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

Université de Provence, Laboratoire de Physique des Interactions Ioniques et Moléculaires, Centre de St-Jérôme, Av Escadrille Normandie-Niemen, 13397 Marseille, France
e-mail: fabrice.duvernay@univ-provence.fr

Received: 10 February 2011
Accepted: 5 April 2011

Abstract

Aims. We investigate the purely thermal formation of hexamethylenetetramine (HMT, C₆H₁₂N₄) in interstellar ice analogs from non-photolysed 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.