Phase Transition of Interstellar CO Ice

Among the over 200 molecular species identified in interstellar clouds, many are organic molecules. It has been proposed that some of these molecules survive the star and planet formation process and are eventually delivered to Earth where they can form the molecular basis of the origin of life. It is now well established that one of the most important factories of these molecules are ice mantles that cover the dust grains in star-forming molecular clouds. Simple atoms and molecules such as H, O, N, and CO condense from the gas phase onto the grain surface and then react with each other in the ice to form increasingly complex molecules. At the extremely low temperature (10-15 K) in these clouds, the widely accepted mechanism to bring reactive species together - diffusion - is severely impeded in the ice, raising the question of the mechanism of their formation. In laboratory experiments we find that the top layers of the ice mantle, which are made primarily of CO, transform from a disordered phase to a polycrystalline phase at such a low temperature. During the phase transition, reactive species buried inside may migrate and react without the need to overcome activation energy for diffusion. By quantifying the kinetics of crystallization, we predict that CO ice in interstellar clouds is mostly in the polycrystalline form. The reorganization of CO ice, which occurs below 10 K, may promote mobility of reactive species, and therefore can be a driving force of molecular complexity in molecular clouds.