Water vapor variations in the Venus mesosphere from microwave spectra

Abstract

Mm-wave spectra of HDO in the Venus mesosphere (65–100 km) were obtained over the period March 1998 to June 2004. Each spectrum is a measurement of the hemispheric-average H 2O 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 = 0 % to 99%), yielding water vapor abundances on 17 dates and over a full range of local solar time (LST) at the sub-Earth point on Venus. Our mesopheric H 2O values are more numerous and far more precise than the earliest mm-derived H 2O measurements [Encrenaz, Th., Lellouch, E., Paubert, G., Gulkis, S., 1991. First detection of HDO in the atmosphere of Venus at radio wavelengths: An estimate of the H 2O vertical distribution. Astron. Astrophys. 246, L63–L66; Encrenaz, Th., Lellouch, E., Cernicharo, J., Paubert, G., Gulkis, S., Spilker, T., 1995. The thermal profile and water abundance in the Venus mesosphere from H 2O and HDO millimeter observations. Icarus 117, 162–172], allowing an analysis of variability that was previously impossible. Measured 65–100 km H 2O ranged from 0.0 ± 0.06 to 3.5 ± 0.3 ppmv , with significantly different variability than found in previous infrared (lower altitude, cloudtop) studies. Strong global variability on a 1–2 month timescale is clear and unambiguous. A limited number of excellent s/n measurements tentatively indicate the 1–2 month variability manifests most rapidly as change in the lower mesosphere, and more slowly as change in the upper mesosphere. Neither long term (1998–2004) nor diurnal variability in 65–100 km H 2O is evident. While six-year and/or diurnal variabilities are not ruled out, they are weaker than the 1–2 month timescale variation. These conclusions are supported by initial (2004) sub-mm measurements.