Aims. This article aims at a quantitative characterization of the phase transition of porous amorphous solid water (ASW) to a non-porous, i.e., more compact structure over an astronomically relevant temperature regime. Methods. A new laboratory based method is described that monitors the ice thickness decrease by combining optical interference with Fourier transform infrared spectroscopy. Three different water ice morphologies are studied; porous ASW as primary target, and less-porous ASW as well as crystalline solid water for comparison. Results. The thickness of the porous ASW sample is found to decrease by 12?±?1% upon heating from 20 to 120 K. The thickness decrease of less-porous ASW is smaller, and negligible for crystalline solid water. Conclusions. Porous ASW, if formed under interstellar conditions, is expected to become less porous with increasing temperature. The thermally induced structural collapse affects the diffusion of the interstellar ice components, and therefore the catalytic properties of the ice.
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