Photochemistry of organic matter in solar system : application to cometary dust

L'étude de la photochimie dans le système solaire est de toute première importance pour appréhender la chimie organique complexe au sein d'un environnement extraterrestre. Parmi ces environnements, les comètes revêtent un intérêt particulier en exobiologie puisqu'elles ont pu, ainsi que leurs grains, être des vecteurs de matière organique sur la Terre primitive et ainsi contribuer à l'émergence de la vie. Mais dans quelle mesure, la matière organique potentiellement présente au sein des grains survit-elle face aux rayonnements solaires? Ma thèse porte sur l'étude de la dégradation photochimique de trois bases azotées (adénine, guanine et uracile) et d'un acide aminé (glycine) dans les conditions du système solaire, c'est à dire soumis à des rayonnements énergétiques VUV/UV ( <300 nm). Les études conduites lors de ce travail peuvent aussi être appliquées à l'interprétation des mesures du le spectromètre de masse COSIMA présent sur l'orbiteur de la mission cométaire ROSETTA et dont l'objectif est l'analyse de la surface de grains cométaires capturés dans l'environnement de la comète 67P/Churyomov-Gerasimenko. Ce travail présente les spectres de sections efficaces d'absorption mesurés dans les domaines VUV/UV pour des films organiques purs. Ces spectres ont mené à la déduction de constantes de photolyse, ainsi qu'à l'élaboration d'un modèle simulant la cinétique de destruction globale d'un film organique optiquement épais. La confrontation entre ce modèle et les données expérimentales d'irradiation en orbite basse terrestre ainsi qu'en laboratoire a permis d'estimer les temps de vie des molécules considérées à 1 ua puis extrapolés à différentes distances héliocentriques. Les résultats obtenus ont montré que la glycine, l'adénine et la guanine, potentiellement présentes au sein des grains cométaires, seraient totalement détruites entre le moment de l'éjection des grains du noyau cométaire et l'arrivée sur Terre si elles sont en surface. En sous-surface, elles sont au contraire très stables, de part la protection efficace que leur confèrent les minéraux constitutifs du grain contre les rayonnements solaires. Dans le cadre de la mission ROSETTA, les résultats diffèrent. Au plus loin du soleil, à 3,5 ua, l'abondance des molécules ne diminuerait pas de façon significative pendant le temps de parcours des grains entre le noyau et l'orbiteur. Au périhélie, la "survie" des molécules dépendra fortement de la distance noyau-orbiteur. Les pertes significatives des 3 molécules par photochimie n'auraient lieu que si l'orbiteur se situe au moins à quelques centaines de kilomètres du noyauThe study of photochemistry in the solar system is of prime importance to assess complex organic chemistry in an extraterrestrial environment. Among those environments, comets are subject to a particular interest in the context of exobiology, along with their grains, as they could have bring organic matter on the primitive earth, and hence contribute to the emergence of life. But to what extent does the organic matter potentially with in grains survive face to solar radiation? My thesis deals with the study of photochemical degradation of three nitrogenous bases (adenine, guanine and uracil) and one amino acid ( glycine) in the conditions of the solar system, which means subject to VUV/UV energetically radiations ( <300 nm). Studies performed during this work can also be applied to the interpretation of COSIMA mass spectrometer, present on the cometary mission ROSETTA, which aims to analyze the surface of cometary grains captured in the environment of the 67P/Churyomov-Gerasimenko comet. This work present absorption cross section spectrum measured in the VUV/UV range, for pure organic films. These spectrum led to the deduction of photolysis rate constants, and to the elaboration of a model simulating the global kinetic of destruction of a optically thick organic film. The comparison between this model and experimental data of low earth orbit irradiation as well as laboratory data allowed to estimates lifetimes for the considered molecules at 1 AU, and then extrapolated at different heliocentrically distances. Results show that glycine, adenine and guanine, potentially existing inside the cometary grains, would be entirely destroyed between the ejection of the grains and the arrival on earth if they exist at the surface. Below the surface, they are at the contrary very stable, thanks the effective protection of the mineral constitutive of the grain against solar radiations. In the frame of ROSETTA mission, results differ. At the farther of the sun, at 3.5 AU, the abundance of the molecule would not significantly decrease during the time of travel of grains between the core and the orbiter. At the perihelia, the survival of molecule strongly depends of the core-orbiter distance. Significant loss of the 3 molecules by photochemistry would only occurred if the orbiter is at more than hundred of kilometers from the cor

