IS DISULFUR MONOXIDE A 2ND ABSORBER ON VENUS

The diminished spherical albedo of Venus from 200 to 500 mn requires a t least two different absorbers in the cloud layer, SO2 has been ident ified as the dominant absorber between 200 and 320 nm; however, the id entity of the second UV absorber which darkens the planet from 320 to 500 nm remains a mystery. We present the results from photochemical ca lculations of sulfur oxides in the atmosphere of Venus which indicate that S2O is a strong candidate for the second absorber. The photochemi cal calculations are constrained by recent observations of SO and SO2 by a CU/LASP sounding rocket experiment carried out on 29 March 1991. We calculate that the photochemical lifetime of S2O in the middle atmo sphere of Venus is much less than the dynamical mixing time scale, and this can explain the shortest time scales for changes in the dark mar kings. The calculated vertical profile of S2O from 58 to 96 km is cons istent with constraints on the second absorber. In addition, since S2O is chemically derived from SO2, the observed spatial correlation betw een the second absorber and SO2 can be directly explained. The conclus ion that S2O is an excellent candidate for the second absorber could b e confirmed by laboratory measurements of its absorption cross section . (C) 1997 Academic Press.