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B.J. Sandor, R.T. Clancy (Space Science Institute)
Mm-wave spectra of HDO in the Venus mesosphere (70-100 km) were obtained over the period March 1998 to June 2004. Each spectrum is a proxy measurement of the hemispheric-average water vapor mixing ratio in the Venus mesosphere. Observations were conducted for wide ranges of Venus solar elongations (46 W to 47 E, and fractional disk illuminations (f= 1% to 99%), yielding water vapor abundances on 14 dates and over a full range of local solar time (LST) at the sub-Earth point on Venus. Measured 70-100 km H2O ranged from 0.0±0.06 to 3.5±0.3 ppmv, significantly drier, and with different variability, than found in previous IR (lower altitude, cloudtop) studies. Variability is seen on timescales as short as 1-2 months. Over the full 1998-2004 period, the Venus mesosphere became much drier. This long-term pattern appears bimodal, wet before and dry after 2001. A more gradual drying trend over 1998-2004 is not ruled out. Mm-wave spectra indicate no diurnal pattern in 70-100 km H2O variability, a conclusion supported by initial (2004) submm measurements.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.