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Polar Clouds from Space Shuttle Exhaust
NAVAL RESEARCH LAB WASHINGTON DC
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The Earths mesosphere lies above the stratosphere in the region between 50 and 90 km altitude, far above where airplanes or balloons can fly. In the unique polar summer mesosphere, the temperature plummets below 150 K -190 degrees F, making this region the coldest on Earth, so inaccessible that it is sometimes like studying the atmosphere of another planet. In this extremely rarefied and dry place, water ice particles are found in narrow layers near 82 km altitude called polar mesospheric clouds PMCs. PMCs are not known in the published record until 1885. The specific processes leading to the formation of PMCs are disputed. However, some evidence indicates that they became brighter and more frequent in the late 20th century, leading some scientists to argue that they are indicators of global climate change. One hypothesis reasons that increasing amounts of methane CH4 emitted at the Earths surface by industrial and agricultural processes increase the humidity of the upper atmosphere as the methane is broken down by ultraviolet sunlight to form water vapor. Recently, NRLs Space Science Division scientists have complicated this hypothesis by identifying a new source for PMCs, challenging long-held beliefs about the meteorology of the upper atmosphere. Using satellite observations from NRLs Middle Atmosphere High Resolution Spectrograph Investigation MAHRSI, Stevens et al. showed that the exhaust plume from the space shuttle can be transported all the way from the east coast of the United States to the Arctic summer mesosphere to form PMCs. Additional data from other experiments now reveal that this phenomenon has occurred over both poles. Here we present the initial observations leading to the discovery, and discuss its scientific impact.
APPROVED FOR PUBLIC RELEASE