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To understand the evolution of organic molecules involved in extraterrestrial environments and with exobiological implications, many experimental programs in the laboratory are devoted to photochemical studies in the gaseous phase as well as in the solid state. The validity of such studies and their applications to extraterrestrial environments can be questioned as long as experiments conducted in space conditions, with the full solar spectrum, especially in the short wavelength domain, have not been implemented. The experiments that are described here will be carried out on a FOTON capsule, using the BIOPAN facility, and on the International Space Station, using the EXPOSE facility. Vented and sealed exposition cells will be used, which will allow us to study the chemical evolution in the gaseous phase as well as heterogeneous processes, such as the degradation of solid compounds and the release of gaseous fragments. Four kinds of experiments will be carried out. The first deal with comets and are related to the Rosetta mission, the second with Titan and are related to the Cassini–Huygens mission, the third with the search for life-related organic compounds on Mars and, finally, the fourth are a continuation of previous studies concerning the behavior of amino acids in space.

To date, there is nothing that could allow us to figure out if we will ever know whether the Earth is the only inhabited planet in the universe. However, astrobiology has gone a long way since the very first experiments by Miller in 1950 and since then, questions related to astrobiology are feeding the exploration of our solar system, and even beyond with ambitious programs for the observation of extrasolar planets (TPF, Darwin). The answer is maybe already at hand, by a rock analyzed with a Martian rover, or further away, under a few kilometers of ice in the Jupiter neighborhood. . . This chapter deals with the preliminary steps of the astrobiological exploration of the solar system, we are currently witnessing. We focused on the search of organic matter and liquid water, but none of them is a clear indicator of past or present life. We did not discuss the next issue about measurement(s) (chemical, geological, spectroscopic), which could allow us to claim that we have finally detected life on another planet. The scientific debate about it is still at its very first stages, and opens the most fascinating prospects.

The ESA mission Rosetta, launched on March 2nd, 2004, carries an instrument suite to the comet 67P/Churyumov-Gerasimenko. The COmetary Secondary Ion Mass Anaylzer – COSIMA – is one of three cometary dust analyzing instruments onboard Rosetta. COSIMA is based on the analytic measurement method of secondary ion mass spectrometry (SIMS). The experiment’s goal is in-situ analysis of the elemental composition (and isotopic composition of key elements) of cometary grains. The chemical characterization will include the main organic components, present homologous and functional groups, as well as the mineralogical and petrographical classification of the inorganic phases. All this analysis is closely related to the chemistry and history of the early solar system. COSIMA covers a mass range from 1 to 3500 amu with a mass resolution m/Δm @ 50% of 2000 at mass 100 amu. Cometary dust is collected on special, metal covered, targets, which are handled by a target manipulation unit. Once exposed to the cometary dust environment, the collected dust grains are located on the target by a microscopic camera. A pulsed primary indium ion beam (among other entities) releases secondary ions from the dust grains. These ions, either positive or negative, are selected and accelerated by electrical fields and travel a well-defined distance through a drift tube and an ion reflector. A microsphere plate with dedicated amplifier is used to detect the ions. The arrival times of the ions are digitized, and the mass spectra of the secondary ions are calculated from these time-of-flight spectra. Through the instrument commissioning, COSIMA took the very first SIMS spectra of the targets in space. COSIMA will be the first instrument applying the SIMS technique in-situ to cometary grain analysis as Rosetta approaches the comet 67P/Churyumov-Gerasimenko, after a long journey of 10 years, in 2014.

Low-temperature microwave-powered plasma based on hydrogen and hydrogen with noble gas mixtures are widely used as a continuous vacuum ultraviolet (VUV) source in laboratory experiments carried out to mimic the photochemistry in astrophysical environments. In this work, we present a study dedicated to optimizing such sources in terms of mono-chromaticity at Lyα (H(Lyα) line at 121.6 nm ~ 10.2 eV) and high spectral irradiance. We report the influence on the emission spectrum of a wide range of experimental conditions including gas composition (pure H2, pure He, and H2/He mixture), gas pressure, flow rates, and microwave power. The absolute spectral irradiance delivered by this VUV light source has been measured. With a microwave input power of 100 W, the best conditions for producing a quasi-monochromatic source are a 1% H2/He gas mixture at a total pressure of 5 mbar and a flow rate of 2 sccm. By changing the microwave input power from 30 to 120 W, H(Lyα) increases by more than one order of magnitude. A comparison between the current measurements and the solar VUV spectral irradiance is reported over 115–170 nm.

