Cosmic Ray Compaction of Porous Interstellar Ices

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 [image] eV Aa super(-1). The experiments roughly follow a universal dependence of ion-induced compaction with restricted dose (eV molecule super(-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 sub(2), we estimate that porous ice mantles on grains in dense molecular clouds are compacted by cosmic rays in [image]10-50 million years.