Venus - Chemical and dynamical processes in the stratosphere and mesosphere
Photochemical models for the Venus clouds are presented and discussed. We illustrate models for sulfuric acid density as a function of altitude based on a proposed photochemical scheme. Emphasis is placed on two competing removal mechanisms for sulfur atoms above the visible clouds: S + O2 yields SO + O, and S + COS yields S2 + CO. The first reaction (which forms the major oxygen sink in the visible cloud region) requires reasonable O2 concentrations and leads to sulfuric acid production. The second reaction occurs in regions where O2 is severely depleted and leads to elemental sulfur production. Quantitative estimates of the balance between these two competing processes are presented together with a discussion of the complete sulfur and oxygen cycles on the planet. We propose that the dark regions in the ultraviolet of Venus are oxygen-depleted regions where a significant amount of ultraviolet-absorbing sulfur is being produced. We also discuss observations of particle densities on Venus and their implications for vertical mixing rates.