Why should we be interested in comets when studying the origin of life on Earth? First, comets are rich in water and carbon, two essential constituents of terrestrial life; part of Earth’s water and carbon might be of cometary origin. But comets might also have brought Earth one step further on the way to the emergence of life. Early last century, Chamberlin and Chamberlin proposed that infalling carbonaceous chondrite meteorites could have been an important source of terrestrial organic compounds (Chamberlin and Chamberlin 1908). Oro was the first in 1961 to propose from observations of carbonand nitrogen-containing radicals in cometary comae that comets may have played a similar role (Oro 1961)

International audience; The origin of CN radicals in comets is not completely understood so far. We present a study of CN and HCN production rates and CN Haser scale lengths showing that: (1) at heliocentric distances larger than 3 AU, CN radicals could be entirely produced by HCN photolysis; (2) closer to the Sun, for a fraction of comets CN production rates are higher than HCN ones whereas (3) in the others, CN distribution cannot be explained by the HCN photolysis although CN and HCN production rates seem to be similar. Thus, when the comets are closer than 3 AU to the Sun, an additional process to the HCN photolysis seems to be required to explain the CN density in some comets. The photolysis of HC 3N or C 2N 2 could explain the CN origin. But the HC 3N production rate is probably too low to reproduce CN density profile, even if uncertainties on its photolysis leave the place for all possible conclusions. The presence of C 2N 2 in comets is a reliable hypothesis to explain the CN origin; thus, its detection is a challenging issue. Since C 2N 2 is very difficult to detect from ground-based observations, only in situ measurements or space observations could determine the contribution of this compound in the CN origin. Another hypothesis is a direct production of CN radicals by the photo- or thermal degradation of complex refractory organic compounds present on cometary grains. This process could explain the spatial profile of CN inside jets and the discrepancy noted in the isotopic ratio 14N/ 15N between CN and HCN. Laboratory studies of the thermal and UV-induced degradation of solid nitrogenated compounds are required to model and validate this hypothesis.

Most of the molecules observed in the cometary environment are directly produced by sublimation from the nucleus, and the main fraction of the observed radicals is the result of the photodissociation of gaseous “parent” molecules already observed. However some of these species (especially CO, H2CO, CN…) have a spatial distribution which can not be explained by these processes. They are produced by an unknown “extended source” in the coma. If one could infer the nature of the material involved in this phenomenon, this should allow to constraint the chemical composition of cometary grains and nucleus which is of prime interest for the exobiology studies.

[1] The formaldehyde (H2CO) observed in cometary atmospheres presents a so-called “extended source,” meaning that its distribution in the coma cannot be explained by sublimation from the nucleus alone; a production inside the coma has to be included. Polyoxymethylene (formaldehyde polymers: (-CH2-O-)n, also called POM) is sometimes evoked as a parent molecule for this extended source. This solid polymer on cometary grains could release gaseous formaldehyde through thermal and photolytic degradation. We have developed an experimental program in order to study the chemical reactions of degradation of POM by UV photolysis and heating. It provides identification of the degradation products and the determination of the photo degradation quantum yields or the thermal degradation kinetics. In this paper we present the improvements of our experimental setup and new measurements on the kinetics of gaseous formaldehyde production by thermal degradation of two types of commercial POM on a greater temperature range than the previous ones. These new data are then included in a model of the outer coma in order to interpret observations. This model takes into account the production of gaseous molecules from solid material present on grains and leads to a very good agreement with Giotto observations of comet 1P/Halley, if we assume that the cometary grains contain 1–16% of POM by mass. Thus, without being final evidence of this polymer presence on comets, we conclude that the degradation of solid POM-like polymers on cometary grains is to date the best explanation of the H2CO extended source.

