Interstellar Ices and Solid-state Chemistry as a Route to Molecular Complexity

Abstract

Dust grains in shielded regions of quiescent interstellar clouds are observed to accumulate mantles of simple ices containing molecules of H2O, CO, and CO2, with minor amounts of CH3OH, NH3, and a few other species. Some of these simple ice molecules (e.g., CO) may simply adhere to the surface from the gas. H-additions to surface species such as atomic oxygen may generate abundant ice molecules such as H2O. Other reactions in these simple ices may generate species such as CH3OH and CO2. All of these simple ice species are important in interstellar chemistry because they can be the molecular feedstock from which more complex species may form in solid-state chemistry that occurs when the ice is suitably energized by UV, X-ray, or energetic particle irradiations. Products of greater complexity promoted by these energy inputs may include acetaldehyde (CH3CHO), dimethyl ether (CH3OCH3), ethanol (CH3CH2OH), and ethylene glycol (CH2OH)2. We discuss some experimental evidence for such chemical processing. Some of these more complex species arising in these experiments may even be of astrobiological interest, such as glycine (NH2CH2COOH) and formamide (HCONH2).