Laboratory studies of the formation of methanol and other organic molecules by water plus carbon monoxide radiolysis: Relevance to comets, icy satellites, and interstellar ices

Radiation processing of cometary, planetary, and interstellar ices has been investigated by irradiating mixtures of H(2)O and CO near 16 K with 0.8-MeV protons. IR spectroscopy and isotopic substitution showed that H and OH, from H(2)O, added to CO to form HCO, H(2)CO, HCOOH, and CH(3)OH. A values (integrated spectral absorbances) for HCOOH and HCO trapped in H(2)O ice were measured for the first time. These new values, along with published A's for H(2)CO and CH(3)OH, were used to calculate radiation yields (G values) and conventional percentage yields. Significantly higher percentage yields of HCOOH and CH(3)OH were observed, compared to previous solid-state H(2)O + 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.