  Context. The synthesis of water is one necessary step in the origin and
development of life. It is believed that pristine water is formed and grows on
the surface of icy dust grains in dark interstellar clouds. Until now, there
has been no experimental evidence whether this scenario is feasible or not on
an astrophysically relevant template and by hydrogen and oxygen atom reactions.
  Aims. We present here the first experimental evidence of water synthesis by
such a process on a realistic grain surface analogue in dense clouds, i.e.,
amorphous water ice.
  Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice
substrate (H2O) held at 10 K. Products are analyzed by the
temperature-programmed desorption technique.
  Results. We observe production of HDO and D2O, indicating that water is
formed under conditions of the dense interstellar medium from hydrogen and
oxygen atoms. This experiment opens up the field of a little explored complex
chemistry that could occur on dust grains, believed to be the site where key
processes lead to the molecular diversity and complexity observed in the
Universe.
