Mysterious clouds caused by cosmoids?
Noctilucent clouds have puzzled and awed cloud watchers since they were discovered a century ago. These clouds, which are seen from the ground in summer after sunset or before sunrise at high latitudes, are unusual because they form in the mesopause, at altitudes of 80 kilometers or more--far above where scientists expect water vapor and other cloud material from the earth to be able to reach. Theories have been proposed to explain the origin of noctilucent clouds, but none is wholely consistent with observations.
Now Maurice Dubin, a physicist at NASA Goddard Space Flight Center in Greenbelt, Md., thinks he has found the solution. He suggests that "cosmoids,' or cosmic meteoroids, breaking up near the earth bring from space both the dust and water needed to form the clouds.
Scientists had concluded previously that noctilucent dust has a cosmic origin. But their theories also assumed that the water that condenses on the dust comes from the earth. According to Dubin, this presents a number of problems. The theories predict, for example, times for condensation that are much longer than the observed times for the formation and evolution of the clouds. Moreover, the theories require that the mesopause be cold enough for condensation to occur. And while, in the presence of noctilucent clouds, temperatures in the polar mesopause do indeed get down to -260| F--the lowest atmospheric temperature recorded--clouds have also been observed by Soviet cosmonauts over the equator, where mesopause temperatures are too warm for condensation.
In Dubin's model, extremely cold and icy cosmoids approach the dark side of the earth, become electrically charged and disintegrate into a stream of small particles, which are funneled by the geomagnetic field into a polar region. Clouds could also form, although for much shorter periods, at equatorial regions if the rate of incoming cosmoids is great enough. The cosmoid idea may also explain why the polar mesopause gets so cold. Dubin thinks that the cosmoids themselves quickly cool the atmosphere as they vaporize.
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|Date:||May 31, 1986|
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