Methylammonium methylcarbamate thermal formation in interstellar ice analogs: a glycine salt precursor in protostellar environments

Context. Analyses of dust cometary grains collected by the Stardust spacecraft have shown the presence of amines and amino acids molecules, and among them glycine (NH2CH2COOH). We show how the glycine molecule could be produced in the protostellar environments before its introduction into comets. Aims. We study the evolution of the interstellar ice analogues affected by both thermal heating and vacuum ultraviolet (VUV) photons, in addition to the nature of the formed molecules and the confrontation of our experimental results with astronomical observations. Methods. Infrared spectroscopy and mass spectrometry are used to monitor the evolution of the H2O:CO2:CH3NH2 and CO2:CH3NH2 ice mixtures during both warming processes and VUV photolysis. Results. We first show how carbon dioxide (CO2) and methylamine (CH3NH2) thermally react in water-dominated ice to form methylammonium methylcarbamate [CH3NH3+][CH3NHCOO-] noted C. We then determine the reaction rate and activation energy. We show that C thermal formation can occurs in the 50-70 K temperature range of a protostellar environment. Secondly, we report that a VUV photolysis of a pure C sample produces a glycine salt, methylammonium glycinate [CH3NH3+][NH2CH2COO-] noted G. We propose a scenario explaining how C and subsequently G can be synthesized in interstellar ices and precometary grains. Conclusions. [CH3NH3+][CH3NHCOO-] could be readily formed and would act as a glycine salt precursor in protostellar environments dominated by thermal and UV processing. We propose a new pathway leading to a glycine salt, which is consistent with the detection of glycine and methylamine within the returned samples of comet 81P/Wild 2 from the Stardust mission.