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From atoms to prebiotic molecules:
The solid phase of interstellar medium
Part of my research is focused on the formation of complex organic molecules in interstellar ices, by experimental investigations. Formed in the most unlivable place during stellar formation, interstellar molecules are relevant for the origin of life.
Illustration of stellar formation
Stellar evolution starts with giant molecular clouds where the star will born. These molecular clouds are the reservoir of gas and dust available for the formation of new planetary systems (star/planets/small bodies...). Gas (mainly H2) represents 99% in mass of the molecular cloud, while dust (or grains), represents the 1% rest. Despite the small quantity of dust, these small grains (up to 100 micron) are very important for the formation of molecules, starting with H2.
Due to low temperature in the molecular cloud environments (10 Kelvin), atoms and molecules from the gas phase tend to freeze out on the solid phase, the grain. An icy mantle is so formed on the silicate/carbonaceous grain and reactions can start between the atoms/molecules in ice. During stellar formation, the icy grains will be submit to many different energetic processes, affecting the initial chemical composition. These processes are irradiations (UV, particles), heating, leading to reactions, sublimations, desorptions, and so on...
Finally all these processes modify the chemical composition of the solid phase, forming complex organic molecules that can be incorporated to the solid matter of the new planetary systems. Primitives bodies of our solar system (comets, asteroides) have accreted the grains from the protosolar disk and so the initial matter. The earth was bombarded during the few millions years by these small bodies, bringing extraterrestrial matter that could have participate to the prebiotic chemistry and so contributed to the origin of life.
The chemical reactions that could happen during all these steps and under all these energetic processes in the interstellar ices are investigated by laboratory experiments. Starting from molecules observed by infrared spectroscopy in molecules clouds, such as water, ammonia and methanol, we reproduce in the lab interstellar ice analogues. Usually, after irradiation at low temperature (10 K), and heating until 300 K, a residue is left over, (in the past called yellow stuff). This organic residue is composed of thousand of different molecules (Danger et al., 2013, GCA, 118 (184-201) and Danger et al. 2016, 189 (184-196). Due to its diversity and for being a result of interstellar ice evolution during solar formation, such organic residue has been proposed to be part of the accreted organic matter in solar system bodies. Comets can contain organic matter coming from interstellar ices less altered than in the organic residue since they have accreted materials of lower temperature (50 K- 200 K). Yet, the formation of these molecules found in organic residue is still not fully understand.
Among the energetic processes that affects the organic matter in ices, I focused my studies on the thermal process. I found that many complexes organics molecules could be formed in the conditions of the interstellar medium, from simple molecules (methanol, water, ammonia, formaldehyde, methylamine) submitted only to thermal process (between 10 K and 300 K).
The figure on the right represents a part of these different chemical reactions, studied in our team, that could happen during stellar formation (ASTRO-PIIM credits)
My work has concerned the black arrows and some red arrows, which all correspond to thermal reactions (acid-base reactions, polymerization, addition, cyclization). Especially I precisely determined the chemical mechanism formation of the hexamethylenetetramine (HMT) molecule in solid phase. HMT is a molecule typically found in the organic residues, with more than 50%wt, and represent a great interest for small body composition and prebiotic chemistry. HMT has been found in meteorites quite recently, and confirmed the interstellar heritage of a part of organic matter in solar system objects.
Network reaction in interstellar ices, PIIM Astrochemistry Laboratory
Many more researches are under going on the evolution of interstellar ices to, may be one day, we will understand the pathway of the thousand of molecules formed during the evolution of ice, and found in the organic residue.
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