Detection of SO 2 in the UV spectrum of Venus

The broad absorption feature below 3300Å in the Venus uv spectrum previously reported by Barker is identified as primarily due to SO 2 absorption based on new higher resolution spectra of the 3000‐3400Å region showing broad (10Å), unresolved absorptions in the regions at all SO 2 band origins between 3000 and 3300Å. SO 2 mixing ratios vary from 5 × 10 −7 down to an upper limit of 2 × 10 −8 at a phase angle of 138°. Previous observational determinations of the SO 2 mixing ratio were biased toward large phase angles, and consequently did not detect any SO 2 absorption at the 10 −8 level. The upper limit derived from the CS 2 band head at 3206Å is ≤ 5 × 10 −8 . The observed range of SO 2 mixing ratios (<2 × 10 −8 to 5 × 10 −7 ) is consistent with model predictions based on the sulfur photo‐chemistry at the cloud tops. Ground‐based observations of SO 2 mixing ratio will provide constraints on models and check on the Venera and Pioneer Venus measurements of the mixing ratios of SO 2 and other sulfur‐bearing gases with altitude.