Reflectivity of Venus' dayside disk during the 2020 observation campaign: outcomes and future perspectives

TL;DR

This study conducted a comprehensive observation campaign of Venus' dayside disk to analyze its reflectivity and absorption properties across a broad wavelength range, aiming to understand the unknown cloud absorber and sulfur dioxide levels.

Contribution

It presents the first detailed reflectivity spectrum analysis of Venus' disk over 283-800 nm, revealing absorption features and comparing them with previous flyby data, and discusses future observational strategies.

Findings

Detected significant absorption in 350-450 nm range linked to the unknown absorber.

Found wavelength dependence of optical depth consistent with low-latitude observations.

Established a framework for future campaigns to measure SO₂ and cloud properties.

Abstract

We performed a unique Venus observation campaign to measure the disk brightness of Venus over a broad range of wavelengths in August and September 2020. The primary goal of the campaign is to investigate the absorption properties of the unknown absorber in the clouds. The secondary goal is to extract a disk mean SO$_2$ gas abundance, whose absorption spectral feature is entangled with that of the unknown absorber at the ultraviolet (UV) wavelengths. A total of 3 spacecraft and 6 ground-based telescopes participated in this campaign, covering the 52 to 1700~nm wavelength range. After careful evaluation of the observational data, we focused on the data sets acquired by 4 facilities. We accomplished our primary goal by analyzing the reflectivity spectrum of the Venus disk over the 283-800 nm wavelengths. Considerable absorption is present in the 350-450 nm range, for which we retrieved the corresponding optical depth by the unknown absorber. The result shows a consistent wavelength dependence of the relative optical depth with that at low latitudes during the Venus flyby by MESSENGER in 2007 (P\'erez-Hoyos et al. 2018), which was expected because the overall disk reflectivity is dominated by low latitudes. Last, we summarize the experience obtained during this first campaign that should enable us to accomplish our second goal in future campaigns.