Predicted CN column density for different initial distributions of nitrogen (columns) and sublimating ices (rows). Horizontal black dotted line indicates the upper limit on the CN column density derived from the observations. The initial H₂O abundance needs to be at least 1.9 × 10⁻⁵ (if all nitrogen starts as N₂) or 3.8 × 10⁻⁵ (if nitrogen starts as NH₃, N, or a mix of NH₃, N and N₂. Similarly, the initial (additional) abundance of NH₃ is ≲ 2.6 × 10⁻⁴ to match the CN upper limit.