Sulfur dioxide at the Venus cloud tops, 1978 - 1986.

Ultraviolet spectroscopy from the Pioneer Venus Orbiter shows a decline in the cloud top abundance of SO2 from about 100 ppb to about 10 ppb in the period 1978-1986. A consistent decline in polar haze has occurred over the same period, with the correlation coefficient between these two observables of r = 0.8. Star calibrations determine the instrument sensitivity to within 10 percent which rules out the possibility that this is an instrumental effect. Systematic errors could increase the SO2 abundance to twice the inferred values in later orbits. Tracking of SO2 features and power spectral analysis give rotation periods for the longer-lived features of 3.6-5.2 days, consistent with cloud-tracked winds observed at other wavelengths. The behavior of SO2 and polar haze can be plausibly explained by episodic injection of SO2 into the cloud top regions, for example, by active volcanism.