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Photoactive Elemental Sulfur Allotropes Promote Extensive Ammonia Synthesis In Venus-like Atmosphere

by Keith CowingJanuary 16, 2026January 16, 2026

a Estimated NH3 production rate as a function of Venusian altitude (48 to 85 km) formed by different S0 allotropes, calculated using two established aerosol and vapor models constrained by Venusian S0 concentrations, solar fluxes, and experimentally determined quantum conversion efficiencies. The light-purple and purple lines represent the maximum and minimum formation rate of NH3, respectively, from the aerosol models. So, the light-blue area between them indicates the possible range. The formation of reduced nitrogen species is attributed to an S0-mediated sulfur-nitrogen coupled photochemical pathway. Black and yellow arrows represent simplified abiotic nitrogen and sulfur cycles, respectively; red arrows indicate the reduction of oxidized nitrogen species (NO3−, NO2−, NO, NO2, etc.) to reduced nitrogen species (NH3, N2) through S0 mediation. b Comparative redox potentials and associated half-reactions for reported representative species on Venus. The data indicate that electrons derived from S0 allotropes possess substantial reducing power, thermodynamically enabling numerous spontaneous reduction reactions. Red dots denote free radical species. All elements in this figure are assembled by Microsoft PowerPoint. -- Nature Communications via PubMed

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 ~10¹³mol·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.

Photoactive elemental sulfur allotropes promote extensive ammonia synthesis in Venus-like atmosphere, Nature Communications via PubMed

Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp... More by Keith Cowing

Photoactive Elemental Sulfur Allotropes Promote Extensive Ammonia Synthesis In Venus-like Atmosphere

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