Investigation of UV absorbers on Venus using the 283 and 365 nm phase curves obtained from Akatsuki

TL;DR

This study analyzes Venus's cloud absorption properties using Akatsuki's UV phase curves, suggesting the unknown absorber is either well-mixed or confined to a thin layer, but its exact nature remains uncertain.

Contribution

It provides new constraints on the vertical distribution and absorption characteristics of the unknown UV absorber in Venus's clouds.

Findings

The unknown absorber can be well-mixed or confined to a thin layer.

Efficient absorption below the cloud top is necessary to explain the 365-nm phase curve.

The absorption coefficient of the unknown absorber at 283 nm must be over twice that at 365 nm.

Abstract

The 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 SO$_2$ abundance from the 283-nm phase curve. However we cannot disentangle the absorption by SO$_2$ and by the unknown absorber. Considering previous SO$_2$ abundance measurements at mid-infrared wavelengths, the required absorption coefficient of the unknown absorber at 283~nm must be more than twice that at 365~nm.