The dry stratosphere: A limit on cometary water influx

The stratosphere accumulates the bulk of meteoric and cometary material that impacts the Earth and provides a measure of that flux. Recent models and measurements of the effective age of stratospheric air demonstrate our ability to simulate this buildup. Current observations of H₂O in the stratosphere and mesosphere are consistent with an extraterrestrial source of H₂O, but limit its influx to less than 2 Tg/yr. Such a flux is consistent with standard estimates but is 100 times less than the small-comet hypothesis.