Modelling Venus's Spectropolarimetric Signatures for EnVision

Polarimetry is a powerful tool to characterise a planet's clouds and hazes. The degree of polarisation of sunlight that is reflected by a planet is very sensitive to the illumination and viewing angles, the wavelength of the light, and the composition, size, and shape of the cloud and haze particles. Additionally, the degree of polarisation is rather insensitive to instrumental errors and to uncertainties in the total flux of sunlight reaching the planet.

EnVision, ESA’s next Venus orbiter, will carry the spectrometers VenSpec-U and VenSpec-H, both of which are polarisation-sensitive. Accurate measurements of the total (polarised + unpolarised) flux of the sunlight that Venus reflects therefore require information about the degree of polarisation of the incoming light. VenSpec-H includes polarisation filters that, apart from correcting for the polarisation sensitivity, will also provide valuable science data.

To support the total flux and polarisation measurements, we have developed a state-of-the-art Fortran radiative transfer code based on the Monte Carlo technique. This code enables us to simulate VenSpec-U and -H observations, fully accounting for the polarisation of light. With our model simulations, we can investigate how the measurements should be taken to minimise the errors and to maximise the amount of atmospheric information that can be retrieved. The code also accounts for the sphericity of Venus’ atmosphere, which is important for accurate simulations in twilight and polar regions. In this talk, we will show simulations of the total flux and polarisation signals of the sunlight that is reflected by Venus and discuss the spectropolarimetric signatures of the clouds and hazes.