SO2 in the Middle Atmosphere of Venus: IR Measurements from Venera-15 and Comparison to UV Data

Abstract

We present and compare two sets of Venus SO2 measurements: IR spectra from the USSR Venera 15 orbiter and UV spectra from the US Pioneer Venus orbiter and sounding rockets. We choose the 40-mb pressure level for this comparison. At low and mid latitudes, both data sets give a mixing ratio (ƒso2) of tens of parts per billion and SO2 scale height (Hso2) of 1.5-2.5 km, which are in good agreement with photochemically predicted values. This confirms that photochemical processes dominate in this latitude range. Both data sets show the SO2 abundance increasing to several hundred parts per billion at high latitudes. The derived scale heights, however, are discrepant for high latitudes: IR data show great variability associated with dynamic features such as the cold collar, high diffuse clouds, the hot dipole and the polar cap; UV data show a decrease to Hso2 = 1 km. From the IR data, we derive Hso2 = 3-5 km outside the cold collar, and ƒso2 = 100-200 ppb in the hot dipole (increasing of ƒso2 = 1000 ppb in regions with high diffuse clouds). For the cold collar, the IR data yield ƒso2 = 1-10 ppb and Hso2 = 1 km. To explain the differences between the IR and UV results, we conclude that the SO2 scale height may decrease above 69 km altitude (P = 40 mb): the scale height differences are explained by the different viewing angles and the different opacity at UV and IR wavelengths. It is possible that temporal variations also contribute.