Ultraviolet Contrasts and the Absorbers Near the Venus Cloud Tops

The absorbers in the Venus atmosphere are not merely markers for the atmospheric motions; their absorption of incident sunlight determines the deposition of the energy which drives these motions. Measurements of the ultraviolet contrasts constrain the location, altitude, and identity of these absorbers near the Venus cloud tops. Spin-scan images from the Pioneer Venus orbiter UV spectrometer (UVS) and the cloud photopolarimeter (CPP) provide a set of planetary contrast measurements in the wavelength range from 1990 angstrom to 3650 angstrom and phase angles from 33 to 130. The planet is darkest at the point where the UVS line of sight penetrates perpendicular to the cloud tops: thus the absorbing material responsible must be deep in the atmosphere. Sulfur dioxide absorption can explain the amount of contrast seen between 2000 A and 3200 angstrom. At the longer wavelengths (where the SO2 cross section has dropped by 2 orders of magnitude), the persistence of contrast requires another absorber which is deeper in the atmosphere than the SO2 (at about the 75-mbar level). This additional absorption is strongly (but not identically) associated with the location of the SO2; we find the short and long wavelength images are remarkably similar. Part of the observed contrast is due to the high-lying haze discovered from Pioneer Venus polarimetry, most importantly at the shortest wavelengths where CO2 is strongly absorbing. The correlation between planetary contrast and polarization does not require large-scale clearing or major vertical motions of the cloud, tops as the sole cause of the observed contrast. However, a scheme in which absorbers subject to photochemical destruction are mixed upward into the cloud top region provides a consistent explanation for the origin of these markings.