LONG-TERM (1979-1990) CHANGES IN THE THERMAL, DYNAMIC, AND COMPOSITIONAL STRUCTURE OF THE VENUS MESOSPHERE AS INFERRED FROM MICROWAVE SPECTRAL-LINE OBSERVATIONS OF (CO)-C-12, (CO)-C-13, AND C18O

Microwave spectral line observations of 12CO (1.3, 2.6 mm) and 13CO (1.4 mm) were obtained for Venus near inferior conjunctions in 1982, 1985, 1986, 1988, and 1990. Detailed analyses of these whole-disk spectra were performed to derive low to midlatitude average CO mixing profiles (75-105 km) and temperature profiles (85-100 km) for the nightside Venus mesosphere. Mesospheric temperatures determined for the 1982, 1988, and 1990 periods are in good agreement with those returned by Pioneer Venus spacecraft observations in 1979. However, the 1985 and 1986 observations indicate 20-40 K increases in low to midlatitude atmospheric temperatures at 85- to 100-km altitude, relative to the 1979 Pioneer-Venus measurements. The 1985-1986 temperature profiles are similar to the mesospheric temperature profile returned by Venera 10 in 1975. Comparisons with 60- to 90-km temperatures observed between 1978 and 1987 by Pioneer-Venus radio occultations indicate a reversal in the latitudinal gradient of atmospheric pressure within the Venus mesosphere in late 1984, early 1985. Cyclostrophic balance appears to persist throughout the Venus mesosphere at this time, as opposed to the breakdown of cyclostrophic balance above 80-km altitude observed by Pioneer Venus in 1979. Mesospheric CO abundances inferred from the microwave observations also exhibit large changes in 1985-1986. The CO mixing ratio at 80-km altitude increased by several hundred percent between 1982 and 1985. The CO column density above 1 mbar (∼86 km) presented a similar increase between 1985 and 1986. Furthermore, the appearance of an extreme solar-zenith-angle gradient in the nightside distribution of mesospheric CO is inferred from the 1986 observations. We suggest that these 1985-1986 perturbations in mesospheric CO and temperatures, the 5- to 10-year cyclical variations in cloud top dynamics, and the 1979 decay of cloud top SO2 are all related phenomena of interannual variations in the global-scale dynamics within and above the cloud layers of the Venus atmosphere. We also present a 1.4-mm spectrum of C18O observed in 1986 which, combined with analysis of the 1986 13CO spectrum, yields a terrestrial ratio (5.6 ± 0.8) for 13CO/C18O in the Venus mesosphere. © 1991.