  Surface processes on cosmic solids in cold astrophysical environments lead to
gas phase depletion and molecular complexity. Most astrophysical models assume
that the molecular ice forms a thick multilayer substrate, not interacting with
the dust surface. In contrast, we present experimental results demonstrating
the importance of the surface for porous grains. We show that cosmic dust
grains may be covered by a few monolayers of ice only. This implies that the
role of dust surface structure, composition, and reactivity in models
describing surface processes in cold interstellar, protostellar, and
protoplanetary environments has to be reevaluated.