Evolution of organic molecules under Mars surface-like UV radiation conditions simulated in space and laboratory

International audienceThe detection and identification of organic molecules at Mars are of primary importance for astrobiology, as some of these molecules are life precursors and components. While in situ exploration missions are searching for them at the surface of the planet, it is essential to understand how organic molecules evolve and are preserved at the surface of Mars. Indeed the harsh conditions of the environment of Mars, such as ultraviolet (UV) radiation or oxidative processes, due for example to iron-bearing minerals and perchlorate ions, could explain the low abundance and diversity of organic molecules detected so far [1][2]. In order to get a better understanding of the evolution of organic matter at the surface of Mars, we exposed organic molecules under Mars-like UV radiation and oxidative environments. Similar organic samples were exposed in two experiments: directly to the Sun radiation, outside the International Space Station (ISS) on the EXPOSE R2 facility; and under the flux of a wide range UV lamp in the laboratory with the MOMIE experiment. The EXPOSE R2 facility was placed in low Earth orbit (LEO), outside the International Space Station (ISS) in 2014, to be exposed to the Solar radiation. One of the EXPOSE R2 experiment, called PSS (Photochemistry on the Space Station), was dedicated to astrobiology- and astrochemistry-related studies. Several of the PSS samples were prepared for the study of the evolution of organic molecules under Mars-like surface radiation conditions. Organic samples have been exposed directly to the Sun under KBr filters (UV transmission >200 nm) from November 2014 to February 2016, mimicking the UV radiation conditions of the surface of Mars. Four types of samples were exposed in the form of thin layers of solid molecules: pure deposits of adenine, adenine mixed with nontronite (a clay mineral detected on Mars), pure deposits of chrysene and glycine mixed with nontronite.The MOMIE (Mars Organic Matter Irradiation and Evolution) experiment has been set up to study the evolution of organic matter under simulated martian UV radiation within the laboratory [3][4]. Organic samples are in this case exposed under a UV lamp (200-400 nm) as thin homogenous films (a fraction of μm thick). Adenine, chrysene and glycine were studied in this experiment as well, allowing us to compare their evolution with the results in LEO. Uracil was also studied. Perchlorate salts effect on organic evolution was investigated. To characterize the evolution of our samples, analyses by infrared spectroscopy (IRTF) were performed: before and after exposure in LEO, for the PSS samples, and continuously along the experiment for the MOMIE samples. These analyses allowed determining whether each molecule is preserved or photodegraded, and if so, its photolysis rate. Most of the studied molecules were rapidly degraded under Mars-like UV radiation. On the other hand, uracil seems to form new bigger compounds in Mars-like UV radiation conditions. The effect of nontronite and perchlorates on organic molecules preservation has been investigated as well. We also compared results from LEO with laboratory data

Evolution of organic molecules under Mars-like UV radiation conditions in space and laboratory

International audienceThe detection and identification of organic molecules at Mars are of prime importance, as some of these molecules are life precursors and components. While in situ planetary missions are searching for them, it is essential to understand how organic molecules evolve and are preserved at the surface of Mars. Indeed the harsh conditions of the environment of Mars such as ultraviolet (UV) radiation or oxidative processes could explain the low abundance and diversity of organic molecules detected by now [1]. In order to get a better understanding of the evolution of organic matter at the surface of Mars, we exposed organic molecules under a Mars-like UV radiation environment. Similar organic samples were exposed to the Sun radiation, outside the International Space Station (ISS), and under a UV lamp (martian pressure and temperature conditions) in the laboratory

The AMINO experiment: methane photolysis under Solar VUV irradiation on the EXPOSE-R facility of the International Space Station

International audienceThe scientific aim of the present campaign is to study the whole chain of methane photo-degradation, as initiated by Solar vacuum-ultraviolet irradiation in Titan's atmosphere. For this purpose, the AMINO experiment on the EXPOSE-R mission has loaded closed cells for gas-phase photochemistry in space conditions. Two different gas mixtures have been exposed, named Titan 1 and Titan 2, involving both N2-CH4 gas mixtures, without and with CO2, respectively. CO2 is added as a source of reactive oxygen in the cells. The cell contents were analysed thanks to infrared absorption spectroscopy, gas chromatography and mass spectrometry. Methane consumption leads to the formation of saturated hydrocarbons, with no detectable influence of CO2. This successful campaign provides a first benchmark for characterizing the whole methane photochemical system in space conditions. A thin film of tholin-like compounds appears to form on the cell walls of the exposed cells

