymĂ©thylĂšne (POM) sont clairement identifiables dans le mode positif de l'instrument. Le POM Ă©tant thermiquement instable, une Ă©tude cinĂ©tique de sa dĂ©gradation thermique a Ă©tĂ© rĂ©alisĂ©e en vue de contraindre la stratĂ©gie opĂ©rationnelle Ă adopter pour sa dĂ©tection. Pour maximiser les chances de le dĂ©tecter dans les grains comĂ©taires, les analyses par COSIMA doivent ĂȘtre effectuĂ©es dans les quinze jours aprĂšs la collecte des grains
COSIMA calibration for the detection and characterization of the cometary solid organic matter
On the orbiter of the Rosetta spacecraft, the Cometary Secondary Ion Mass Analyser (COSIMA) will provide new in situ insights about the chemical composition of cometary grains all along 67P/ChuryumovâGerasimenko (67P/CG) journey until the end of December 2015 nominally. The aim of this paper is to present the pre-calibration which has already been performed as well as the different methods which have been developed in order to facilitate the interpretation of the COSIMA mass spectra and more especially of their organic content. The first step was to establish a mass spectra library in positive and negative ion mode of targeted molecules and to determine the specific features of each compound and chemical family analyzed. As the exact nature of the refractory cometary organic matter is nowadays unknown, this library is obviously not exhaustive. Therefore this library has also been the starting point for the research of indicators, which enable to highlight the presence of compounds containing specific atom or structure. These indicators correspond to the intensity ratio of specific peaks in the mass spectrum. They have allowed us to identify sample containing nitrogen atom, aliphatic chains or those containing polyaromatic hydrocarbons. From these indicators, a preliminary calibration line, from which the N/C ratio could be derived, has also been established. The research of specific mass difference could also be helpful to identify peaks related to quasi-molecular ions in an unknown mass spectrum. The Bayesian Positive Source Separation (BPSS) technique will also be very helpful for data analysis. This work is the starting point for the analysis of the cometary refractory organic matter. Nevertheless, calibration work will continue in order to reach the best possible interpretation of the COSIMA observations
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