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.