Venus: Vertical Structure of Stratospheric Hazes from Mariner 10 Pictures

Abstract

High-resolution pictures of the limb of Venus taken by the Mariner 10 television camera indicate the presence of tenuous haze layers high in the stratosphere. At least two distinct layers separated by a few kilometers in altitude appear in pictures taken in both orange and ultraviolet light and extend laterally for several thousand kilometers from the equator to high latitudes. Photometric profiles of these hazes have been analyzed to determine their vertical distribution. An “optical barometer” technique for determining the altitudes of the hazes is presented wherein the Rayleigh-scattering component is derived by comparing orange and UV brightness profiles for nearby picture pairs. This technique appears to work very well for the orange/UV pairs which were studied. The derived scale height for CO₂gas is 4.2 km, corresponding to a temperature of 200 K, in good agreement with radio occultation data. The optical barometer yields a pressure of 4 mb for the level at which the stint path optical depth, τ_slant, at the limb is unity. This level corresponds to a distance R from the center of Venue equal to 6131 km which is accurate to within 1 km provided that there is no appreciable contribution to the brightness by Rayleigh-scattering aerosols which mimic CO₂ gas. It is possible that the limb haze layering observed between R = 6130 and 6140 km could be correlated with temperature inversions detected by the Mariner 5 radio occultation experiment. A model is proposed wherein the concentration of particles increases rapidly with an effective scale height of about 2 km as we descend about 10 km from the limb haze (τ_slant = 1) to the main polarization cloud deck (τ_vort = 1).