Evolution of the effective IR band strengths, normalised to the corresponding band strength in the pure ice, as a function of the CO₂:H₂O molecular ratio. Filled symbols indicate results from this work. Squares are for H₂O:CO₂ binary mixtures, inverted triangles are for H₂O:CO₂:CO ternary mixtures with 20% CO:H₂O, circles are for H₂O:CO₂:CO ternary mixtures with 40% CO:H₂O ratio. The coloured lines indicate the main trends and are drawn only as a guide for the eye. Open squares correspond to CO₂:H₂O binary mixtures from the literature, which in the upper panel are from Oberg et al. (2007), while in the middle panel they are from Gerakines et al. (1995)