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The atmosphere of Venus contains a large amount of CO in the upper mesosphere. The abundance at 100 Km altitude is about 1000 ppm. The millimeter rotational lines of CO at 115 and 230 GHz have been used to study the abundance and time variations of the Venus upper atmosphere (Clancy and Muhleman Icarus 64, 157; 64, 183; 89, 129). At the center of the CO line is a narrow Doppler core which can be used to measure the velocity of the mesospheric winds (Buhl, Chin and Goldstein, ApJ 369, L17). The millimeter CO lines arise from the 100 km altitude region in the Venus mesosphere. Observations were made of this line, using the NRAO 12 meter telescope at Kitt Peak, before the inferior conjunction of January 18, 1990 and before and after the inferior conjunction of April 1, 1993. Wind velocities of approximately 100 m/s were measured and two distinct circulation flows were observed. During all three observing periods (December 1989, March 1993 and April 1993) a Zonal flow was measured with an equatorial velocity of ~100 m/s in the same direction as the planets rotation (retrograde). The second global wind flow was first seen by Goldstein 1989 in the infrared using lines of CO2 and is directed from the subsolar point on the planet to the antisolar point. This wind flow is at a higher altitude than the 100 km winds studied with the CO line. For the first two observing periods we measured an antisolar to subsolar wind or return flow of about 120 m/s. During the third observing period this flow reversed from a return flow to a direct flow of about 50 m/s. This change in the wind pattern is most likely part of a cycle taking place in the Venus mesosphere. An alternative explanation is that the altitude of the line we observed changed dramatically from March to April 1993, hence sampling a different wind field.
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