UV-photoprocessing of interstellar ice analogs: Detection of hexamethylenetetramine-based species

The physical conditions governing the dense cloud environment are reproduced in a high vacuum experimental setup at low temperature T approximate to 12 K. The accretion and photoprocessing of ices on grain surfaces is simulated by depositing an ice layer on a cold finger, while it is irradiated by ultraviolet (UV) photons. After irradiation the sample is slowly warmed to room temperature; a residue remains, containing the most refractory products of photo- and thermal processing. In this paper we report on the analysis of the residues performed by means of gas chromatography-mass spectrometry (GC-MS). A number of new molecules based on hexamethylenetetramine (HMT, C6H12N4), the most abundant component of the residues reported here, were detected: methyl-HMT (C6H11N4-CH3), hydroxy-HMT (C6H11N4-OH), methanyl-HMT (C6H11N4-CH2OH), amin-aldehyd-HMT (C6H11N4-NH-CHO) and methanyl-aldehyd-HMT (C6H11N4-CHOH-CHO). To the best of our knowledge, this is the first reported synthesis of these molecules. Currently, these are the heaviest identified components of the residue. These species might also be present in the interstellar medium, given that the ice was submitted to high temperatures, of the order of 300 K, and form part of comets. Our work serves as preparation for the ESA-Rosetta mission, which plans to do in situ analysis of the composition of a comet nucleus with the COSAC instrumentation.