We report hexamethylenetetramine (HMT) detection in organic residues resulting from H2 O:C O :C H 3 OH :N H 3 ices submitted to UV photolysis or proton irradiation. This is the first time HMT is detected after proton irradiation of an interstellar or cometary ice analog, whereas this molecule was suspected to be a characteristic signature of UV versus ion processing. This result strengthens the probability of HMT presence in the interstellar medium and in comets, where it may eventually be detected with the COSAC (COmetary Sampling And Composition) experiment on board the upcoming Rosetta mission. Subject headings: astrochemistry — comets: general — ISM: molecules — methods: laboratory — space vehicles: instruments

After a ten-year journey, the European spacecraft Rosetta arrived at comet 67P/Churyumov-Gerasimenko (67P/C-G) on August 6, 2014. In order to conduct intensive research for twenty six months, a total of 21 instruments were on board the Rosetta orbiter and the Philae lander.The time-of-flight secondary ions mass spectrometer, COSIMA (COmetary Secondary Ions Mass Analyzer), was one of the orbiter instruments and was designed for in situ analysis of the chemical composition of cometary particles ejected from the comet 67P/C-G. This thesis aims to characterize the refractory organic component of the cometary dust, with the COSIMA instrument.As part of this work, I have developed and applied a methodology to analyze the cometary mass spectra, more specifically in the positive mode of the instrument. Using this methodology, five organic ions of cometary origin were detected and identified: C+ (m/z = 12.00), CH+ (m/z = 13.01), CH2+ (m/z = 14.02), CH3+ (m/z = 15.02) and C2H3+ (m/z = 27.02).The characterization of this organic component has been carried out with calibration mass spectra. The reference compounds used during this thesis include pure organic molecules and cometary organic matter analogs, such as natural samples (carbonaceous chondrites, ultracarbonaceous micrometeorite, insoluble organic matter extracted from carbonaceous chondrites) and one sample synthetized in the laboratory. The best analogue found so far to the refractory component of 67P/C-G particles is the insoluble organic matter extracted from carbonaceous chondrites. The cometary organic matter would be of high-molecular-weight and the carbon bounded in very large macromolecular compounds.Carbon is also one of the most abundant element in the dust particles with an elemental ratio of C⁄Si = 5,5 +1.4⁄-1.2. Comet 67P/C-G is among the most carbon-rich bodies of the solar system.Finally, a specific compound was sought, the polyoxymethylene. However, indicators developed and applied to the cometary data do not allow to conclude on the presence of polyoxymethylene in the particles analyzed by COSIMA., La sonde spatiale européenne Rosetta, après un voyage de dix années, a rejoint le 6 août 2014 son objet d’étude, la comète 67P/Churyumov-Gerasimenko (67P/C-G). Afin de l’étudier de manière intensive durant vingt-six mois, un total de 21 instruments était embarqué sur l’orbiteur Rosetta et l’atterrisseur Philae.Le spectromètre de masse d’ions secondaires à temps de vol, COSIMA (COmetary Secondary Ions Mass Analyzer), était l’un des instruments de l’orbiteur et a été conçu pour analyser in situ la composition chimique des particules solides éjectées de la comète 67P/C-G. L’objectif de cette thèse est de caractériser la composante organique réfractaire contenue dans les poussières cométaires, à l’aide de l’instrument COSIMA.Dans le cadre de ces travaux, j’ai mis au point au point et appliqué une méthodologie pour analyser les spectres de masse cométaires, plus particulièrement dans le mode positif de l’instrument. À l’aide de cette méthodologie, cinq ions organiques d’origine cométaire ont été détectés et identifiés : C+ (m/z = 12,00), CH+ (m/z = 13,01), CH2+ (m/z = 14,02), CH3+ (m/z = 15,02) et C2H3+ (m/z = 27,02).La caractérisation de cette composante organique a été effectuée à l’aide de spectres de masse de calibration. Les composés de référence utilisés dans ces travaux de thèse incluent des molécules organiques pures et des analogues de la matière organique cométaire, tels que des échantillons naturels (des météorites carbonées, une micrométéorite ultracarbonée, de la matière organique insoluble extraite de météorites carbonées) et un échantillon synthétisé en laboratoire. La matière organique insoluble extraite des chondrites carbonées est, à ce jour, le meilleur analogue à la matière organique réfractaire contenue dans les particules de 67P/C-G. Ainsi, la matière organique cométaire serait de haut poids moléculaire et le carbone lié dans de grandes structures macromoléculaires.Le carbone est également l’un des éléments les plus abondants dans les particules avec un rapport C⁄Si = 5,5 +1,4⁄-1,2. La comète 67P/C-G figure ainsi parmi les corps les plus riches en carbone du système solaire.Enfin, un composé spécifique a été recherché, le polyoxyméthylène. Cependant, les indicateurs développés et appliqués aux données cométaires ne permettent pas de conclure quant à sa présence dans les particules analysées par COSIMA.

