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R.T. Clancy, B.J. Sandor (Space Science Institute), G. Moriarty-Schieven (James Clerk Maxwell Telescope)
Circulation and photochemical behaviors of the Venus middle atmosphere (mesosphere, 65-105 km altitudes) exhibit remarkable temporal variations that are often defined by ground-based studies. Sub-millimeter spectral line observations in particular play and important role in the investigation of the Venus mesosphere due to relatively strong transitions for CO, HDO, SO2, and SO in the wavelength region and the pressure-broadened lineshapes of these absorptions. Venus nightside sub-millimeter 12CO spectra (345 GHz) exhibit sharp, deep absorption cores that yield excellent temperature weighting functions about the Venus mesopause (Clancy et al., 2003) and maximum sensitivity to Doppler line shifts, a modest 10 m/sec line-of-sight wind is easily detectable in short integrations (5-10 minutes). An accumulated set of James Clerk Maxwell Telescope (JCMT) observations during Venus inferior conjunctions in 2000, 2002, and 2004 provide nightside mapping of Doppler winds, CO, and temperatures over the 95-105 km altitude region. The nightside distribution of winds varies over all observed timescales. For periods separated by one week, the Venus nightside global circulation changes character for zonal rotation to subsolar-to-antisolar (SS-AS) flow. On hourly timescales, wind velocities may vary by > 50 m/sec over 3000 km spatial scales. The instantaneous nightside circulation field is extremely asymmetric in latitude and local time. During the June 2004 inferior conjunction of Venus, we also obtained the first detection of mesospheric SO2 and a very sensitive upper limit for SO; indicating Venus mesospheric SO2 abundances roughly twice that predicted by the preferred photochemical model of Yung and DeMore (1982), and an SO2/SO ratio at least 8 times the same model predictions. These departures from the model are probably due in large part to the fixed water vapor abundance of 1 ppmv throughout the Venus mesosphere, employed in the Yung and DeMore model for lack of data. As an accumulated set of millimeter HDO measurements show (Sandor and Clancy, 2005), water vapor in the Venus mesosphere is highly (> factors-of-ten) variable on global scales and fell below 0.1 ppmv at the time of the 2004 SO2, SO measurements.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.