Observations of the ultraviolet dayglow of atomic oxygen from the DE-1 spacecraft (Frank et al., 1986a,b) have revealed the existence of frequent localized decreases (''holes'') in the emitted intensity. These have been interpreted as evidence of a steady influx of small ''comets'' whose icy envelope would provide a source of H2O molecules in the upper thermosphere. Using a one-dimensional model of middle-atmosphere composition, we show that the mesospheric water-vapor concentration implied by this influx is much larger than the range of observed values.

We suggest that the entry of such an icy object into the atmosphere at meteoric speeds is likely to lead to dissociation of H2O molecules, and to a primary downward flux of OH rather than H2O. This would reduce significantly the predicted increase in H2O concentrations in the mesosphere, and would introduce a different mechanism for the production of the airglow holes, i.e., chemical destruction of atomic oxygen by the fast reaction with OH.

The constraints imposed on this scenario by the mesospheric water-vapor measurements are examined, and we conclude that the premissible flux is some 30 times smaller than that proposed by Frank et al. The individual airglow holes would then have diameters of order 10 km instead of 50 km, and would be much less intense as seen by the DE-1 instrument than those reported. We conclude that the mechanism, through relaxing the constraints on extraterrestrial water fluxes considerably, falls well short of explaining the observations.