  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. We present here
the first experimental evidence of water synthesis under interstellar
conditions. After D and O deposition on a water ice substrate (HO) held at 10
K, we observe production of HDO and DO. The water substrate itself has an
active role in water formation, which appears to be more complicated than
previously thought. Amorphous water ice layers are the matrices where complex
organic prebiotic species may be synthesized. This experiment opens up the
field of a little explored complex chemistry that could occur on interstellar
dust grains, believed to be the site of key processes leading to the molecular
diversity and complexity observed in our universe.
