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The morphology of water ice in the interstellar medium is still an openquestion. Although accretion of gaseous water could not be the only possibleorigin of the observed icy mantles covering dust grains in cold molecularclouds, it is well known that water accreted from the gas phase on surfaceskept at 10 K forms ice films that exhibit a very high porosity. It is alsoknown that in the dark clouds H2 formation occurs on the icy surface of dustgrains and that part of the energy (4.48 eV) released when adsorbed atoms reactto form H2 is deposited in the ice. The experimental study described in thepresent work focuses on how relevant changes of the ice morphology result fromatomic hydrogen exposure and subsequent recombination. Using thetemperature-programmed desorption (TPD) technique and a method of inversionanalysis of TPD spectra, we show that there is an exponential decrease in theporosity of the amorphous water ice sample following D-atom irradiation. Thisdecrease is inversely proportional to the thickness of the ice and has a valueof Phi_0 = 2 x 10^16 D-atoms/cm^2 per layer of H2O. We also use a model whichconfirms that the binding sites on the porous ice are destroyed regardless oftheir energy depth, and that the reduction of the porosity corresponds in factto a reduction of the effective area. This reduction appears to be compatiblewith the fraction of D2 formation energy transferred to the porous ice network.Under interstellar conditions, this effect is likely to be efficient and,together with other compaction processes, provides a good argument to believethat interstellar ice is amorphous and non-porous.

Changes in the morphology of interstellar ice analogues after hydrogen atom exposure