3D Monte-Carlo modeling of Venus' reflectance in UV

Observations of solar light backscattered by Venus' cloud top in the ultraviolet domain carry a rich scientific content, since they can be used to monitor absorbing species column densities, namely trace species like SO₂ and SO[1,2], but also the well-mixed main constituent CO₂ (which helps in constraining cloud top altitude) as well as the infamous UV absorber whose composition is still unknown as of 2025. These observations have revealed a large spatial and temporal variability at various scales, whose origin is still debated (atmospheric oscillations and/or volcanic plume forcings?)

Data analysis was usually performed using plane-parallel or pseudo-spherical radiative transfer models based on publicly available codes such as e.g. DISORT[3]. However, applying such models was not relevant at high emission or solar zenith angles (e.g. near dusk or dawn, or at very high latitudes), nor at very small horizontal scales comparable to the photon free mean path between two scattering events. To circumvent these limits, we used a 3D Monte Carlo solver provided by the MesoStar> company, and previously used to study Titan's atmosphere[4].

Preliminary results show that:

• 1D pseudo-spherical and spherical Monte-Carlo models agree for nadir observations up to solar zenith angles as high as 75°;
• Analysis of SPICAV-UV/Venus Express data near the terminator exhibit a already known decrease in SO₂ with increasing latitude, as well as dusk/dawn asymmetry in SO₂ column density (to be further investigated and confirmed);
• Horizontal UV contrasts at Venus' cloud top are typically blurred over a typical scale of ~10 km due to multiple scattering in the horizontal direction by the upper haze.

This work has been funded by the French National Research Agency (ANR), project RaD3-net, grant number ANR-21-CE49-0020-01.

References
[1] Marcq et al., Icarus (2020)
[2] Marcq et al., Nat Geosci (2013)
[3] Laszlo et al., in Light Scattering reviews (2016)
[4] He et al., JQSRT (2025, accepted)