A spectroscopic search for molecular oxygen in the Venus middle atmosphere

The results from a new attempt to detect ground state molecular oxygen in the Venus middle atmosphere are presented. The upper limit inferred from the January 1995 observations using the Ultra High Resolution Facility (UHRF) spectrometer at the Anglo-Australian Observatory (AAO) is equivalent to a uniform volume mixing ratio of 3 × 10^-6 at and above the 300 mbar level. This is comparable to the upper limit inferred from observations in the early 1970s and is a factor of 10 larger than the upper limit inferred from observations in 1982. These observations indicate the observed secular decrease in the abundance of SO₂ and SO from the early 1980s to 1995 did not result in an O₂ abundance that could be detected within the sensitivity of the present observations. Radiative transfer calculations show the sensitivity of observations to the vertical profile of O₂. These calculations show that all three spectroscopic observations (1973, 1982, and 1995) would have detected the presence of O₂ if its mixing ratio near 50 km altitude had been 10^-5 as was reported by in situ measurements in 1980.