The Particulate Medium in the Atmosphere of Venus

Abstract

An extension of a radiative transfer scheme based on the Chandrasekhar ψ- and ϕ-functions of zero order is used to interpret the 8–14 μ limb darkening observations of Venus. An inversion of the transfer equation, including both anisotropic multiple scattering and arbitrary thermal emission, is used to obtain information about the thermal structure and scattering properties of the Cytherean atmosphere in the region around the tropopause. The results, when normalized to the thermal profile deduced from the Mariner 5 S-band radio occultation experiment, imply the following atmospheric properties: 1) the 8–14 μ optical thickness of the lower stratosphere is about τ1=2, of which it is estimated that 20-35% is contributed by CO2, the remainder presumably being due to a particulate medium; and 2) the single-scattering albedo of a volume element is found to be ω̃0≲0.2 implying that the single-scattering albedo due to particles alone is ω̃0≲0.3. The calculation provided no information about the particle size distribution or refractive index. Information about the diffuseness of the medium is less certain, but mean free photon path lengths of l∼5-10 km are implied for both the lower stratosphere and upper troposphere. This in turn implies an aerosol-like structure of the clouds on either side of the tropopause. On the assumption of a uniformly- mixed atmosphere, an integration to the T=550 K level yields a total optical thickness down to this level of τ1∼20-100, or possibly somewhat higher, depending upon the scattering properties of the lower troposphere. A range of dust concentrations from M=0.05-0.5 gm cm−1 in an atmospheric column would supply the required opacity. Finally, solutions to the radiative transfer equation with radiative equilibrium reveal that, for an atmosphere of zero heat capacity, a particulate medium alone is capable of producing sub-solar, mean equatorial, and average planetary surface temperatures of 725, 507 and 480K, respectively. A single-scattering albedo in the “visible” part of the spectrum of ω̃0=0.998 implies a Bond albedo of Ā=0.74 for the same models. Convective instability is found to exist from almost the top of the optically active part of the atmosphere to depths of τ=100-200.