OBSERVATIONS OF THE CO BULGE ON VENUS AND IMPLICATIONS FOR MESOSPHERIC WINDS

Observations of CO at 2.6 mm (115.27 GHz) were made with the OVRO mill imeter interferometer in 1986 and 1988, yielding high-quality disk-res olved spectra which were inverted to determine the CO mixing ratio pro file from distinct regions on the disk, allowing us to map the distrib ution of CO in the upper mesosphere of Venus both horizontally and ver tically. The 1986 observations were of the morning terminator and were particularly useful in searching for a suspected CO maximum (''bulge' ') on the nightside. The resulting CO mixing ratio profiles were mappe d for various altitudes as functions of latitude and local time, and w e report that we have resolved the previously inferred CO bulge. The b ulge increases in magnitude from a small day-night variation at 90 km to an extensive nightside peak at 100 km, the upper limit of our obser vations. The peak bulge-to-dayside ratio approached 20-30 at 100 km in 1986 and may have been as large as 50-100, assuming late-afternoon CO abundances found in 1988 were similar to those in 1986. Three-dimensi onal mapping shows that in the upper mesosphere the bulge was displace d from local midnight toward the morning equator, centered at 3:30 AM local time. Using the qualitative model of mesospheric circulation on Venus proposed by Clancy and Muhleman (1985, Icarus 64, 183-204; 1991, Icarus 89, 129-146), we explain this shift in terms of strong retrogr ade zonal winds throughout the mesosphere, matching the directly detec ted mesospheric circulation (Shah, K., D. O. Muhleman, and G. L. Berge 1991, Icarus 93, 96-121) observed with the same dataset in 1988. (C) 1995 Academic Press, Inc.