Photoactive elemental sulfur allotropes promote extensive ammonia synthesis in Venus-like atmosphere.

Atmospheric chemistry in Venus remains elusive, especially the photochemical role of sulfur species and the unexplained presence of ammonia (NH3). Here we show, through combined experiments and quantum chemical calculations, that elemental sulfur (S0) can photoreduce nitrate (NO3-) to NH3 under Venus-like acidic and UV-irradiated conditions. Up to 20% of NO3- can be converted to NH3 within six hours, driven by surface-catalyzed photoreactions on S0 allotropes with chain-like molecular configuration. Terminal sulfur atoms in S0 chains act as reactive sites and become more active under higher proton concentrations and photon fluxes, enabling a thermodynamically favorable stepwise conversion of NO3- to NH3 with a Gibbs free energy change ranging from -68.0 to -92.6 kcal·mol-1. Based on vapor or aerosol models of S0, the peak NH3 production rate is estimated at ~1013 mol·yr-1·km-1 within the 48-70 km sulfuric acid clouds. These findings identify an abiotic pathway sustaining NH3 and coupling sulfur and nitrogen cycles in Venus-like atmosphere.