C₂O (blue circles)and C₃O (red triangles) column densities with respect to the initial CO column density (CO_i) as a functionof fluence (ions cm⁻², bottom x-axis). The top x-axes gives 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 the timescales are reported near each x-axis in the figure.