Formed since the creation of the Solar System, comets are of particular interest. They could have brought organic compounds to the primitive Earth that contributed to the chemical evolution that lead to the origin of life on Earth. Thanks to ground based observations and space missions, it is possible to measure the composition of these small bodies. Although most of the gaseous molecules detected in cometary atmospheres are produced from the sublimation of nucleus ices, but the distribution of some of them is more complex.This thesis work is part of the study of distributed sources in comets. Two molecules were studied. On the one hand, formaldehyde which presents an atypical behavior in most of the comets in which it has been detected. The case of six comets is discussed: C/2001 Q4 (NEAT), C/2004 Q2 (Machholz), 8P/Tuttle, C/1996 B2 (Hyakutake), C/2012 S1 (ISON) and C/2012 F6 (Lemmon). On the other hand, glycine, for which a profile of the density as a function of the distance of the nucleus has been measured in comet 67P/Churyumov-Gerasimenko during the Rosetta mission. This density profile shows a particular behavior.In order to interpret these observations, I have developed a numerical model applied first to the production of formaldehyde in cometary atmospheres, and then extended to the case of glycine. Two cases have been considered for the study of formadehyde: a. it is produced from the sublimation of nucleus ices, b. it is produced from a distributed source involving thermal and photochemical degradation of polyoxymethylene on particles ejected from the nucleus. Concerning glycine, three cases have been considered: a. it is emitted directly and only from the sublimation of nucleus ice, b. it is produced from the sublimation of solid glycine on dust particles ejected from the nucleus, c. glycine is embedded in water ice and emitted from the sublimation of this ice from the particles ejected from the nucleus. These last two cases being distributed sources.A parent source from the nucleus does not explain the density profiles of formaldehyde measured in comets C/2001 Q4 (NEAT), C/2004 Q2 (Machholz), 8P/Tuttle, C/1996 B2 (Hyakutake), C/2012 S1 (ISON) and C/2012 F6 (Lemmon). This case also does not explain the glycine observations in comet 67P/Churyumov-Gerasimenko. Concerning formaldehyde, the best solution proposed corresponds to a low production from the nucleus, i.e. between 0 and 0.2% in relation to the total gas production, and a majority of formaldehyde coming from a distributed source from the degradation of its polymer, polyoxymethylene. The abundance, from one comet to another, varies between 0.25 and 2.7% of polyoxymethylene by mass in particles. Regarding glycine, the observations can be adjusted if the sublimation of water ice from dust particles in the atmosphere is considered with an abundance of glycine of about 170 ppb by mass in water ice, Présentes depuis la formation du Système Solaire, les comètes suscitent un intérêt particulier. Elles ont pu apporter sur la Terre primitive des composés organiques, et ainsi favoriser l’apparition de la vie. Des observations depuis la Terre et des missions spatiales ont permis de mesurer la composition de ces petits corps, et en particulier celle de son atmosphère. Un grand nombre des molécules observées dans l’environnement cométaire provient directement de la sublimation des glaces du noyau, mais la distribution de certaines d’entre elles est plus complexe.Ce travail de thèse entre dans le cadre de l’étude des sources distribuées dans les comètes. Deux molécules ont été étudiées au cours de cette thèse. D’une part, le formaldéhyde qui présente un comportement atypique dans la plupart des comètes dans lesquelles il a été détecté. Le cas de six comètes est discuté : C/2001 Q4 (NEAT), C/2004 Q2 (Machholz), 8P/Tuttle, C/1996 B2 (Hyakutake), C/2012 S1 (ISON) et C/2012 F6 (Lemmon). D’autre part la glycine, pour laquelle un profil de densité en fonction de la distance au noyau a été mesuré dans la comète 67P/Churyumov-Gerasimenko lors de la mission Rosetta. Ce profil de densité présente un comportement particulier.Afin d’interpréter ces observations, j’ai développé un modèle numérique appliqué dans un premier temps à la production de formaldéhyde dans les atmosphères cométaires, puis étendu au cas de la glycine. Deux cas ont été considérés pour l’étude du formaldéhyde : a. celui-ci est formé à partir de sublimation des glaces du noyau, b. celui-ci est produit à partir d’une source distribuée impliquant la dégradation thermique et photochimique du polyoxyméthylène présent sur les particules éjectées du noyau. Concernant la glycine, trois cas ont été pris en compte : a. celle-ci est émise directement et uniquement à partir de la sublimation des glaces du noyau, b. celle-ci est produite à partir de la sublimation de glycine solide présente sur les particules de poussière éjectées du noyau, c. celle-ci serait incluse dans la matrice de glace d’eau présente sur les particules de poussière éjectées du noyau et serait émise en phase gazeuse en même temps que la glace d’eau sublimerait. Ces deux derniers cas s’apparentent à une source distribuée.Une source unique à partir du noyau n’explique pas les profils de densité mesurés de formaldéhyde dans les comètes C/2001 Q4 (NEAT), C/2004 Q2 (Machholz), 8P/Tuttle, C/1996 B2 (Hyakutake), C/2012 S1 (ISON) et C/2012 F6 (Lemmon). Ce cas ne permet pas non plus d’expliquer les observations de glycine faites dans la comète 67P/Churyumov-Gerasimenko. La meilleure hypothèse concernant le formaldéhyde correspond donc, d’une part, à une faible production à partir du noyau, soit entre 0 et 0,2% par rapport à la production de gaz total. Et d’autre part à une majorité de formaldéhyde provenant d’une source distribuée à partir de la dégradation de son polymère, le polyoxyméthylène pour des quantités qui varient, d’une comète à l’autre, entre 0,25 et 2,7 % de polyoxyméthylène en masse dans les particules. En ce qui concerne la glycine, les observations peuvent être ajustées si l’on considère la sublimation de glace d’eau à partir des particules de poussière dans l’atmosphère avec une abondance de glycine d’environ 170 ppb en masse au sein de la glace d’eau

