Predicted NO column density for different distributions of nitrogen and sublimating ices. Nitrogen initially starts all in N₂ (left column), N (second column), NH₃ (third column), and a mixture (fourth column). The effect of sublimating ices is presented in the different rows: sublimating water (top row), water depletion (second row), extra OH (third row), sublimating water and NH₃ simultaneously (fourth row), sublimating NH₃ (fifth row), and sublimating NO (final row). The horizontal black line indicates the NO column density derived from the observations. Only the models with a high initial NO abundance and all nitrogen initially in N₂ or NH₃ match the observed NO column density.