Tracing gas-giant global and local atmospheric processes through photo-kinetic chemistry

Recent JWST detections of SO₂ and H₂S in hydrogen-dominated exoplanet atmospheres have established sulphur as a powerful tracer of photochemistry and planetary evolution. Interpreting these detections often relies on one-dimensional (1D), limb-averaged models that neglect the strong three-dimensional (3D) thermal and radiative asymmetries predicted for tidally locked gas giants. Such asymmetries are expected to strongly influence the distribution of disequilibrium species, especially those produced photochemically.

 

We investigate how 3D atmospheric structure shapes gas-phase chemistry in warm gas-giant exoplanets by coupling photochemical kinetics calculations with the ARGO model to temperature–pressure profiles extracted from the ExoRad 3D global circulation model. Our study focuses on the warm Saturn WASP-69 b, a JWST target lying near the proposed SO₂ “shoreline,” and uses WASP-39 b as a benchmark case. By performing column-by-column 1D chemistry calculations across latitude and longitude, we isolate the impact of 3D climate features, such as hotspot offsets and high-latitude Rossby gyres, on chemically active species.

 

We find that methane (CH₄) is impacted at quench level (p=0.25 bar) at the morning terminator by particularly cold gyre structures at the poles and  by a hotspot offset at the equator. CH₄ also exhibits strong day-night column density variations (of an order of magnitude) driven by photodissociation (p=10^-3 -10^-4 bar). SO₂ forms photochemically at 10^-3-10^-5 bar, and traces the dayside hotspot shift such that production peaks westwards from the substellar point. SO₂ is less sensitive to the off-equatorial cold gyres dominating over large parts of the morning limb. Ammonia (NH₃) and carbon dioxide (CO₂) show only weak spatial sensitivity. These 3D chemical contrasts are comparable to, or larger than, variations caused by changes in metallicity or C/O ratio.

 

Our results demonstrate that SO₂ and CH₄ provide sensitive tracers of exoplanet climate regimes and highlight the need to incorporate the effect of 3D atmospheric structure in the interpretation of JWST spectra, and future CHEOPS–PLATO synergies. This work directly supports ongoing efforts to link observations and theory in the characterization of gas-giant exoplanet atmospheres.