Sulfur allotropes and sulfur hydrides on the Venus cloud chemistry

. IntroductionThe cloud layer of Venus between 47 and 70 km is home to a vivid sulfur chemistry and microphysics, with SO2 as the major gas species and condensed phase composed of H2SO4 and H2O. This cloud layer has been extensively observed and modelled. The main discrepancy between the observation and models in the SO2 vertical gradient throughout the cloud, no chemical model is able to reproduce the 3 order of magnitude decrease between the base and top of the cloud layer. Polysulfur chemistry could a candidate for buffering significant sulfur atoms, as it can grow into long chain and possibly condense. Strong absorption in Spectrophotometer data from VENERA 11, 12, 13 and 14 at 450–600 nm between 10 and 30 km recorded strong absorptions attributed most commonly to gaseous elemental sulfur Sx (Maiorov et al., 2005), and correlating S3 spectral features from Venera-11 with an increase with altitude from 0.03 ppbv at 3 km to 0.1 ppbv at 19 km (Maiorov et al., 2005). The presence of S3 and other sulfur allotropes polysulfur (Sx) in the clouds has been hypothesized from Vega probes (Porshnev et al., 1987), but no definitive detection of Sx in this region has been performed. Sx is a potential candidate to the UV unknown absorbed (Toon et al., 1982; Pérez-Hoyos et al., 2018). H2S were measured at ppm values below the clouds by the Pioneer Venus mission. (Hoffman et al., 1980; Oyama et al., 1980). DAVINCI will measure sulfur allotropes and H2S in the deep atmosphere (Garvin et al., 2022). For the first time in 3D and with realistic photolysis, we studied the impact of these species into the Venus cloud chemistry.2. ModelThe 3D Venus Planetary Climate Model (PCM) is used (Garate-Lopez and Lebonnois, 2018). The photochemical model describes the comprehensive chemistries of CO2, CO, hydrogen, oxygen, chlorine, sulfur and nitrogen down to roughly 35 km (Stolzenbach et al., 2023; Streel et al., 2025). In this study we added five species H2S, HS, S3, S4, S8 and 34 reactions to the gas phase chemistry. The PCM takes into account the condensation of H2SO4 and H2O based on the hypothesis that clouds are at all times in a state of thermodynamic equilibrium, i.e. following exactly the saturation pressure profile of the calculated equilibrium H2SO4 aqueous solution. The same philosophy is used for the polysufur condensation/evaporation in the model. We allow S2, S3, S4 and S8 to condense following (Zahnle et al., 2016). Photolysis rates are calculated online, following the same formalism as in the Mars version of the PCM (Lefèvre et al., 2021). Thescreening effect of all ultraviolet-absorbing species in the computation of the photolysis rates is taken into account. The radiative transfer computation is performed over the range of 0-815 nm. Four photodissociations were added following recent theoretical calculations, and the spectra of S2 was updated.3. ResultsFig 1 shows the zonal-mean distribution of condensed S2, reaching 0.25 ppm at the cloud-top. It is the dominating species of the sulfur allotrope due to its low saturation mixing ratio. In total this chemistry can store up 0.5 ppm with a low SO2 value below the clouds, and around 4 ppm with realistic SO2 values. Polysulfur cloud chemistry appears to be a substantial sulfur buffers but not large enough to explain the SO2 cloud vertical gradient. H2S is stable with few tenths of ppm in the lower clouds.Fig 1: Zonal-mean distribution of condensed S2 in ppmvReferencesGarate-Lopez, I., & Lebonnois, S. (2018). Icarus, 314:1–11.Garvin, J. B., Getty, S. A., Arney, G. N., et al. (2022). The Planetary Science Journal, 3(5):117.Hoffman, J. H., Hodges, R. R., Donahue, T. M., et al. (1980). Journal of Geophysical Research, 85:7882–7890.Lefèvre, F., Trokhimovskiy, A., Fedorova, A., et al. (2021). Journal of Geophysical Research (Planets), 126(4):e06838.Maiorov, B. S., Ignat’ev, N. I., Moroz, V. I., et al. (2005). Solar System Research, 39(4):267–282.Oyama, V. I., Carle, G. C., Woeller, F., et al. (1980). Journal of Geophysical Research, 85:7891–7902.Pérez-Hoyos, S., Sánchez-Lavega, A., García-Muñoz, A., et al. (2018). Journal of Geophysical Research (Planets), 123:145–162.Porshnev, N. V., Mukhin, L. M., Gel’Man, B. G., et al. (1987). Kosmicheskie Issledovaniia, 25:715–720.Stolzenbach, A., Lefèvre, F., Lebonnois, S., et al. (2023). Icarus, 395:115447.Streel, N., Lefèvre, F., Martinez, A., et al. (2025). Submitted to JGR: Planets.Toon, O. B., Turco, R. P., & Pollack, J. B. (1982). Icarus, 51:358–373. Zahnle, K., Marley, M. S., Morley, C. V., et al. (2016). The Astrophysical Journal, 824:137.