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Accueil > Français > Équipes > Astrochimie, Spectroscopie, Théorie, Réactivité, Origine > Projets

Projet RARICI : Radical Analysis and Reactivity in Interstellar and Cometary Ices

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T. Butscher (PhD student), Pr. T. Chiavassa, Dr. G. Danger, Dr. F. Duvernay

Through this project, we focus on the evolution and the increase of complexity of the organic matter of evolution in the stellar nebulae until it is incorporated in the various constituents a planetary system objects such as comets and asteroids. Understanding the evolution and composition of this organic matter is important because comets form a reservoir exogenous organic matter. This reservoir is also considered to have played a significant role in the chemical evolution that preceded the emergence of biochemical system in the environment of the early Earth. More specifically, in this project we want to investigate radical chemistry in interstellar analogues by the means of low temperature electron paramagnetic resonance spectroscopy (EPR).

The objective of the project is to understand the chemical evolution of organic matter formed in the interstellar medium until it is incorporated within planetary systems and determine how this material could play a role in the development of a prebiotic chemistry on the surface of terrestrial planets. Part of our research is therefore to reproduce the first materials of the universe that are the grains of interstellar dust/cometary grains and the various energy processes (thermal, photochemical effects ..) to which they were subjected. Our goal here is to understand the chemistry leading to the formation of complex organic molecules (COMs) detected in the star-forming regions, comets or meteorites.
Among these molecules, some interest in the development of a prebiotic chemistry in terrestrial planet surface were detected during chemical analysis of some meteorites (carbonaceous chondrites) or cometary grains (Stardust mission), such as amino acids that are "building blocks" of life on Earth. [1] However, questions regarding their origin and formation mechanisms are still unanswered. With innovative laboratory simulations, we will determine the reaction pathways to explain the presence of complex organic molecules in the cometary environments, but also to propose new molecules to be detected. Ultimately, the results from these laboratory simulations may be useful in the analysis of data from space missions (i.e the Rosetta mission).
The originality of our research lies in the development of experimental laboratory systems to simulate the evolution of organic matter in different astrophysical environments from the interstellar medium to the chemistry within cometary environments. Our research has further aims to connect these different environments to establish an overall pattern of evolution of the organic material (Fig. 1). Thus, thought this project, we will get a better understanding of various processes that lead to the formation of this organic matter and determine to what extent this development can be seen as a precursor to life on the early Earth. [2] In this project we intend to develop an analysis Electronic Paramagnetic Resonance (EPR) at low temperature of irradiated cometary ice analogs, to study more precisely the reactivity of radicals formed in the cometary ices by VUV radiation of our Sun. Indeed complex organic molecules formed in cometary nuclei come from a radical initiated by short wavelength VUV photons chemistry (λ <200 nm) (Fig 1). While it is admitted by the scientific community of the importance of such reactions in the formation of complex organic molecules, we lack experimental data on reactivity of these species at low temperature conditions simulating those of cometary nuclei [3]. Our goal here, thanks to the low temperature EPR spectrometry (4K) to characterize the radicals formed (life time, reactivity ...) during the UV irradiation analogs cometary ices. These new experiences will allow us to more accurately understand the complex chemistry of cometary environments.

Fig. 1 : evolution of organic matter in cometary environments and formation of complex organic molecules.

Experimental studies on the reactivity of radicals will be supplemented by theoretical studies by the team and Theoretical Chemistry Model (OCTs) of ISM2 laboratory at the Aix-Marseille University. Indeed, some kinetic and mechanistic parameters will not be accessible by the experience because of the short lifetime of some reaction intermediates. Quantum calculations made by methods ab-initio and DFT will allow us to lift the shadows and refine our understanding of the stability and reactivity of radical species formed after VUV irradiation. Moreover, these theoretical methods to calculate the infrared spectra of these species that can be directly compared with our experimental infrared spectra. This complementarity between experience and theory has been successfully undertaken in the past. We actually determined by the contribution of theoretical chemistry the formation mechanism of the hexamethylentetramine (HMT) under conditions simulating cometary environments [4].

Originality of this project

The analyzes of meteoritic and cometary samples revealed the presence of a previously unsuspected molecular diversity. Over one hundred was indeed detected complex organic molecules in meteorites including amino acids and nucleosides, often regarded as elements of the living blocks.
Many laboratories in France or abroad (NASA, Leiden observatory ...) work on laboratory simulations to train and to characterize this complex organic matter comet. But these experiences are mainly oriented towards identifying COMs (not radical species) by spectroscopic techniques (IR, MS) or chromatographic (GC-MS, LC-MS). In this project, our approach will be different. We want to understand the mechanisms of formation of COMs and specifically the role of radical chemistry within interstellar ices/ cometary grains. For this, we will deposit at low temperature (4 K-10 K) ice analogs with compositions close to those of interstellar/cometary ices containing H2O, CO, CO2, CH3OH. They will be then irradiated with VUV plasma source to form the radical precursors of complex organic molecules. The originality of this project lies in the choice of the method of analysis. Traditionally this type of study, infrared (IR) spectroscopy is used for in situ characterization of irradiated cometary ices because results are directly comparable to IR spectra recorded by space telescopes. However by this analytical technique, only stable products are observed and little information on the nature and reactivity of radicals can be obtained. Our choice concerning the analytical method has therefore focused on the EPR which allows the characterization of the radicals. This technique is commonly used in many areas of research for the characterization of radicals but not yet in the field of astrochemistry. Using this technique, we can thus study the radical species formed during the irradiation to determine their life and their reactivity depending on the temperature of the ice. Furthermore the determination of reaction pathways leading to the formation of complex organic molecules will be also studied.