Magazine article Science News

Reservoir of Water Hides High above Earth

Magazine article Science News

Reservoir of Water Hides High above Earth

Article excerpt

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. …

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