Willem J. Knibbe

A biwavelength analysis of Pioneer Venus polarization observations

A new method for analyzing Pioneer Venus polarimetry data on a pixel-wise basis is presented. Quasi-simultaneous observations at two wavelengths (550 and 935 nm) are combined and compared with results of multiple scattering calculations. In this manner, hypotheses about particle size distributions in the upper part of the Venus atmosphere are tested. Particles composed of a sulfuric acid solution are considered, and a distinction is made between large and small particles, called cloud and haze particles, respectively. Three model atmospheres have been investigated: (1) a single layer containing cloud particles, (2) a single layer containing a mixture of cloud and haze particles, and (3) a two layer model with an upper layer composed of haze particles and a lower layer containing cloud particles. It is found that all three models agree with the observations at phase angles near 20°, but that the first model cannot be made to agree with the observations for phase angles near 90°. This confirms the presence of haze particles in the Venus atmosphere found earlier by Kawabata et al. [1980]. We find that the haze particles may be situated either above or mixed within the main cloud deck of Venus. We derived effective radii between 0.85 and 1.15 μm for the cloud particles, and effective radii of 0.2 or 0.3 μm for the haze particles. When the haze particles are situated above the cloud layer, the haze optical thickness can take values of up to 0.6 at 550 nm. When the haze particles are mixed with the cloud particles, their contribution to the total atmospheric scattering coefficient at 550 nm can become as large as 70%.

Spatial variations of Venus' cloud properties derived from polarimetry

We study satellite polarization data of the clouds of Venus obtained by the Pioneer Venus Orbiter from 1978 through 1990. We present a new method for comparing these data to results of exact multiple scattering computations. This method has been applied to the analysis of a single disk distribution of the polarization at wavelengths 550 and 935 nm, using a simple model for the atmosphere of Venus. We find little variation in the cloud particle size distribution for the equatorial part of the disk. For this region, the effective particle radius is about 1.0 micrometers and the width of the size distribution decreases when approaching the terminator. However, our analysis of observations at higher latitudes suggests that for these regions a different explanation is needed. Here, an upper haze layer with smaller particles than those of the underlying cloud but having the same composition explains the observations well. The optical thickness of this haze is between 0.1 and 0.5 for an effective haze particle radius of 0.40 micrometers .

Derivation of cloud properties from biwavelength polarization measurements

The influence of the top altitude of terrestrial liquid water clouds on the polarization of sunlight reflected by a cloudy atmosphere is theoretically investigated. It is compared to the influences of other cloud properties and of stratospheric aerosols. Using a typical atmosphere model, we show how accurately the cloud top pressure may be derived from nadir measurements performed by satellites of the polarization at 350 nm. The accuracy of the derived pressure is about 5 mb under favorable conditions, when the accuracy of polarization measurements is 0.1%, and it depends mainly on knowledge of the density of the cloud and of the stratospheric aerosol optical thickness. The stratospheric aerosol optical thickness may be estimated with an accuracy of 0.02 using observations of the polarization at 670 nm having an accuracy of 0.1%.