CARBON DIOXIDE INFLUENCE ON THE THERMAL FORMATION OF COMPLEX ORGANIC MOLECULES IN INTERSTELLAR ICE ANALOGS
Résumé
Interstellar ices are submitted to energetic processes (thermal, UV, and cosmic-ray radiations) producing complex
organic molecules. Laboratory experiments aim to reproduce the evolution of interstellar ices to better understand
the chemical changes leading to the reaction, formation, and desorption of molecules. In this context, the thermal
evolution of an interstellar ice analogue composed of water, carbon dioxide, ammonia, and formaldehyde is
investigated. The ice evolution during the warming has been monitored by IR spectroscopy. The formation of
hexamethylenetetramine (HMT) and polymethylenimine (PMI) are observed in the organic refractory residue left
after ice sublimation. A better understanding of this result is realized with the study of another ice mixture
containing methylenimine (a precursor of HMT) with carbon dioxide and ammonia. It appears that carbamic acid, a
reaction product of carbon dioxide and ammonia, plays the role of catalyst, allowing the reactions toward HMT
and PMI formation. This is the first time that such complex organic molecules (HMT, PMI) are produced from the
warming (without VUV photolysis or irradiation with energetic particles) of abundant molecules observed in
interstellar ices (H2O, NH3, CO2, H2CO). This result strengthens the importance of thermal reactions in the ices’
evolution. HMT and PMI, likely components of interstellar ices, should be searched for in the pristine objects of
our solar system, such as comets and carbonaceous chondrites.
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