Infrared study of ion-irradiated water-ice mixtures with hydrocarbons relevant to comets

Ethane, methane, and acetylene were detected in Comets C/1996 B2 Hyakutake and C/1995 O1 Hale-Bopp. In order to investigate the possibility of condensed-phase synthesis of C2H6 in interstellar ices, we have examined the role of CH4 and C2H2 in the radiation synthesis of C2H6 in laboratory-formed low-temperature ices. Mixtures such as H2O + CH4, H2O + C2H2, H2O + CH4 + C2H2 were irradiated. Many mixtures were investigated using different concentrations of hydrocarbons. Results showed that yields of C2H6 from CH4 due to dimerization, were similar to yields of C2H6 from C2H2 due to H-addition reactions. However, CH4 was not a source of C2H2, suggesting that C2H2 is part of the natal chemistry of these comets. Both CH3OH and C2H5OH were identified in our experiments along with CO, CO2, HCO, H2CO, and CH3CHO. (C) 1998 Academic Press.