Implications of enhanced mesospheric water vapor observed by HALOE

Recently reprocessed water vapor data from the Halogen Occultation Experiment (HALOE) on the UARS satellite show significant deviations from the expected constant value of 2*CH4 + H2O. An unusual enhancement in the H2O is seen from 65-70 km which exceeds the stratospheric the value of 2*CH4 + H2O by 0.6-0.8 ppmv. This is inconsistent with the conventional view of transport of H2O and CH4 up from the lower stratosphere and photodissociation in the mesosphere and suggests the presence of another source of H2O. We re-evaluate the hypothesis that there is an extraterrestrial source of water deposited at mesospheric altitudes. The HALOE data above 60 km provide stringent new constraints on the magnitude and altitude of such a source, but cannot rule out the possibility of a small contribution in the middle mesosphere.