Organic molecule detection at Mars is one of the main goals of the current and future Mars exploration space missions, Mars Science Laboratory (MSL, NASA) and ExoMars 2020 (ESA). Several organic sources exist : abiotic sources (interplantary medium, hydrothermalism and atmospheric synthesis) but also biotic sources such as potential past biological activity. Curiosity from the MSL mission detected chlorinated and sulfur organic compounds. However these compounds can not be linked to any biological activity and do not represent the meteoritical organic diversity.The main hypothesis to explain the low diversity of detected organic compounds at Mars is that the martian environment degrade organic matter. In order to understand organic molecule evolution at the Martian surface and be able to guide and help interpret in situ analysis, I worked on two experimental simulations mimicking some of the martian environmental conditions (UV radiation, pressure, temperature and mineral composition). MOMIE, for Mars Organic Matter Irradiation at Mars, is a laboratory experiment set up at the LISA laboratory (Créteil, France). PSS, for Photochemistry on the Space Station, has been set up on the International Space Station (ISS) in low Earth orbit, using directly filtered UV photons from the Sun.I studied the evolution of four organic molecules likely to be present at Mars with ou without a mineral phase : glycine (an amino acid), adenine and uracil (two nucleobases), and chrysene (a polycyclic aromatic hydrocarbon). Glycine, adenine and chrysene are degraded at Mars surface with quantum efficiencies of photodecomposition from 6,4 ± 1,4 x 10-6 to 2,3 ± 1,0 x 10-3 molecule.photon-1. Uracil evolve into more stable photoproducts with a production efficiency of 1,64 ± 1,43 x 10-1 molécule.photon-1. Four uracil dimers have been identified as uracil photoproducts. Finally, the studied mineral phases, an amorphous iron-rich phase and perchlorates, accelerate organics evolution or degradation, La recherche et la détection de molécules organique à la surface de Mars est l’un des objectifs des missions martiennes actuelles (MSL, Mars Science Laboratory) et futures (ExoMars 2020). Plusieurs sources de matière organique peuvent être considérées telles que les sources abiotiques (milieu interplanétaire, hydrothermalisme, synthèses atmosphériques…) mais également les sources biotiques telles qu’une potentielle activité biologique martienne passée. A ce titre, le rover Curiosity de la mission MSL a permis la détection de composés organiques d’origine martienne chlorés et soufrés, bien que ces molécules ne soient pas liées à une quelconque activité biologique ou bien ne reflètent pas la diversité moléculaire de sources abiotiques avérées telles que le milieu interplanétaire. L’une des hypothèses pour expliquer cette faible diversité consiste à considérer que l’environnement martien n’est pas favorable à la préservation de la matière organique. Afin de comprendre l’évolution des molécules organiques à la surface de Mars et donc de guider et aider les interprétations des analyses menées in situ, j’ai travaillé sur deux expériences de simulation simulant certains paramètres de la surface de Mars (rayonnement UV, pression, température, composition minérale). La première, MOMIE (Mars Organic Matter Irradiation and Evolution), est une simulation de laboratoire mise en place au LISA (Créteil, France). La seconde est l’expérience PSS (Photochemistry on the Space Station), mise en place sur la plateforme EXPOSE R2 sur la Station Spatiale Internationale (ISS) en orbite basse terrestre, utilisant directement le flux de photons UV du Soleil filtré.J’ai étudié l’évolution de quatre molécules organiques susceptibles d’être présentes sur Mars, pures ou en présence de phases minérales analogues martiennes : la glycine (un acide aminé), l’adénine et l’uracile (deux bases azotées), et le chrysène (un hydrocarbure aromatique polycyclique). La glycine, l’adénine et le chrysène se dégradent en surface directe de Mars avec des rendements quantiques de photodissociation ϕ200-280 compris entre 6,4 ± 1,4 x 10-6 et 2,3 ± 1,0 x 10-3 molécule.photon-1. L’uracile forme des photoproduits plus stables, selon un rendement de production élevé de 1,64 ± 1,43 x 10-1 molécule.photon-1. Quatre dimères d’uracile ont pu être identifiés comme des photoproduits. Pour finir, l’ajout de phases minérales amorphe et riche en fer ou bien de perchlorates accélère la dégradation ou l’évolution des molécules organiques

