Surface recombination of O and H₂ on meteoric dust as a source of mesospheric water vapor

One of the most provocative recent developments in mesospheric science is the discovery of a narrow layer of water vapor located near 70 km altitude where the H 2 O mixing ratio reaches the highest values observed in the middle atmosphere [ Summers et al. , 1997a]. This layer can only be explained by a local source of H 2 O, which is contrary to conventional gas phase chemistry that predicts net H 2 O destruction in that part of the mesosphere. Although suggestions have been made that this layer is produced by an influx of small comets releasing water in the upper atmosphere, we propose that the reaction O + H 2 → H 2 O on the surface of meteoric dust can account for the observed H 2 O layer. Using a chemical‐transport model we find that the inclusion of this reaction with a reaction probability of ∼0.01 yields a model H 2 O layer with the observed characteristics. We suggest that the key to understanding the origin of the H 2 O layer lies in coincident observations of mesospheric H 2 and H 2 O.