Is Disulfur Monoxide a Second Absorber on Venus?

Abstract

The diminished spherical albedo of Venus from 200 to 500 nm requires at least two different absorbers in the cloud layer. SO2has been identified as the dominant absorber between 200 and 320 nm; however, the identity of the second UV absorber which darkens the planet from 320 to 500 nm remains a mystery. We present the results from photochemical calculations of sulfur oxides in the atmosphere of Venus which indicate that S2O is a strong candidate for the second absorber. The photochemical calculations are constrained by recent observations of SO and SO2by a CU/LASP sounding rocket experiment carried out on 29 March 1991. We calculate that the photochemical lifetime of S2O in the middle atmosphere of Venus is much less than the dynamical mixing time scale, and this can explain the shortest time scales for changes in the dark markings. The calculated vertical profile of S2O from 58 to 96 km is consistent with constraints on the second absorber. In addition, since S2O is chemically derived from SO2, the observed spatial correlation between the second absorber and SO2can be directly explained. The conclusion that S2O is an excellent candidate for the second absorber could be confirmed by laboratory measurements of its absorption cross section.