HMT production and sublimation during thermal process of cometary organic analogs. Implications for its detection with the ROSETTA instruments

Abstract

One important component of refractory organic residues synthesized from interstellar/cometary ice analogues is hexamethylenetetramine (HMT, C6H12N4). However, HMT has never been observed in any astrophysical or planetary environment so far. We investigated thermal evolution of HMT above ambient temperature. The synthesis of the organic residue (ice deposition, photolysis and warming) as well as its heating to temperatures higher than 300K are performed by means of the same experimental apparatus. The later also allows in situ continuous monitoring of both the solid organic residue (by FTIR spectrometry) and of the gas species (by mass spectrometry).Two different ice mixtures, composed of H2O:CH3OH:NH3=10:1:1 and H2O:CH3OH:NH3:CO2=10:1:1:2, were deposited and simultaneously photolyzed at 29K. Warming these photolyzed ices up to 300K allows the production of refractory organic residues. At 300K the organic residues clearly show the presence of HMT, but also some difference, in particular in their oxygenated components. Different evolutions of the organic residues are observed for temperatures >300K.We characterized the organic residue thermal evolution for temperatures up to 500K. We observed that HMT is still produced at temperatures higher than 300K. Production of solid HMT and sublimation are simultaneous. HMT observed in the solid phase could be only a minor fraction of the total HMT production, the major fraction being sublimated. The kinetics of the HMT thermal evolution strongly depends on the organic residue composition at 300K and seems to depend on the exact nature of the oxygenated fraction of the organic residue. The maximum temperature at which solid HMT is observed is 450K. As HMT forms only for temperatures greater than 280K in laboratory conditions, it implies that the detection of solid HMT in extraterrestrial samples will provide a strong indication of their thermal history. Consequently, the search for HMT in both solid cometary grains and gaseous phases in the coma of Comet 67P/Churyumov–Gerasimenko will have to be performed by the Rosetta space mission, in particular with the COSIMA and ROSINA instruments.