Spacecraft observation of Venus Infra-red limb-darkening

Abstract

The limb-darkening laws predicted for several models of the atmosphere and clouds of Venus are compared with limb-darkening observations in the 8–13μ interval made from the ground and from Mariner II. Model A attributes the limb-darkening to pure molecular absorption by the atmosphere above the Gytherean cloud layer; Model B, to multiple scattering by cloud particles of arbitrary albedo and altitude distribution. In each model, the temperature regime is considered specified, alternatively by convective and by radiative equilibrium. In Model A, limb-darkening arises from the increase of temperature with atmospheric depth. In Model B, both the T — τ relation and the dependence of emissivity on the angle of emergence contribute to the limb-darkening. Each model can be made to agree with observation, with an appropriate specification of atmospheric and cloud parameters. With the angular resolution available from the vicinity of the Earth, a discrimination among these models is difficult to achieve. But a Venus flyby or orbiter which performs only somewhat better than Mariner II can potentially determine uniquely the cause of the limb-darkening, and provide significant new information on the structure and composition of the atmosphere and clouds of Venus.