One Hundred Years of Venus Polarimetry: PICSARR Observations of the Phase Curves

Abstract

We report new high-precision observations of the polarization of light scattered from the atmosphere of Venus, made 100 years after the pioneering studies by Bernard Lyot. The new observations include disk-integrated observations in a range of filters as well as imaging polarimetry. We compare the new results with past observations and models. We have reproduced the 1974 modelling of the Venus polarization by Hansen and Hovenier using modern radiative transfer codes. We show that the new models are in good agreement with the originals, and enable us to calculate the polarization for wavelengths not covered by the original study and to model the polarization distribution across the disk. The new observations are in good agreement with past determinations of the size distribution of the predominant particle mode. They agree with past studies in showing variability of the phase curve between synodic cycles and also polarization variability on short timescales, particularly at higher phase angles (crescent phases). Imaging polarimetry observations show good agreement with models for the redder wavelengths. However, observations in the ultraviolet show very different polarization behavior in the polar regions (within about 30 degrees of the north and south poles). The simplest explanation of this result is that there is a larger Rayleigh scattering component in the polar regions than in the equatorial and mid-latitudes and this could be explained by a lower cloud-top height in agreement with previous spacecraft observations. These ultraviolet polarization observations are inconsistent with horizontally homogeneous atmospheric models.

Figures (18)

• Figure 1: Linear polarization observations of Venus obtained with PICSARR polarimeters over 2021-2024. Crosses are observations from Pindari Observatory (Sydney, Australia) and circles are from the Weaver Student Observatory (Marina, California). As is conventional for such observations polarization is expressed relative to the scattering plane (the plane containing the Sun, the planet and the observer), such that positive polarization is perpendicular to the plane, and negative polarization is parallel to the plane. The solid lines are based on the HH74 models hh74 recalculated for our filter wavelengths as described in Section \ref{['sec:repro']}.
• Figure 2: Comparison of models of the Venus disk-integrated polarization from HH74 hh74 and using our version of the same single layer model using VSTAR/VLIDORT bailey18. Circles are models from HH74, digitized from their published figures. Solid lines are equivalent models recalculated using VSTAR/VLIDORT as described in Section \ref{['sec:repro']} and using the parameters listed in Table \ref{['tab:hh_models']}.
• Figure 3: Polarization models calculated with VSTAR/VLIDORT for wavelengths from 300 nm to 1050 nm as described in Section \ref{['sec:repro']}. The strong peak at the left (phase angle $\sim$ 15 to 20 degrees) is the primary rainbow from the cloud particles. The peak at $\sim$80 degrees in UV wavelengths is mostly due to Rayleigh scattering.
• Figure 4: Comparison of new PICSARR observations with selected past observations and models in different wavelength regions. Solid lines are disk-integrated polarization models selected from those used in Figs \ref{['fig:picsarr_obs']} and \ref{['fig:mod_wavelengths']}. For 3 wavelengths (378nm, 491nm and 999nm) we also plot as dashed lines the modelled polarization of the center of the image.
• Figure 5: Comparison of the effect of the three components included in the polarization models (see Section \ref{['sec:models']}). At 400 nm all three components are needed to provide a reasonable match to the observations. At 600 nm the effect of Rayleigh scattering and the UV absorber are relatively small compared with the cloud particles.