Venus upper clouds and the UV-absorber from MESSENGER/MASCS observations

TL;DR

This study analyzes MESSENGER/MASCS data to investigate the UV-absorber in Venus' upper clouds, revealing its spectral properties and potential chemical candidates, thereby advancing understanding of Venus's atmospheric composition.

Contribution

The paper provides the first detailed spectral characterization of the UV-absorber using MESSENGER data and compares candidate molecules, offering new insights into its possible chemical nature.

Findings

UV-absorber centered at 0.34 μm

Cloud tops at 75 km altitude

Disulfur oxide and dioxide disulfur are plausible candidates

Abstract

One of the most intriguing, long-standing questions regarding Venus' atmosphere is the origin and distribution of the unknown UV-absorber, responsible for the absorption band detected at the near-UV and blue range of Venus' spectrum. In this work, we use data collected by MASCS spectrograph on board the MESSENGER mission during its second Venus flyby in June 2007 to address this issue. Spectra range from 0.3 {\mu}m to 1.5 {\mu}m including some gaseous H2O and CO2 bands, as well as part of the SO2 absorption band and the core of the UV absorption. We used the NEMESIS radiative transfer code and retrieval suite to investigate the vertical distribution of particles in the Equatorial atmosphere and to retrieve the imaginary refractive indices of the UV-absorber, assumed to be well mixed with Venus' small mode-1 particles. The results show an homogeneous Equatorial atmosphere, with cloud tops (height for unity optical depth) at 75+/-2 km above surface. The UV absorption is found to be centered at 0.34+/-0.03 {\mu}m with a full width half maximum of 0.14+/-0.01 {\mu}m. Our values are compared with previous candidates for the UV aerosol absorber, among which disulfur oxide (S2O) and dioxide disulfur (S2O2) provide the best agreement with our results.