New insight into the formation of hexamethylenetetramine (HMT) in interstellar and cometary ice analogs

Aims. We investigate the purely thermal formation of hexamethylenetetramine (HMT, C6H12N4) in interstellar ice analogs from nonphotolysed ice and compare our results with those for the formation from photolysed ice. Methods. We use Fourier transform-infrared spectroscopy to follow residue formation from VUV irradiation of H2CO:NH3 ice mixture in different concentration ratios. We also report the warming of the H2CO:NH3:HCOOH ice mixture. Results. We present the characterization of organic residues obtained at 330 K from VUV irradiation of H2CO:NH3 ice mixtures. The organic residues contain compounds related to polyoxomethylene (POM, [-CH2-O-]n) and HMT (C6H12N4). We report, for the first time, the formation of HMT from the warming of an interstellar ice analogs, H2CO:NH3:HCOOH, without any energetic processing (i.e. photons or particles). New insights into HMT formation mechanism are proposed. These results strengthen the hypothesis that HMT is present in interstellar grains or in comets, where it may be detected with the COSAC instrument of the Rosetta mission.