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We studied the compaction of microporous vapor-deposited ice films under irradiation with different ions in the 80-400 keV energy range. We found that porosity decreases exponentially with irradiation fluence, with a mean compaction area per ion that scales linearly with the stopping power of the projectile S above a threshold S t = 4 eV Å −1 . The experiments roughly follow a universal dependence of ion-induced compaction with restricted dose (eV molecule −1 ). This behavior can be used to extrapolate our results to conditions of the interstellar medium. Relating our results to ionization rates of interstellar H 2 , we estimate that porous ice mantles on grains in dense molecular clouds are compacted by cosmic rays in ~10-50 million years.
Raut et al. (2008) studied this question.