Icy grain mantles

Many kinds of organic molecules liave been detected in interstellar clouds. The formation mechanisms of these molecules have been one of the most significant issues in cosmochemistiy. Recent tlieoretical and experimental studies suggest that tlie important mechanisms are ion-molecular reactions in tlie gas phase [59], grain surface reaction in dusts [60], gas-pliase pyrolysis [61], and photochemical reaction in icy grain mantles [62]. The ion-molecular reaction results from a collision of ions and molecules in a gas phase. Because of tlie lack... 

Ice mantles are important constituents of interstellar grains in molecular clouds, and icy bodies dominate the outer reaches of the solar system. The region of the solar system where ices were stable increased with time as the solar system formed, as accretion rates of materials to the disk waned and the disk cooled. The giant planets and their satellites formed, in part, from these ices, and probably also from the nebular gas itself. 

Interstellar grains with ice mantles probably comprised a significant amount of the material that collapsed to form the solar nebula. Heating of this material caused the icy mantles to sublimate, producing a vapor that subsequently condensed as crystalline ices as the nebula cooled. By mass, H20 ice rivals rock in terms of potentially condensable matter from a gas of cosmic composition. The amount of water ice depends, of course, on the extent to which oxygen is otherwise tied up with carbon as CO and/or C02 (Prinn,... 

Solid icy surfaces are observed both in the interstellar medium as mantles on silicatic or carbonaceous grains and on many objects in the Solar System." In space, these icy targets are continuously bombarded by energetic ions from solar wind and flares, planetary magnetospheres, stellar winds and galactic cosmic rays. When an energetic ion collides with an icy target produces physico-chemical modifications in the latter. The study of those effects is based on laboratory ion irradiation experiments carried out under physical conditions as close as possible to the astrophysical ones. 

Chemical models of dense interstellar clouds have grown in complexity in recent years (see Herbst, this volume). However, since molecules are widely observed in the gas phase, many modellers have chosen to ignore the effects of accretion. Gas phase atoms and radicals, upon collision with cold dust grains present in the interstellar medium, are expected to stick to the dust forming an icy mantle and thus leaving the gas phase. This is thought to occur on a timescale comparable to the expected lifetime of the clouds. 

---