χ² contour plot for fitting the model parameters (parameterizing the ability of CO₂ to switch position with a H₂O molecule) and c_CO2 (parameterizing the CO₂ ice thickness where swapping is fast compared to the investigated heating rates) using the experimentally determined amount of CO₂ trapped in a binary H₂O:CO₂ ice (Exps. 4 to 12). These two parameters regulate the distribution of the volatile molecules in the gas, surface and mantle according to Eqs. (11) and (10) of Sect. 2.1.