  At the low temperatures of interstellar dust grains, it is well established
that surface chemistry proceeds via diffusive mechanisms of H atoms weakly
bound (physisorbed) to the surface. Until recently, however, it was unknown
whether atoms heavier than hydrogen could diffuse rapidly enough on
interstellar grains to react with other accreted species. In addition, models
still require simple reduction as well as oxidation reactions to occur on
grains to explain the abundances of various molecules. In this paper we
investigate O-atom diffusion and reactivity on a variety of astrophysically
relevant surfaces (water ice of three different morphologies, silicate, and
graphite) in the 6.5 - 25 K temperature range. Experimental values were used to
derive a diffusion law that emphasizes that O atoms diffuse by quantum
mechanical tunnelling at temperatures as low as 6.5 K. The rate of diffusion on
each surface, based on modelling results, were calculated and an empirical law
is given as a function of the surface temperature. Relative diffusion rates are
k_H2Oice > k_sil > k_graph >> k_expected. The implications of an efficient
O-atom diffusion over astrophysically relevant time-scales are discussed. Our
findings show that O atoms can scan any available reaction partners (e.g.,
either another H atom, if available, or a surface radical like O or OH) at a
faster rate than that of accretion. Also, as dense clouds mature H2 becomes far
more abundant than H and the O/H ratio grows, the reactivity of O atoms on
grains is such that O becomes one of the dominant reactive partners together
with H.
