  Any evolving system can change of state via thermal mechanisms (hopping a
barrier) or via quantum tunneling. Most of the time, efficient classical
mechanisms dominate at high temperatures. This is why an increase of the
temperature can initiate the chemistry. We present here an experimental
investigation of O-atom diffusion and reactivity on water ice. We explore the
6-25 K temperature range at sub-monolayer surface coverages. We derive the
diffusion temperature law and observe the transition from quantum to classical
diffusion. Despite of the high mass of O, quantum tunneling is efficient even
at 6 K. As a consequence, the solid-state astrochemistry of cold regions should
be reconsidered and should include the possibility of forming larger organic
molecules than previously expected.
