  Aims. The mobility of H atoms on the surface of interstellar dust grains at
low temperature is still a matter of debate. In dense clouds, the hydrogenation
of adsorbed species (i.e., CO), as well as the subsequent deuteration of the
accreted molecules depend on the mobility of H atoms on water ice.
Astrochemical models widely assume that H atoms are mobile on the surface of
dust grains even if controversy still exists. We present here direct
experimental evidence of the mobility of H atoms on porous water ice surfaces
at 10 K. Methods. In a UHV chamber, O2 is deposited on a porous amorphous water
ice substrate. Then D atoms are deposited onto the surface held at 10 K.
Temperature-Programmed Desorption (TPD) is used and desorptions of O2 and D2
are simultaneously monitored. Results. We find that the amount of O2 that
desorb during the TPD diminishes if we increase the deposition time of D atoms.
O2 is thus destroyed by D atoms even though these molecules have previously
diffused inside the pores of thick water ice. Our results can be easily
interpreted if D is mobile at 10 K on the water ice surface. A simple rate
equation model fits our experimental data and best fit curves were obtained for
a D atoms diffusion barrier of 22(+-)2 meV. Therefore hydrogenation can take
place efficiently on interstellar dust grains. These experimental results are
in line with most calculations and validate the hypothesis used in several
models.
