CO Diffusion in Layered Water Ices

The mobility of molecules in ice mantles on interstellar dust grains regulates the ice chemistry, and therefore much of the overall chemical evolution during star formation. Interstellar ices are typically dominated by water, followed by CO and CO2. This study aims to quantify the rate and barrier for CO diffusing into water ice and a water:CO2 ice mixture at low temperatures (15-23K), by measuring the mixing rate of CO and water in initially layered ices maintained at a constant temperature. The mixing fraction of CO as a function of time is determined by monitoring the infrared CO stretching band. Mixing is observed at all investigated temperatures on minute to hour time scales. The mixing rate and final mixing fraction depends on ice temperature, porosity, thickness and composition. Under the assumption that mixing is due to CO diffusion, the temperature dependence of the experiments are used to calculate the CO diffusion barrier to 100–200 K, significantly lower than the value used in current astrochemical models. Fick’s diffusion equation is then applied to determine an analytic solution to CO diffusion in water ice and discuss the implications for our understanding of diffusion in ices more generally as well as the astrochemical implications.