The PROCESS Experiment: An Astrochemistry Laboratory for Solid and Gaseous Organic Samples in Low-Earth Orbit

International audienceThe PROCESS (PRebiotic Organic ChEmistry on the Space Station) experiment was part of the EXPOSE-E payload outside the European Columbus module of the International Space Station from February 2008 to August 2009. During this interval, organic samples were exposed to space conditions to simulate their evolution in various astrophysical environments. The samples used represent organic species related to the evolution of organic matter on the small bodies of the Solar System (carbonaceous asteroids and comets), the photolysis of methane in the atmosphere of Titan, and the search for organic matter at the surface of Mars. This paper describes the hardware developed for this experiment as well as the results for the glycine solid-phase samples and the gas-phase samples that were used with regard to the atmosphere of Titan. Lessons learned from this experiment are also presented for future low-Earth orbit astrochemistry investigations

Photochemistry of organic molecules in the Solar System: experimental studies outside the International Space Station. The cases of glycine, and nucleobases

Solar UV radiation is a major source of energy for initiating chemical evolution towards complex organic structures, but it can also photo-dissociate even the most complex molecules. Thus, solar UV can erase the organic traces of past life at the surface of planets, such as Mars, destroy organic molecules present on meteorites and micrometeorites, influence the production of distributed sources in comets or initiate chemistry in Titan's atmosphere. In the interstellar medium, the UV radiation field emitted by stars in the galaxy is also responsible for the chemical evolution and the extraordinary diversity of organic molecules detected. PSS (Photochemistry on the Space Station) was a Low Earth Orbit (LEO) experiment, implemented from mid-2014 to early 2016 on the EXPOSE-R2 platform outside the International Space Station. Its goal was to improve our knowledge about the chemical nature and evolution of organic molecules with astrobiological implications in space environments. It was a new step in a series of experiments conducted outside the MIR space station, in the ESA BIOPAN and previous EXPOSE facilities. In PSS, both vented and sealed cells were used allowing exposure of both solid and gaseous samples. Five kinds of experiments was carried out exposing molecules related to different environmental factors of astrobiological significance: the interstellar medium, comets & meteorites, Titan, Mars, as well as a set of samples to test the stability of biochips in space. In this talk we will describe the PSS experiment and focus on some results related to the stability of some prebioticaly relevant compounds such as glycine, the simplest amino acid, and nucleobases such as uracil, guanine and adenine. These molecules were both exposed in Low Earth Orbit and studied in the laboratory in order to derive their photochemical lifetime if they are ejected from comets on dust particles and orbit around the Sun before reaching the Earth as micrometeorites. The results can lead to better understand the contribution of cometary particles in the establishment of an organic reservoir on primitive Earth

Photochemical studies in low Earth orbit for organic compounds related to small bodies, Titan and Mars. Current and future facilities.

The study of the evolution of organic matter subjected to space conditions, and more specifically to solar photons in the vacuum ultraviolet range (120-200 nm) has been undertaken in low Earth Orbit since the 90's, and implemented on various space platforms. The most recent exposure facilities are BIOPAN outside the Russian automatic capsules FOTON, and EXPOSE-E & -R (1&2) outside the International Space Station. They allow the photolysis of many different samples simultaneously, and provide us with valuable data about the formation and evolution of organic matter in the Solar System (meteorites, comets, Titan's atmosphere, the Martian surface...) and in the Interstellar Medium. They have been used by European teams in the recent past(ORGANIC on BIOPAN V-FOTON M2 and UVolution on BIOPAN VI-FOTON M3, PROCESS on EXPOSE-E, AMINO and ORGANICS on EXPOSE-R), and a new EXPOSE set is currently exposed outside the ISS (PSS on EXPOSE-R2). These existing tools are very valuable; however, they have significant limitations that limit their capabilities and scientific return. One of the most critical issues for current studies is the lack of any in-situ analysis of the evolution of the samples as a function of time. Only two measurements are available for the experiment: one before and one after the exposure. A significant step forward has been achieved with the O/OREOS NASA nanosatellite and the OREOcube ESA project with onboard UV-visible measurements. However, for organic samples, following the evolution of the samples would be more informative and provide greater insight with infrared measurements, which display specific patterns characteristic of major organic functionalities in the mid-infrared range (4000-1000 cm-1)