Are Small Comets Dampening the Atmosphere?

The controversial theory that fluffy, house-size comets are pummeling the outer reaches of the atmosphere enjoyed a boost last week when a satellite instrument detected signs of as much as 50% more water vapor than expected around 70 kilometers up. Cometary snowballs might be a source of this mesospheric dampness, but most experts are not yet ready to concede that's the case.

Space physicist Louis Frank of the University of Iowa in Iowa City made headlines this spring when he announced that ultraviolet satellite images showed strange dark spots in the upper atmosphere. Earlier hints of excess water in the mesosphere had come late last year when James Russell of Hampton University in Hampton, Virginia, and his colleagues reported a reanalysis of data gathered by a satellite-borne instrument, the Halogen Occultation Experiment. The data revealed a peak in water vapor at an altitude of about 70 kilometers.

Now an instrument deployed from the space shuttle, the Middle Atmosphere High Resolution Spectrograph Investigation, has found abundant hydroxyl radicals, a breakdown product of water, at the same altitude. "There's a startling amount of water above altitudes of 65 kilometers," says Robert Conway of the Naval Research Laboratory (NRL) in Washington, D.C.

Theoretician Michael Summers of NRL says that "definitely something very unusual [is] going on in the mesosphere that we don't understand at all." But he adds that even if the mesosphere is damp, it's not nearly as damp as Frank's theory would have it. Calculations by Summers and others suggest that the small comets would produce three times more water than observed. These researchers are also skeptical that giant blobs of water vapor could plunge from an altitude of 800 kilometers, where Frank says the comets would break up, to less than 100 kilometers while leaving hardly a trace of water along their way.

The criticisms don't worry Frank. "The most important thing is the finding of excess water up there," he says. "It's a big step."