  The morphology of water ice in the interstellar medium is still an open
question. Although accretion of gaseous water could not be the only possible
origin of the observed icy mantles covering dust grains in cold molecular
clouds, it is well known that water accreted from the gas phase on surfaces
kept at 10 K forms ice films that exhibit a very high porosity. It is also
known that in the dark clouds H2 formation occurs on the icy surface of dust
grains and that part of the energy (4.48 eV) released when adsorbed atoms react
to form H2 is deposited in the ice. The experimental study described in the
present work focuses on how relevant changes of the ice morphology result from
atomic hydrogen exposure and subsequent recombination. Using the
temperature-programmed desorption (TPD) technique and a method of inversion
analysis of TPD spectra, we show that there is an exponential decrease in the
porosity of the amorphous water ice sample following D-atom irradiation. This
decrease is inversely proportional to the thickness of the ice and has a value
of Phi_0 = 2 x 10^16 D-atoms/cm^2 per layer of H2O. We also use a model which
confirms that the binding sites on the porous ice are destroyed regardless of
their energy depth, and that the reduction of the porosity corresponds in fact
to a reduction of the effective area. This reduction appears to be compatible
with 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 believe
that interstellar ice is amorphous and non-porous.
