Reservoir of water hides high above Earth.

Earth's upper atmosphere, a region drier than the Sahara Desert, harbors unexpected amounts of water vapor, according to data from a pair of satellites. The discovery could bolster a controversial theory that thousands of house-size comets are hitting the atmosphere each day.

"What we have found is astonishing. We really don't have answers," says Robert R. Conway of the Naval Research Laboratory in Washington, D.C.

Conway and his colleagues made their discovery last week using an instrument called the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI), part of a satellite released and later picked up by the space shuttle Discovery. The instrument measures hydroxyl (OH) ions in the upper stratosphere and overlying mesosphere, from 35 to 100 kilometers above Earth. The amount of hydroxyl is directly related to the atmosphere's humidity, because at these high altitudes the ion forms when ultraviolet light splits apart molecules of water vapor.

The MAHRSI data revealed a layer with a surprising abundance of hydroxyl in the upper mesosphere above the Arctic, at altitudes of 60 to 80 km. The finding corroborates MAHRSI measurements made during a shuttle flight in 1994. At the time, Conway and his colleagues had questioned their data, which contradicted established atmospheric theory.

The new findings also back up observations made by the Halogen Occultation Experiment (HALOE) on a satellite in orbit since 1991. The HALOE instrument measures water vapor directly by peering through the atmosphere while the sun rises and sets behind Earth.

Originally, HALOE investigators paid scant attention to the upper mesosphere; they expected it would contain too little water vapor for their instrument to detect reliably. While reprocessing their data earlier this year, however, they discovered up to 50 percent more water vapor than expected at altitudes of 75 km. The moisture appeared during summertime above the sunlit polar area.

Many atmospheric scientists discounted the HALOE data at first. "They said it can't be true because it violates our ideas. Now we come along with MAHRSI, and we have clear, strong substantiating evidence," says Conway.

Water vapor is abundant in the stormy lower atmosphere, but it cannot drift upward easily, so the stratosphere and mesosphere remain dry. In theory, water vapor ascends only in the tropics, where lofty thunderheads push moisture into the stratosphere. The scant water vapor then drifts upward into the mesosphere.

Louis A. Frank of the University of Iowa in Iowa City has proposed an additional source of water vapor in the upper atmosphere (SN: 5/31/97, p. 332). Thousands of small comets break up high above Earth and deposit a fine spray of water in the mesosphere, from which it rains down onto Earth's surface, he says. Most researchers dismissed this theory when Frank first proposed it a decade ago, but a NASA satellite recently recorded evidence of what appear to be incoming cometlike bodies.

Frank views the new water vapor observations as further support. "When you get that excess of water vapor up there, it just can't come from the Earth. It must come from space," he says.

Other researchers are searching for a more down-to-earth explanation. "I can't deny the fact that Frank predicted what we're observing and what HALOE is seeing. But I have a very hard time with his model, and I can't believe that his explanation is the right answer," says Conway.

HALOE investigators agree. "There's more than one way to explain this," says James M. Russell III, HALOE's lead scientist and an atmospheric physicist at Hampton (Va.) University