Analysis of historical polarization data of Venus

Abstract

Aims: Analysing historical disk-integrated polarization data of Venus with the help of numerical simulations to learn about the time variability in cloud and haze properties, the position of the UV-absorber and the polar cap regions.
Method: With numerical simulations, polarization curves will be calculated for specific input conditions. We compare numerically computed polarization curves to the historical polarization data to derive the microphysical properties of Venus's cloud and haze particles. Results: From our research we obtain values for the imaginary part of the refractive index of the cloud particles of the order 10^(-4). It is most likely that the absorber is mainly located at the equator or in patchy clouds covering 80% of the planet. It is found that in the presence of precise disk-integrated polarization data, it is possible to detect variations in the polar regions. For Venus we found that it is likely that there exist larger particles at higher altitudes at the Venus poles. Unfortunately, it is very hard to say something about the long term variations on the planet. For the Venus case, it can be roughly stated that: the polarization was higher in 1975/1976 than in 1965/1968; the polarization was also higher in 1968/1970 than 1965/1968 but not higher than in 1976; there was a decrease in the polarization of the ultraviolet region in the years 1964/1965. Conclusions: Future observations should be made in a broad range of wavelengths, at as many phase angles as possible and in time frames similar to the orbital period of the (exo)planet. Since Venus can be observed as if it were an exoplanet (broad range of observable phase angles and disk-integrated data), we applied this research to exoplanets as well. We concluded with the help of the Venus data and simple models that it is possible to obtain cloud properties, such as the refractive index and the optical thickness of the cloud, of exoplanets using polarization observations. It was found that those properties are harder to distinguish using flux measurements since the geometric albedo and the distance to the planet are unknown.