DISCOVERY OF A DOPPLER-LIMITED CO LINE IN THE UPPER MESOSPHERE OF VENUS - A NEW DYNAMIC PROBE

The presence of CO in the upper atmosphere of Venus is a consequence of the photochemistry of CO2, the dominant atmospheric constituent. In 1989 December we observed the J = 1-2 transition of CO at 230 GHz. In addition to the broad absorption line first reported by Kakar et al. in 1976, we discovered a narrow absorption feature at the center of the line due to upper mesospheric CO, where the temperature profile starts to exhibit diurnal variation with altitude (approximately 100 km). The narrow feature is approximately 600 kHz wide and is predominantly Doppler-broadened. The Doppler core provides a new means of measuring wind velocities at these altitudes in the atmosphere of Venus. Detection of small Doppler shifts in the line core can in principle be used to measure winds with an accuracy of 10 m s-1. Results are presently limited by the 17 kHz uncertainty in the measured rest frequency, corresponding to a systematic error in wind velocity up to 22 m s-1, and the absence of laboratory measurements of the pressure shift in CO by CO2.