THE ABUNDANCE OF SULFUR-DIOXIDE BELOW THE CLOUDS OF VENUS

We present a new method for determining the abundance of sulfur dioxide below the clouds of Venus. Absorption by the 3nu3 band of SO2 near 2.45 mum has been detected in high-resolution spectra of the night side of Venus recorded at the Canada-France-Hawaii telescope in 1989 and 1991. The inferred SO2 abundance is 130 +/- 40 ppm at all observed locations and pertains to the 35-45 km region. These values are comparable to those measured by the Pioneer Venus and Venera 11/12 entry probes in 1978. This stability stands in contrast to the apparent massive decrease in SO2 observed at the cloud tops since these space missions. These results are consistent with laboratory and modelling studies of the SO2 destruction rates in the lower atmosphere of Venus. The new spectroscopic technique presented here allows a remote monitoring of the SO2 abundance below the clouds, a likely tracer of Venusian volcanism.