Abstract
Cometary nuclei are the least modified solar system bodies remaining from the time of planetary formation. Their current compositions are thought to be representative of the materials from which they formed. In the observation of comets Hyakutake1) and Hale-Bopp, C2H4 has not been detected by infrared spectroscopy despite of the strong intensities of C2 hydrocarbons, C2H2 and C2H6. In the present work, the reaction of H atoms with solid thin film of C2H2 at 10 K has been studied by means of thermal desorption mass spectrometry and FT-IR spectroscopy in order to obtain information on the chemical evolution on the cold dust grains. It was found that C2H6 is only the major product and the intermediate C2H4 could not be detected. This finding is in accord with the failure of the observation of C2H4 in the coma of the comets Hyakutake and Hale-Bopp.
