Investigation of UV Absorbers on Venus Using the 283 and 365 nm Phase Curves Obtained From Akatsuki

T. Encrenaz,A. García Muñoz,S. Watanabe,A. Yamazaki,Y. J. Lee,M. Yamada

doi:10.1029/2020gl090577

Abstract

AbstractThe so‐called unknown absorber in the clouds of Venus is an important absorber of solar energy, but its vertical distribution remains poorly quantified. We analyze the 283 and 365 nm phase curves of the disk‐integrated albedo measured by Akatsuki. Based on our models, we find that the unknown absorber can exist either well mixed over the entire upper cloud or within a thin layer. The necessary condition to explain the 365 nm phase curve is that the unknown absorber must absorb efficiently within the cloud scale height immediately below the cloud top. Using this constraint, we attempt to extract the SO2 abundance from the 283 nm phase curve. However, we cannot disentangle the absorption by SO2 and by the unknown absorber. Considering previous SO2 abundance measurements at midinfrared wavelengths, the required absorption coefficient of the unknown absorber at 283 nm must be more than twice that at 365 nm.

Highlights

The 80% of solar energy incident on Venus is reflected back to space by the thick and highly reflective clouds that cover the entire planet (Moroz et al, 1985; Tomasko, Smith, et al, 1980)
We find that the unknown absorber can exist either well mixed over the entire upper cloud or within a thin layer
We find that the UA could exist in the entire upper cloud or in a thin layer in the cloud, but the necessary condition for all UA scenarios is that sufficient absorption occurs within a few kilometers immediately below the cloud top

Summary

Introduction

The 80% of solar energy incident on Venus is reflected back to space by the thick and highly reflective clouds that cover the entire planet (Moroz et al, 1985; Tomasko, Smith, et al, 1980) This reflected radiation has a strong directional dependency, which has been used to retrieve the microphysical properties of the cloud aerosols (Arking & Potter, 1968; García Muñoz et al, 2014; Lee et al, 2017; Mallama et al, 2006; Markiewicz et al, 2014; Petrova et al, 2015; Shalygina et al, 2015). The UA is reported to have its maximum absorption at 340 nm (Pérez-Hoyos et al, 2018) and becomes less absorbing toward shorter wavelengths (Marcq et al, 2020; Pérez-Hoyos et al, 2018), where it overlaps with SO2 absorption

Results

Discussion

Conclusion