Laboratory Studies of the Formation of Methanol and Other OrganicMolecules by Water+Carbon Monoxide Radiolysis: Relevanceto Comets, Icy Satellites, and Interstellar Ices

Radiation processing of cometary, planetary, and interstellar ices has been investigated by irradiating mixtures of H2O and CO near 16 K with 0.8-MeV protons. IR spectroscopy and isotopic substitution showed that H and OH, from H2O, added to CO to form HCO, H2CO, HCOOH, and CH3OH. A values (integrated spectral absorbances) for HCOOH and HCO trapped in H2O ice were measured for the first time. These new values, along with published A’s for H2CO and CH3OH, were used to calculate radiation yields (G values) and conventional percentage yields. Significantly higher percentage yields of HCOOH and CH3OH were observed, compared to previous solid-state H2O+CO experiments. This suggests that radiation processing may help explain the discrepancy between the observed gas-phase abundances of small organic molecules and the abundances predicted by current theoretical models and previous laboratory experiments. In contrast to previous experiments on one-component ices, no spectral evidence for long-chain molecules was found in the present work. This demonstrates that ice composition is a critical factor in applying laboratory results to cometary, icy satellite, and interstellar ices. c © 1999 Academic Press