Temporal evolution of the number of water molecules within the FOV computed with our time-dependent models of sublimating icy grains. Curves are displayed for pure ice grains (solid line), three-layer grains with a fractional mass of the mantle component α_ice = 0.9 (dash-dotted-dotted line) and two-layer (Ol) grains with α_ice = 0.98 (long-dashed line). The outputs from the models with rarefied medium and dense medium are shown in red and black, respectively. The filled triangles are the number of water molecules derived from the analysis of the Spitzer spectra. The model results have been adjusted so that the curves peak at (3.5  ±  0.8)  ×  10³¹ molecules, which corresponds to the observed value at TI + 0.67 h.