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Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability¹. However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program^2,3found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO₂) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M_J) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref. ⁴). The most plausible way of generating SO₂in such an atmosphere is through photochemical processes^5,6. Here we show that the SO₂distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations⁷with NIRSpec PRISM (2.7σ)⁸and G395H (4.5σ)⁹. SO₂is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H₂S) is destroyed. The sensitivity of the SO₂feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO₂also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.

The current Venus climate is largely regulated by globally covered concentrated sulfuric acid clouds from binary condensation of sulfuric acid (H₂SO₄) and water (H₂O). To understand this complicated H₂SO₄‐H₂O gas‐cloud system, previous theoretical studies either adopted complicated microphysical calculations or assumed that both H₂SO₄and H₂O vapor follow their saturation vapor pressure. In this study, we developed a simple one‐dimensional cloud condensation model including condensation, diffusion and sedimentation of H₂SO₄and H₂O but without detailed microphysics. Our model is able to explain the observed vertical structure of cloud and upper haze mass loading, cloud acidity, H₂SO₄, and H₂O vapor, and the mode‐2 particle size on Venus. We found that most H₂SO₄is stored in the condensed phase above 48 km, while the partitioning of H₂O between the vapor and clouds is complicated. The cloud cycle is mostly driven by evaporation and condensation of H₂SO₄rather than H₂O and is about seven times stronger than the H₂SO₄photochemical cycle. Most of the condensed H₂O in the upper clouds is evaporated before the falling particles reach the middle clouds. The cloud acidity is affected by the temperature and the condensation‐evaporation cycles of both H₂SO₄and H₂O. Because of the large chemical production of H₂SO₄vapor and relatively inefficient cloud condensation, the simulated H₂SO₄vapor above 60 km is largely supersaturated by more than two orders of magnitude, which could be tested by future observations.

Sulfur‐water chemistry plays an important role in the middle atmosphere of Venus. Ground‐based observations have found that simultaneously observed SO₂and H₂O at ~64 km vary with time and are temporally anticorrelated. To understand these observations, we explore the sulfur‐water chemical system using a one‐dimensional chemistry‐diffusion model. We find that SO₂and H₂O mixing ratios above the clouds are highly dependent on mixing ratios of the two species at the middle cloud top (58 km). The behavior of sulfur‐water chemical system can be classified into three regimes, but there is no abrupt transition among these regimes. In particular, there is no bifurcation behavior as previously claimed. We also find that the SO₂self‐shielding effect causes H₂O above the clouds to respond to the middle cloud top in a nonmonotonic fashion. Through comparison with observations, we find that mixing ratio variations at the middle cloud top can explain the observed variability of SO₂and H₂O. The sulfur‐water chemistry in the middle atmosphere is responsible for the H₂O‐SO₂anticorrelation at 64 km. Eddy transport change alone cannot explain the variations of both species. These results imply that variations of species abundance in the middle atmosphere are significantly influenced by the lower atmospheric processes. Continued ground‐based measurements of the coevolution of SO₂and H₂O above the clouds and new spacecraft missions will be crucial for uncovering the complicated processes underlying the interaction among the lower atmosphere, the clouds, and the middle atmosphere of Venus.