Organic molecule detection at Mars is one of the main goals of the current and future Mars exploration space missions, Mars Science Laboratory (MSL, NASA) and ExoMars 2020 (ESA). Several organic sources exist : abiotic sources (interplantary medium, hydrothermalism and atmospheric synthesis) but also biotic sources such as potential past biological activity. Curiosity from the MSL mission detected chlorinated and sulfur organic compounds. However these compounds can not be linked to any biological activity and do not represent the meteoritical organic diversity.The main hypothesis to explain the low diversity of detected organic compounds at Mars is that the martian environment degrade organic matter. In order to understand organic molecule evolution at the Martian surface and be able to guide and help interpret in situ analysis, I worked on two experimental simulations mimicking some of the martian environmental conditions (UV radiation, pressure, temperature and mineral composition). MOMIE, for Mars Organic Matter Irradiation at Mars, is a laboratory experiment set up at the LISA laboratory (Créteil, France). PSS, for Photochemistry on the Space Station, has been set up on the International Space Station (ISS) in low Earth orbit, using directly filtered UV photons from the Sun.I studied the evolution of four organic molecules likely to be present at Mars with ou without a mineral phase : glycine (an amino acid), adenine and uracil (two nucleobases), and chrysene (a polycyclic aromatic hydrocarbon). Glycine, adenine and chrysene are degraded at Mars surface with quantum efficiencies of photodecomposition from 6,4 ± 1,4 x 10-6 to 2,3 ± 1,0 x 10-3 molecule.photon-1. Uracil evolve into more stable photoproducts with a production efficiency of 1,64 ± 1,43 x 10-1 molécule.photon-1. Four uracil dimers have been identified as uracil photoproducts. Finally, the studied mineral phases, an amorphous iron-rich phase and perchlorates, accelerate organics evolution or degradation; La recherche et la détection de molécules organique à la surface de Mars est l’un des objectifs des missions martiennes actuelles (MSL, Mars Science Laboratory) et futures (ExoMars 2020). Plusieurs sources de matière organique peuvent être considérées telles que les sources abiotiques (milieu interplanétaire, hydrothermalisme, synthèses atmosphériques…) mais également les sources biotiques telles qu’une potentielle activité biologique martienne passée. A ce titre, le rover Curiosity de la mission MSL a permis la détection de composés organiques d’origine martienne chlorés et soufrés, bien que ces molécules ne soient pas liées à une quelconque activité biologique ou bien ne reflètent pas la diversité moléculaire de sources abiotiques avérées telles que le milieu interplanétaire. L’une des hypothèses pour expliquer cette faible diversité consiste à considérer que l’environnement martien n’est pas favorable à la préservation de la matière organique. Afin de comprendre l’évolution des molécules organiques à la surface de Mars et donc de guider et aider les interprétations des analyses menées in situ, j’ai travaillé sur deux expériences de simulation simulant certains paramètres de la surface de Mars (rayonnement UV, pression, température, composition minérale). La première, MOMIE (Mars Organic Matter Irradiation and Evolution), est une simulation de laboratoire mise en place au LISA (Créteil, France). La seconde est l’expérience PSS (Photochemistry on the Space Station), mise en place sur la plateforme EXPOSE R2 sur la Station Spatiale Internationale (ISS) en orbite basse terrestre, utilisant directement le flux de photons UV du Soleil filtré.J’ai étudié l’évolution de quatre molécules organiques susceptibles d’être présentes sur Mars, pures ou en présence de phases minérales analogues martiennes : la glycine (un acide aminé), l’adénine et l’uracile (deux bases azotées), et le chrysène (un hydrocarbure aromatique polycyclique). La glycine, l’adénine et le chrysène se dégradent en surface directe de Mars avec des rendements quantiques de photodissociation ϕ200-280 compris entre 6,4 ± 1,4 x 10-6 et 2,3 ± 1,0 x 10-3 molécule.photon-1. L’uracile forme des photoproduits plus stables, selon un rendement de production élevé de 1,64 ± 1,43 x 10-1 molécule.photon-1. Quatre dimères d’uracile ont pu être identifiés comme des photoproduits. Pour finir, l’ajout de phases minérales amorphe et riche en fer ou bien de perchlorates accélère la dégradation ou l’évolution des molécules organiques

