  The mobility of O atoms at very low temperatures is not generally taken into
account, despite O diffusion would add to a series of processes leading to the
observed rich molecular diversity in space. We present a study of the mobility
and reactivity of O atoms on an amorphous silicate surface. Our results are in
the form of RAIRS and temperature-programmed desorption spectra of O2 and O3
produced via two pathways: O + O and O2 + O, investigated in a submonolayer
regime and in the range of temperature between 6.5 and 30 K. All the
experiments show that ozone is formed efficiently on silicate at any surface
temperature between 6.5 and 30 K. The derived upper limit for the activation
barriers of O + O and O2 + O reactions is 150 K/kb. Ozone formation at low
temperatures indicates that fast diffusion of O atoms is at play even at 6.5 K.
Through a series of rate equations included in our model, we also address the
reaction mechanisms and show that neither the Eley Rideal nor the Hot atom
mechanisms alone can explain the experimental values. The rate of diffusion of
O atoms, based on modeling results, is much higher than the one generally
expected, and the diffusive process proceeds via the Langmuir-Hinshelwood
mechanism enhanced by tunnelling. In fact, quantum effects turn out to be a key
factor that cannot be neglected in our simulations. Astrophysically, efficient
O3 formation on interstellar dust grains would imply the presence of huge
reservoirs of oxygen atoms. Since O3 is a reservoir of elementary oxygen, and
also of OH via its hydrogenation, it could explain the observed concomitance of
CO2 and H2O in the ices.
