Recently, through Venus atmospheric experiments, the Planetary Science Team of Shandong University revealed that gas discharge in the cloud layers of Venus is an important mechanism producing the high concentration of polysulfur and sulfur-oxides, which could be the precursors leading to the formation of the strips with the famous unknown Ultraviolet (UV) absorber in the atmosphere. The related study has been published in the journal Earth and Planetary Science Letters. This paper features Dr. Qu Quincy Hongkun (a postdoctor at Shandong University) as first author, with Prof. Alian Wang from the Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University in St. Louis, and Prof. Ling Zongcheng from the School of Space Science and Technology, Shandong University, as corresponding authors. Shandong University is credited as the first and corresponding author institution.

The origin of the dark strips in the ultraviolet images of Venus has been one of the major challenges in Venusian science for almost a century. The dark and white strips shown in Fig. 1 are attributed to the absorption of UV radiation from the Sun by the famous unknown UV-absorbers, which are both inhomogeneously distributed and temporally variable. Three absorption bands are visible in the reflectance spectrum of Venus between 200 and 400 nm. It has been suggested that at least two chemical substances would be needed to contribute to these absorption bands. SO2 causes the absorption feature in the short UV range (nm) in the atmosphere of Venus. However, the contributors for NUV absorption (320 - 400 nm)and their generation mechanisms have been in debate for almost a century, which is typically referred to as the unknown UV-absorber. Ultraviolet absorption in Venus' atmosphere is one of the hot topics in Venusian science. It is of great significance for studying the morphology, motion, and dynamical state of Venus' cloud layers, serving as a key piece in understanding the atmospheric chemistry and revealing the evolutionary mechanisms of Venus' atmosphere. Many candidates have been proposed, e.g., S₈,NOHSO₄,Cl₂,(NH₄)₂S₂O₅,FeCl_3,SCl₂,C₆O₅H₂,S₂O,(SO)₂,S_n and iron-sulfur mineral chemistry. They are primarily made up of sulfur compounds. Moreover, the NUV absorption was found to be spatially correlated with that of SO₂, suggesting the mysterious UV-absorber may connect with the chemical processes involving SO₂.

Fig 1 Albedo of the Venusian atmosphere and image at 365 nm

To address this scientific question, we conducted a new set of electrical discharge experiments within the gas mixtures of SO2, various mixing ratios with the major Venus gases (major Venus gases: 96.5 % CO₂+3.5 % N₂) under the conditions relevant to the Venus cloud layer (~ 52 - 75 km) anddetectedthe free radicals that are simultaneously generated by discharge in the gaseous mixture. The free radicals N2^*, CO₂⁺, CO^*, C_I^*, C_II^*, O_I^*, O_II^*, CN^*, S_I^*, S_II^*, S₂^*, and SO^* were identified by their emission lines, indicating that the two major breakdown paths of SO₂ by electrical discharge (SO₂→SO+Oand SO₂→S+ O₂) are likely to occur during electrical events on Venus. These highly reactive free radicals are unstable and will react further with ambient gases to generate new compounds. Combining the above observations on experimental products, we generated Fig.2that, which shows the involved electrochemical processes and the breakdown products of SO₂. Two major groups of candidates for the mysterious UV-absorber—polysulfur (S_n, n = 2-7) and sulfur-oxides ((SO)₂, S₂O, etc.) have been formed during the electrical discharges.

Fig 2 A schematic diagram of the SO2 electrochemical reaction pathway

The projected transient yield of active S-species (SO^∗and S_n^∗) by electrical activity would still be ~3 orders of magnitude higher than the global mixing ratio estimated by a photochemistry model. Electrochemistry usually takes place in a local region and causes a local enhancement of the mixing ratio of S-bearing species in a local region. In this case, the famous unknown absorber may be created and absorb UV light from the Sun, contributing to the dark and white strips in the UV images of Venus.

Fig 3 A comparison of the predicted transient local mixing ratio of S-bearing species (red band) by lightning on Venus with global mixing ratio SO₂ and SO from mission observations and of S (black line) from a photochemical model prediction

This study demonstrated that (1) the direct electrical breakdown products of SO₂ in Venusian atmosphere are possibly important precursors for the mysterious UV-absorber; (2) the local enrichment and the change of distribution with the time of the breakdown products of SO₂ can contribute to the inhomogeneous spatial distribution of the mysterious UV absorber and its temporal changes as observed in Venus UV-images; (3) the formation of H₂SO₄ and S₈during gas discharges indicates that electrochemistry plays significant roles in the sulfur cycle on Venus. It also reveals a path forward for reanalyzing Venus mission data and for more experimental and modeling investigations. This holds significant implications for future international and Chinese Venus exploration missions as well as their data interpretations.

This work was supported by the National Natural Science Foundation of China, China Postdoctoral Science Foundation, and special funding from the McDonnell Center for Space Sciences (MCSS) at WUSTL to maintain a collaboration with planetary scientists and students from Shandong University in China. We are grateful for the instructive advice and for the provision of some lab equipment by Prof. Bruce Fegley at WUSTL.