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 thèse s'inscrit dans le cadre de l'expérience de spectrométrie de masse d'ions secondaires à temps de vol, COSIMA, se trouvant à bord de la mission européenne cométaire Rosetta. Cet instrument a pour vocation de collecter puis de mesurer la nature chimique des grains de la comète 67P/Churyumov-Gerasimenko. L'objectif de ma thèse est de préparer et de faciliter l'interprétation des spectres de masse de COSIMA, et plus particulièrement de leur composante organique réfractaire. Pour cela, à partir d'un instrument de laboratoire conçu spécialement pour présenter des caractéristiques instrumentales similaires à COSIMA, les spectres de masse de deux familles de composés purs (hétérocycles azotés et acides carboxyliques) ont été mesurés. Ces données ont été qualifiées en tant que spectres de masse de référence utilisables par l'équipe COSIMA. Cette base de données est le point de départ de la recherche de marqueurs permettant la distinction des molécules constituées d'atomes particuliers. Deux marqueurs ont été trouvés. Néanmoins ils sont à manipuler avec prudence compte tenu des biais pouvant être induits par la présence de minéraux au sein des grains ou de contaminants sur les cibles d'analyse. L'analyse d'analogues de la matière organique cométaire a également été effectuée. Il en ressort que les mesures réalisées avec COSIMA peuvent présenter des biais importants. L'identification des polymères de HCN en est d'ailleurs compromise. Toutefois, l'analyse d'un grain de la météorite de Murchison montre que la distinction entre les composantes minérale et organique est facilement réalisable. De plus, les signatures de composés clés comme l'hexaméthylènetétramine ou le polyoxyméthylène (POM) sont clairement identifiables dans le mode positif de l'instrument. Le POM étant thermiquement instable, une étude cinétique de sa dégradation thermique a été réalisée en vue de contraindre la stratégie opérationnelle à adopter pour sa détection. Pour maximiser les chances de le détecter dans les grains cométaires, les analyses par COSIMA doivent être effectuées dans les quinze jours après la collecte des grains.