Simulation results of [O₃]/[O₂]₀ irradiated by 5 keV protons at 12 K, based on model A (solid line) and model S19 (dashed line). The shaded gray region shows the range of steady-state [O₃]/[O₂]₀ obtained by Ennis et al. (2011) with an uncertainty of a factor of three. The vertical gray line indicates the fluence of 7.6 × 10¹⁵ ions cm⁻² which was required to reach the steady state in the experiment of Ennis et al. (2011).