2006A&A...453..903P

We used infrared absorption spectroscopy to study the effects of ion irradiation on the morphology/porosity of amorphous water ice. Thin icy films (about 0.25µm) of amorphous water were irradiated with 200keV protons at 15K. Both the behaviour of the OH dangling bond feature and the ability to trap carbon monoxide (CO) were used to investigate the evolution of icy samples after ion irradiation. We show that the intensity of the OH dangling bond feature decreases after ion irradiation and that the amount of absorbed carbon monoxide decreases as the fluence of impinging ions increases. The results obtained indicate that the porosity of amorphous water ice decreases after ion irradiation. Furthermore, icy mixtures such as H₂O:CO₂, H₂O:CO, and H₂O:CH₄ were irradiated with 200keV H⁺, 30 and 200 keV He⁺ ions. Also in these cases, the intensity of the OH dangling bond band decreases after ion irradiation. However, when a second molecular species is present in the ice sample, this decrease is slower. Here we present the experimental results and discuss their relevance to our understanding of the properties of interstellar water ice. In particular, we suggest that, because of cosmic ion bombardment, water ice in interstellar grain mantles is compact in structure.