Critical saturation ratio S_crit as a function of semi-major axis for the full extent of our simulations shown for bare silicate particles (heterogeneous ice nucleation) in black and water-ice-covered particles (depositional ice growth) in blue. Also shown is the ice line location, where S = 1, as a grey dashed line. The upper horizontal axis shows corresponding disc temperatures, increasing inwards as . Critical saturation ratios for silicate in the temperature range 160 ≤ T ≤ 179 are experimental data from Iraci et al. (2010).