  Even though water is the main constituent in interstellar icy mantles, its
chemical origin is not well understood. Three different formation routes have
been proposed following hydrogenation of O, O2, or O3, but experimental
evidence is largely lacking. We present a solid state astrochemical laboratory
study in which one of these routes is tested. For this purpose O2 ice is
bombarded by H- or D-atoms under ultra high vacuum conditions at astronomically
relevant temperatures ranging from 12 to 28 K. The use of reflection absorption
infrared spectroscopy (RAIRS) permits derivation of reaction rates and shows
efficient formation of H2O (D2O) with a rate that is surprisingly independent
of temperature. This formation route converts O2 into H2O via H2O2 and is found
to be orders of magnitude more efficient than previously assumed. It should
therefore be considered as an important channel for interstellar water ice
formation as illustrated by astrochemical model calculations.
