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Summary
In the standard picture of the atmosphere, the mesosphere--the region between 50 and 90 kilometers--is almost bone dry. But last week, a satellite instrument detected signs of as much as 50% more water vapor than was expected at those altitudes. A controversial theory offers an explanation: Fluffy, house-size comets are pummeling the outer reaches of the atmosphere 20 times a minute, releasing water that ultimately ends up in the mesosphere. Many researchers remain skeptical, however.
