C₂O/C₃O ratio as a function of fluence (ions cm⁻², bottom x-axis) obtained after ion irradiation of pure CO (black squares), CO:N₂ = 1:1 mixture (redcircles), CO:N₂ = 8:1 (green triangles) and CO:N₂ = 1:8 (blue rhombus). Top x-axes give an estimate of the time (in 10⁶ yr) necessary for interstellar ices to undergo the effects observed in the laboratory. Values of cosmic-ray ionization rate used to calculate timescales are reported near each x-axis in the figure. The green box represents the range of observed C₂O/C₃O ratios in star-forming regions (including the errors. Ratios are given in Table 1.