tokens: with consistent dust/ice opacities and dust disk thermal structure. We demonstrate that the photodesorption of water ice is key in the quantitative interpretation of the peak strengths of the 45 and 63 μ m features. The water ice features originate from grains smaller than ~10 μ m. The water ice features originate predominantly at the surface snow line of the disk between 10 and 100 au, which means that they are very robust against several disk parameters such as inner and outer radii, the grain size distribution (within limits), and dust settling. However, the feature strength scales with disk dust mass in the range of typical T Tauri disks. The features also change substantially with the thickness of the ice mantle, although the strength levels off for mantles much thicker than 2.5 times the original grain sizes. Most interestingly, the features do carry an imprint of the thermal history of the ice and thus can distinguish between a warmup scenario for the water ice (where water ice is formed in cold, optically thick regions and then moves to warmer areas in the disk) and a cooldown scenario (where water ice is formed in warm areas close to the midplane snow line and then moves outwards to colder areas in the disk). The direct deposit scenario (ice stays where it was formed locally in the disk) resembles qualitatively the warmup scenario and while the peak strength change of the 45 μ m feature maynot allow a discrimination, the shift in the peak wavelengths to shorter values might be a discriminator. Given the results of our ice model series, simulations using the sensitivities for the planned SOFIA/HIRMES instrument show that the lowresolution and the sensitivity will lead to severely limited ice studies – beyond simple detection – in T Tauri disks at the distance of typical star forming regions. The 45 μ m ice feature is strong enough to be detected if it originates from warm grains with thick ice mantles that are crystalline in nature. Similar simulations for the SPICA/SAFARI instrument show that the differences in peak strengths and the wavelength shifts in the peak position can both be easily measured in samples of several hundred T Tauri disks (exposure times of 10 min). At low spectral resolution ( R = 300) weak flat ice features at 45 μ m (e.g., amorphous, direct deposit, warmup of ice) require a good continuum characterization in deep surveys. Appendix A SEDs and disk structures A.1 Disk inner radius and tapering-off radius Figures A.1 and A.2 illustrate the temperature distribution in the disk models with varying inner radius and tapering-off radius. To assess where the water ice features originate, we also show the snow line and the vertical optical depth τ = 1 contours for 45 and 63 μ m and the radial optical depth τ = 1 at 0.55 μ m. A.2 Dust settling Figure A.3 shows the 2D temperature distribution for disk models with various degrees of settling ( α turb = 10 −2 to 10 −4 ). The change in opacities is strongest in the outer
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